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Frontswap provides a "transcendent memory" interface for swap pages.

Browse Source 
In some environments, dramatic performance savings may be obtained because
 swapped pages are saved in RAM (or a RAM-like device) instead of a swap disk.
 This tag provides the basic infrastructure along with some changes to the
 existing backends.
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Merge tag 'stable/frontswap.v16-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/konrad/mm

Pull frontswap feature from Konrad Rzeszutek Wilk:
 "Frontswap provides a "transcendent memory" interface for swap pages.
  In some environments, dramatic performance savings may be obtained
  because swapped pages are saved in RAM (or a RAM-like device) instead
  of a swap disk.  This tag provides the basic infrastructure along with
  some changes to the existing backends."

Fix up trivial conflict in mm/Makefile due to removal of swap token code
changing a line next to the new frontswap entry.

This pull request came in before the merge window even opened, it got
delayed to after the merge window by me just wanting to make sure it had
actual users.  Apparently IBM is using this on their embedded side, and
Jan Beulich says that it's already made available for SLES and OpenSUSE
users.

Also acked by Rik van Riel, and Konrad points to other people liking it
too.  So in it goes.

By Dan Magenheimer (4) and Konrad Rzeszutek Wilk (2)
via Konrad Rzeszutek Wilk
* tag 'stable/frontswap.v16-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/konrad/mm:
  frontswap: s/put_page/store/g s/get_page/load
  MAINTAINER: Add myself for the frontswap API
  mm: frontswap: config and doc files
  mm: frontswap: core frontswap functionality
  mm: frontswap: core swap subsystem hooks and headers
  mm: frontswap: add frontswap header file
This commit is contained in:
Linus Torvalds 2012-06-04 12:28:45 -07:00
commit a3fe778c78
13 changed files with 827 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

278
Documentation/vm/frontswap.txt Normal file
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@ -0,0 +1,278 @@
Frontswap provides a "transcendent memory" interface for swap pages.
In some environments, dramatic performance savings may be obtained because
swapped pages are saved in RAM (or a RAM-like device) instead of a swap disk.
(Note, frontswap -- and cleancache (merged at 3.0) -- are the "frontends"
and the only necessary changes to the core kernel for transcendent memory;
all other supporting code -- the "backends" -- is implemented as drivers.
See the LWN.net article "Transcendent memory in a nutshell" for a detailed
overview of frontswap and related kernel parts:
https://lwn.net/Articles/454795/ )
Frontswap is so named because it can be thought of as the opposite of
a "backing" store for a swap device. The storage is assumed to be
a synchronous concurrency-safe page-oriented "pseudo-RAM device" conforming
to the requirements of transcendent memory (such as Xen's "tmem", or
in-kernel compressed memory, aka "zcache", or future RAM-like devices);
this pseudo-RAM device is not directly accessible or addressable by the
kernel and is of unknown and possibly time-varying size. The driver
links itself to frontswap by calling frontswap_register_ops to set the
frontswap_ops funcs appropriately and the functions it provides must
conform to certain policies as follows:
An "init" prepares the device to receive frontswap pages associated
with the specified swap device number (aka "type"). A "store" will
copy the page to transcendent memory and associate it with the type and
offset associated with the page. A "load" will copy the page, if found,
from transcendent memory into kernel memory, but will NOT remove the page
from from transcendent memory. An "invalidate_page" will remove the page
from transcendent memory and an "invalidate_area" will remove ALL pages
associated with the swap type (e.g., like swapoff) and notify the "device"
to refuse further stores with that swap type.
Once a page is successfully stored, a matching load on the page will normally
succeed. So when the kernel finds itself in a situation where it needs
to swap out a page, it first attempts to use frontswap. If the store returns
success, the data has been successfully saved to transcendent memory and
a disk write and, if the data is later read back, a disk read are avoided.
If a store returns failure, transcendent memory has rejected the data, and the
page can be written to swap as usual.
If a backend chooses, frontswap can be configured as a "writethrough
cache" by calling frontswap_writethrough(). In this mode, the reduction
in swap device writes is lost (and also a non-trivial performance advantage)
in order to allow the backend to arbitrarily "reclaim" space used to
store frontswap pages to more completely manage its memory usage.
Note that if a page is stored and the page already exists in transcendent memory
(a "duplicate" store), either the store succeeds and the data is overwritten,
or the store fails AND the page is invalidated. This ensures stale data may
never be obtained from frontswap.
If properly configured, monitoring of frontswap is done via debugfs in
the /sys/kernel/debug/frontswap directory. The effectiveness of
frontswap can be measured (across all swap devices) with:
failed_stores - how many store attempts have failed
loads - how many loads were attempted (all should succeed)
succ_stores - how many store attempts have succeeded
invalidates - how many invalidates were attempted
A backend implementation may provide additional metrics.
FAQ
1) Where's the value?
When a workload starts swapping, performance falls through the floor.
Frontswap significantly increases performance in many such workloads by
providing a clean, dynamic interface to read and write swap pages to
"transcendent memory" that is otherwise not directly addressable to the kernel.
This interface is ideal when data is transformed to a different form
and size (such as with compression) or secretly moved (as might be
useful for write-balancing for some RAM-like devices). Swap pages (and
evicted page-cache pages) are a great use for this kind of slower-than-RAM-
but-much-faster-than-disk "pseudo-RAM device" and the frontswap (and
cleancache) interface to transcendent memory provides a nice way to read
and write -- and indirectly "name" -- the pages.
Frontswap -- and cleancache -- with a fairly small impact on the kernel,
provides a huge amount of flexibility for more dynamic, flexible RAM
utilization in various system configurations:
In the single kernel case, aka "zcache", pages are compressed and
stored in local memory, thus increasing the total anonymous pages
that can be safely kept in RAM. Zcache essentially trades off CPU
cycles used in compression/decompression for better memory utilization.
Benchmarks have shown little or no impact when memory pressure is
low while providing a significant performance improvement (25%+)
on some workloads under high memory pressure.
"RAMster" builds on zcache by adding "peer-to-peer" transcendent memory
support for clustered systems. Frontswap pages are locally compressed
as in zcache, but then "remotified" to another system's RAM. This
allows RAM to be dynamically load-balanced back-and-forth as needed,
i.e. when system A is overcommitted, it can swap to system B, and
vice versa. RAMster can also be configured as a memory server so
many servers in a cluster can swap, dynamically as needed, to a single
server configured with a large amount of RAM... without pre-configuring
how much of the RAM is available for each of the clients!
In the virtual case, the whole point of virtualization is to statistically
multiplex physical resources acrosst the varying demands of multiple
virtual machines. This is really hard to do with RAM and efforts to do
it well with no kernel changes have essentially failed (except in some
well-publicized special-case workloads).
Specifically, the Xen Transcendent Memory backend allows otherwise
"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple
virtual machines, but the pages can be compressed and deduplicated to
optimize RAM utilization. And when guest OS's are induced to surrender
underutilized RAM (e.g. with "selfballooning"), sudden unexpected
memory pressure may result in swapping; frontswap allows those pages
to be swapped to and from hypervisor RAM (if overall host system memory
conditions allow), thus mitigating the potentially awful performance impact
of unplanned swapping.
A KVM implementation is underway and has been RFC'ed to lkml. And,
using frontswap, investigation is also underway on the use of NVM as
a memory extension technology.
2) Sure there may be performance advantages in some situations, but
what's the space/time overhead of frontswap?
If CONFIG_FRONTSWAP is disabled, every frontswap hook compiles into
nothingness and the only overhead is a few extra bytes per swapon'ed
swap device. If CONFIG_FRONTSWAP is enabled but no frontswap "backend"
registers, there is one extra global variable compared to zero for
every swap page read or written. If CONFIG_FRONTSWAP is enabled
AND a frontswap backend registers AND the backend fails every "store"
request (i.e. provides no memory despite claiming it might),
CPU overhead is still negligible -- and since every frontswap fail
precedes a swap page write-to-disk, the system is highly likely
to be I/O bound and using a small fraction of a percent of a CPU
will be irrelevant anyway.
As for space, if CONFIG_FRONTSWAP is enabled AND a frontswap backend
registers, one bit is allocated for every swap page for every swap
device that is swapon'd. This is added to the EIGHT bits (which
was sixteen until about 2.6.34) that the kernel already allocates
for every swap page for every swap device that is swapon'd. (Hugh
Dickins has observed that frontswap could probably steal one of
the existing eight bits, but let's worry about that minor optimization
later.) For very large swap disks (which are rare) on a standard
4K pagesize, this is 1MB per 32GB swap.
When swap pages are stored in transcendent memory instead of written
out to disk, there is a side effect that this may create more memory
pressure that can potentially outweigh the other advantages. A
backend, such as zcache, must implement policies to carefully (but
dynamically) manage memory limits to ensure this doesn't happen.
3) OK, how about a quick overview of what this frontswap patch does
in terms that a kernel hacker can grok?
Let's assume that a frontswap "backend" has registered during
kernel initialization; this registration indicates that this
frontswap backend has access to some "memory" that is not directly
accessible by the kernel. Exactly how much memory it provides is
entirely dynamic and random.
Whenever a swap-device is swapon'd frontswap_init() is called,
passing the swap device number (aka "type") as a parameter.
This notifies frontswap to expect attempts to "store" swap pages
associated with that number.
Whenever the swap subsystem is readying a page to write to a swap
device (c.f swap_writepage()), frontswap_store is called. Frontswap
consults with the frontswap backend and if the backend says it does NOT
have room, frontswap_store returns -1 and the kernel swaps the page
to the swap device as normal. Note that the response from the frontswap
backend is unpredictable to the kernel; it may choose to never accept a
page, it could accept every ninth page, or it might accept every
page. But if the backend does accept a page, the data from the page
has already been copied and associated with the type and offset,
and the backend guarantees the persistence of the data. In this case,
frontswap sets a bit in the "frontswap_map" for the swap device
corresponding to the page offset on the swap device to which it would
otherwise have written the data.
When the swap subsystem needs to swap-in a page (swap_readpage()),
it first calls frontswap_load() which checks the frontswap_map to
see if the page was earlier accepted by the frontswap backend. If
it was, the page of data is filled from the frontswap backend and
the swap-in is complete. If not, the normal swap-in code is
executed to obtain the page of data from the real swap device.
So every time the frontswap backend accepts a page, a swap device read
and (potentially) a swap device write are replaced by a "frontswap backend
store" and (possibly) a "frontswap backend loads", which are presumably much
faster.
4) Can't frontswap be configured as a "special" swap device that is
just higher priority than any real swap device (e.g. like zswap,
or maybe swap-over-nbd/NFS)?
No. First, the existing swap subsystem doesn't allow for any kind of
swap hierarchy. Perhaps it could be rewritten to accomodate a hierarchy,
but this would require fairly drastic changes. Even if it were
rewritten, the existing swap subsystem uses the block I/O layer which
assumes a swap device is fixed size and any page in it is linearly
addressable. Frontswap barely touches the existing swap subsystem,
and works around the constraints of the block I/O subsystem to provide
a great deal of flexibility and dynamicity.
For example, the acceptance of any swap page by the frontswap backend is
entirely unpredictable. This is critical to the definition of frontswap
backends because it grants completely dynamic discretion to the
backend. In zcache, one cannot know a priori how compressible a page is.
"Poorly" compressible pages can be rejected, and "poorly" can itself be
defined dynamically depending on current memory constraints.
Further, frontswap is entirely synchronous whereas a real swap
device is, by definition, asynchronous and uses block I/O. The
block I/O layer is not only unnecessary, but may perform "optimizations"
that are inappropriate for a RAM-oriented device including delaying
the write of some pages for a significant amount of time. Synchrony is
required to ensure the dynamicity of the backend and to avoid thorny race
conditions that would unnecessarily and greatly complicate frontswap
and/or the block I/O subsystem. That said, only the initial "store"
and "load" operations need be synchronous. A separate asynchronous thread
is free to manipulate the pages stored by frontswap. For example,
the "remotification" thread in RAMster uses standard asynchronous
kernel sockets to move compressed frontswap pages to a remote machine.
Similarly, a KVM guest-side implementation could do in-guest compression
and use "batched" hypercalls.
In a virtualized environment, the dynamicity allows the hypervisor
(or host OS) to do "intelligent overcommit". For example, it can
choose to accept pages only until host-swapping might be imminent,
then force guests to do their own swapping.
There is a downside to the transcendent memory specifications for
frontswap: Since any "store" might fail, there must always be a real
slot on a real swap device to swap the page. Thus frontswap must be
implemented as a "shadow" to every swapon'd device with the potential
capability of holding every page that the swap device might have held
and the possibility that it might hold no pages at all. This means
that frontswap cannot contain more pages than the total of swapon'd
swap devices. For example, if NO swap device is configured on some
installation, frontswap is useless. Swapless portable devices
can still use frontswap but a backend for such devices must configure
some kind of "ghost" swap device and ensure that it is never used.
5) Why this weird definition about "duplicate stores"? If a page
has been previously successfully stored, can't it always be
successfully overwritten?
Nearly always it can, but no, sometimes it cannot. Consider an example
where data is compressed and the original 4K page has been compressed
to 1K. Now an attempt is made to overwrite the page with data that
is non-compressible and so would take the entire 4K. But the backend
has no more space. In this case, the store must be rejected. Whenever
frontswap rejects a store that would overwrite, it also must invalidate
the old data and ensure that it is no longer accessible. Since the
swap subsystem then writes the new data to the read swap device,
this is the correct course of action to ensure coherency.
6) What is frontswap_shrink for?
When the (non-frontswap) swap subsystem swaps out a page to a real
swap device, that page is only taking up low-value pre-allocated disk
space. But if frontswap has placed a page in transcendent memory, that
page may be taking up valuable real estate. The frontswap_shrink
routine allows code outside of the swap subsystem to force pages out
of the memory managed by frontswap and back into kernel-addressable memory.
For example, in RAMster, a "suction driver" thread will attempt
to "repatriate" pages sent to a remote machine back to the local machine;
this is driven using the frontswap_shrink mechanism when memory pressure
subsides.
7) Why does the frontswap patch create the new include file swapfile.h?
The frontswap code depends on some swap-subsystem-internal data
structures that have, over the years, moved back and forth between
static and global. This seemed a reasonable compromise: Define
them as global but declare them in a new include file that isn't
included by the large number of source files that include swap.h.
Dan Magenheimer, last updated April 9, 2012

7
MAINTAINERS
View File

@ -2930,6 +2930,13 @@ F: Documentation/power/freezing-of-tasks.txt
F: include/linux/freezer.h
F: kernel/freezer.c
FRONTSWAP API
M: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
L: linux-kernel@vger.kernel.org
S: Maintained
F: mm/frontswap.c
F: include/linux/frontswap.h
FS-CACHE: LOCAL CACHING FOR NETWORK FILESYSTEMS
M: David Howells <dhowells@redhat.com>
L: linux-cachefs@redhat.com

8
drivers/staging/ramster/zcache-main.c
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@ -3002,7 +3002,7 @@ static inline struct tmem_oid oswiz(unsigned type, u32 ind)
return oid;
}
static int zcache_frontswap_put_page(unsigned type, pgoff_t offset,
static int zcache_frontswap_store(unsigned type, pgoff_t offset,
struct page *page)
{
u64 ind64 = (u64)offset;
@ -3025,7 +3025,7 @@ static int zcache_frontswap_put_page(unsigned type, pgoff_t offset,
/* returns 0 if the page was successfully gotten from frontswap, -1 if
* was not present (should never happen!) */
static int zcache_frontswap_get_page(unsigned type, pgoff_t offset,
static int zcache_frontswap_load(unsigned type, pgoff_t offset,
struct page *page)
{
u64 ind64 = (u64)offset;
@ -3080,8 +3080,8 @@ static void zcache_frontswap_init(unsigned ignored)
}
static struct frontswap_ops zcache_frontswap_ops = {
.put_page = zcache_frontswap_put_page,
.get_page = zcache_frontswap_get_page,
.store = zcache_frontswap_store,
.load = zcache_frontswap_load,
.invalidate_page = zcache_frontswap_flush_page,
.invalidate_area = zcache_frontswap_flush_area,
.init = zcache_frontswap_init

10
drivers/staging/zcache/zcache-main.c
View File

@ -1835,7 +1835,7 @@ static int zcache_frontswap_poolid = -1;
* Swizzling increases objects per swaptype, increasing tmem concurrency
* for heavy swaploads. Later, larger nr_cpus -> larger SWIZ_BITS
* Setting SWIZ_BITS to 27 basically reconstructs the swap entry from
* frontswap_get_page(), but has side-effects. Hence using 8.
* frontswap_load(), but has side-effects. Hence using 8.
*/
#define SWIZ_BITS 8
#define SWIZ_MASK ((1 << SWIZ_BITS) - 1)
@ -1849,7 +1849,7 @@ static inline struct tmem_oid oswiz(unsigned type, u32 ind)
return oid;
}
static int zcache_frontswap_put_page(unsigned type, pgoff_t offset,
static int zcache_frontswap_store(unsigned type, pgoff_t offset,
struct page *page)
{
u64 ind64 = (u64)offset;
@ -1870,7 +1870,7 @@ static int zcache_frontswap_put_page(unsigned type, pgoff_t offset,
/* returns 0 if the page was successfully gotten from frontswap, -1 if
* was not present (should never happen!) */
static int zcache_frontswap_get_page(unsigned type, pgoff_t offset,
static int zcache_frontswap_load(unsigned type, pgoff_t offset,
struct page *page)
{
u64 ind64 = (u64)offset;
@ -1919,8 +1919,8 @@ static void zcache_frontswap_init(unsigned ignored)
}
static struct frontswap_ops zcache_frontswap_ops = {
.put_page = zcache_frontswap_put_page,
.get_page = zcache_frontswap_get_page,
.store = zcache_frontswap_store,
.load = zcache_frontswap_load,
.invalidate_page = zcache_frontswap_flush_page,
.invalidate_area = zcache_frontswap_flush_area,
.init = zcache_frontswap_init

8
drivers/xen/tmem.c
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@ -269,7 +269,7 @@ static inline struct tmem_oid oswiz(unsigned type, u32 ind)
}
/* returns 0 if the page was successfully put into frontswap, -1 if not */
static int tmem_frontswap_put_page(unsigned type, pgoff_t offset,
static int tmem_frontswap_store(unsigned type, pgoff_t offset,
struct page *page)
{
u64 ind64 = (u64)offset;
@ -295,7 +295,7 @@ static int tmem_frontswap_put_page(unsigned type, pgoff_t offset,
* returns 0 if the page was successfully gotten from frontswap, -1 if
* was not present (should never happen!)
*/
static int tmem_frontswap_get_page(unsigned type, pgoff_t offset,
static int tmem_frontswap_load(unsigned type, pgoff_t offset,
struct page *page)
{
u64 ind64 = (u64)offset;
@ -362,8 +362,8 @@ static int __init no_frontswap(char *s)
__setup("nofrontswap", no_frontswap);
static struct frontswap_ops __initdata tmem_frontswap_ops = {
.put_page = tmem_frontswap_put_page,
.get_page = tmem_frontswap_get_page,
.store = tmem_frontswap_store,
.load = tmem_frontswap_load,
.invalidate_page = tmem_frontswap_flush_page,
.invalidate_area = tmem_frontswap_flush_area,
.init = tmem_frontswap_init

127
include/linux/frontswap.h Normal file
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@ -0,0 +1,127 @@
#ifndef _LINUX_FRONTSWAP_H
#define _LINUX_FRONTSWAP_H
#include <linux/swap.h>
#include <linux/mm.h>
#include <linux/bitops.h>
struct frontswap_ops {
void (*init)(unsigned);
int (*store)(unsigned, pgoff_t, struct page *);
int (*load)(unsigned, pgoff_t, struct page *);
void (*invalidate_page)(unsigned, pgoff_t);
void (*invalidate_area)(unsigned);
};
extern bool frontswap_enabled;
extern struct frontswap_ops
frontswap_register_ops(struct frontswap_ops *ops);
extern void frontswap_shrink(unsigned long);
extern unsigned long frontswap_curr_pages(void);
extern void frontswap_writethrough(bool);
extern void __frontswap_init(unsigned type);
extern int __frontswap_store(struct page *page);
extern int __frontswap_load(struct page *page);
extern void __frontswap_invalidate_page(unsigned, pgoff_t);
extern void __frontswap_invalidate_area(unsigned);
#ifdef CONFIG_FRONTSWAP
static inline bool frontswap_test(struct swap_info_struct *sis, pgoff_t offset)
{
bool ret = false;
if (frontswap_enabled && sis->frontswap_map)
ret = test_bit(offset, sis->frontswap_map);
return ret;
}
static inline void frontswap_set(struct swap_info_struct *sis, pgoff_t offset)
{
if (frontswap_enabled && sis->frontswap_map)
set_bit(offset, sis->frontswap_map);
}
static inline void frontswap_clear(struct swap_info_struct *sis, pgoff_t offset)
{
if (frontswap_enabled && sis->frontswap_map)
clear_bit(offset, sis->frontswap_map);
}
static inline void frontswap_map_set(struct swap_info_struct *p,
unsigned long *map)
{
p->frontswap_map = map;
}
static inline unsigned long *frontswap_map_get(struct swap_info_struct *p)
{
return p->frontswap_map;
}
#else
/* all inline routines become no-ops and all externs are ignored */
#define frontswap_enabled (0)
static inline bool frontswap_test(struct swap_info_struct *sis, pgoff_t offset)
{
return false;
}
static inline void frontswap_set(struct swap_info_struct *sis, pgoff_t offset)
{
}
static inline void frontswap_clear(struct swap_info_struct *sis, pgoff_t offset)
{
}
static inline void frontswap_map_set(struct swap_info_struct *p,
unsigned long *map)
{
}
static inline unsigned long *frontswap_map_get(struct swap_info_struct *p)
{
return NULL;
}
#endif
static inline int frontswap_store(struct page *page)
{
int ret = -1;
if (frontswap_enabled)
ret = __frontswap_store(page);
return ret;
}
static inline int frontswap_load(struct page *page)
{
int ret = -1;
if (frontswap_enabled)
ret = __frontswap_load(page);
return ret;
}
static inline void frontswap_invalidate_page(unsigned type, pgoff_t offset)
{
if (frontswap_enabled)
__frontswap_invalidate_page(type, offset);
}
static inline void frontswap_invalidate_area(unsigned type)
{
if (frontswap_enabled)
__frontswap_invalidate_area(type);
}
static inline void frontswap_init(unsigned type)
{
if (frontswap_enabled)
__frontswap_init(type);
}
#endif /* _LINUX_FRONTSWAP_H */

4
include/linux/swap.h
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@ -197,6 +197,10 @@ struct swap_info_struct {
struct block_device *bdev; /* swap device or bdev of swap file */
struct file *swap_file; /* seldom referenced */
unsigned int old_block_size; /* seldom referenced */
#ifdef CONFIG_FRONTSWAP
unsigned long *frontswap_map; /* frontswap in-use, one bit per page */
atomic_t frontswap_pages; /* frontswap pages in-use counter */
#endif
};
struct swap_list_t {

13
include/linux/swapfile.h Normal file
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@ -0,0 +1,13 @@
#ifndef _LINUX_SWAPFILE_H
#define _LINUX_SWAPFILE_H
/*
* these were static in swapfile.c but frontswap.c needs them and we don't
* want to expose them to the dozens of source files that include swap.h
*/
extern spinlock_t swap_lock;
extern struct swap_list_t swap_list;
extern struct swap_info_struct *swap_info[];
extern int try_to_unuse(unsigned int, bool, unsigned long);
#endif /* _LINUX_SWAPFILE_H */

17
mm/Kconfig
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@ -389,3 +389,20 @@ config CLEANCACHE
in a negligible performance hit.
If unsure, say Y to enable cleancache
config FRONTSWAP
bool "Enable frontswap to cache swap pages if tmem is present"
depends on SWAP
default n
help
Frontswap is so named because it can be thought of as the opposite
of a "backing" store for a swap device. The data is stored into
"transcendent memory", memory that is not directly accessible or
addressable by the kernel and is of unknown and possibly
time-varying size. When space in transcendent memory is available,
a significant swap I/O reduction may be achieved. When none is
available, all frontswap calls are reduced to a single pointer-
compare-against-NULL resulting in a negligible performance hit
and swap data is stored as normal on the matching swap device.
If unsure, say Y to enable frontswap.

1
mm/Makefile
View File

@ -29,6 +29,7 @@ obj-$(CONFIG_HAVE_MEMBLOCK) += memblock.o
obj-$(CONFIG_BOUNCE) += bounce.o
obj-$(CONFIG_SWAP) += page_io.o swap_state.o swapfile.o
obj-$(CONFIG_FRONTSWAP) += frontswap.o
obj-$(CONFIG_HAS_DMA) += dmapool.o
obj-$(CONFIG_HUGETLBFS) += hugetlb.o
obj-$(CONFIG_NUMA) += mempolicy.o

314
mm/frontswap.c Normal file
View File

@ -0,0 +1,314 @@
/*
* Frontswap frontend
*
* This code provides the generic "frontend" layer to call a matching
* "backend" driver implementation of frontswap. See
* Documentation/vm/frontswap.txt for more information.
*
* Copyright (C) 2009-2012 Oracle Corp. All rights reserved.
* Author: Dan Magenheimer
*
* This work is licensed under the terms of the GNU GPL, version 2.
*/
#include <linux/mm.h>
#include <linux/mman.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/proc_fs.h>
#include <linux/security.h>
#include <linux/capability.h>
#include <linux/module.h>
#include <linux/uaccess.h>
#include <linux/debugfs.h>
#include <linux/frontswap.h>
#include <linux/swapfile.h>
/*
* frontswap_ops is set by frontswap_register_ops to contain the pointers
* to the frontswap "backend" implementation functions.
*/
static struct frontswap_ops frontswap_ops __read_mostly;
/*
* This global enablement flag reduces overhead on systems where frontswap_ops
* has not been registered, so is preferred to the slower alternative: a
* function call that checks a non-global.
*/
bool frontswap_enabled __read_mostly;
EXPORT_SYMBOL(frontswap_enabled);
/*
* If enabled, frontswap_store will return failure even on success. As
* a result, the swap subsystem will always write the page to swap, in
* effect converting frontswap into a writethrough cache. In this mode,
* there is no direct reduction in swap writes, but a frontswap backend
* can unilaterally "reclaim" any pages in use with no data loss, thus
* providing increases control over maximum memory usage due to frontswap.
*/
static bool frontswap_writethrough_enabled __read_mostly;
#ifdef CONFIG_DEBUG_FS
/*
* Counters available via /sys/kernel/debug/frontswap (if debugfs is
* properly configured). These are for information only so are not protected
* against increment races.
*/
static u64 frontswap_loads;
static u64 frontswap_succ_stores;
static u64 frontswap_failed_stores;
static u64 frontswap_invalidates;
static inline void inc_frontswap_loads(void) {
frontswap_loads++;
}
static inline void inc_frontswap_succ_stores(void) {
frontswap_succ_stores++;
}
static inline void inc_frontswap_failed_stores(void) {
frontswap_failed_stores++;
}
static inline void inc_frontswap_invalidates(void) {
frontswap_invalidates++;
}
#else
static inline void inc_frontswap_loads(void) { }
static inline void inc_frontswap_succ_stores(void) { }
static inline void inc_frontswap_failed_stores(void) { }
static inline void inc_frontswap_invalidates(void) { }
#endif
/*
* Register operations for frontswap, returning previous thus allowing
* detection of multiple backends and possible nesting.
*/
struct frontswap_ops frontswap_register_ops(struct frontswap_ops *ops)
{
struct frontswap_ops old = frontswap_ops;
frontswap_ops = *ops;
frontswap_enabled = true;
return old;
}
EXPORT_SYMBOL(frontswap_register_ops);
/*
* Enable/disable frontswap writethrough (see above).
*/
void frontswap_writethrough(bool enable)
{
frontswap_writethrough_enabled = enable;
}
EXPORT_SYMBOL(frontswap_writethrough);
/*
* Called when a swap device is swapon'd.
*/
void __frontswap_init(unsigned type)
{
struct swap_info_struct *sis = swap_info[type];
BUG_ON(sis == NULL);
if (sis->frontswap_map == NULL)
return;
if (frontswap_enabled)
(*frontswap_ops.init)(type);
}
EXPORT_SYMBOL(__frontswap_init);
/*
* "Store" data from a page to frontswap and associate it with the page's
* swaptype and offset. Page must be locked and in the swap cache.
* If frontswap already contains a page with matching swaptype and
* offset, the frontswap implmentation may either overwrite the data and
* return success or invalidate the page from frontswap and return failure.
*/
int __frontswap_store(struct page *page)
{
int ret = -1, dup = 0;
swp_entry_t entry = { .val = page_private(page), };
int type = swp_type(entry);
struct swap_info_struct *sis = swap_info[type];
pgoff_t offset = swp_offset(entry);
BUG_ON(!PageLocked(page));
BUG_ON(sis == NULL);
if (frontswap_test(sis, offset))
dup = 1;
ret = (*frontswap_ops.store)(type, offset, page);
if (ret == 0) {
frontswap_set(sis, offset);
inc_frontswap_succ_stores();
if (!dup)
atomic_inc(&sis->frontswap_pages);
} else if (dup) {
/*
failed dup always results in automatic invalidate of
the (older) page from frontswap
*/
frontswap_clear(sis, offset);
atomic_dec(&sis->frontswap_pages);
inc_frontswap_failed_stores();
} else
inc_frontswap_failed_stores();
if (frontswap_writethrough_enabled)
/* report failure so swap also writes to swap device */
ret = -1;
return ret;
}
EXPORT_SYMBOL(__frontswap_store);
/*
* "Get" data from frontswap associated with swaptype and offset that were
* specified when the data was put to frontswap and use it to fill the
* specified page with data. Page must be locked and in the swap cache.
*/
int __frontswap_load(struct page *page)
{
int ret = -1;
swp_entry_t entry = { .val = page_private(page), };
int type = swp_type(entry);
struct swap_info_struct *sis = swap_info[type];
pgoff_t offset = swp_offset(entry);
BUG_ON(!PageLocked(page));
BUG_ON(sis == NULL);
if (frontswap_test(sis, offset))
ret = (*frontswap_ops.load)(type, offset, page);
if (ret == 0)
inc_frontswap_loads();
return ret;
}
EXPORT_SYMBOL(__frontswap_load);
/*
* Invalidate any data from frontswap associated with the specified swaptype
* and offset so that a subsequent "get" will fail.
*/
void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
{
struct swap_info_struct *sis = swap_info[type];
BUG_ON(sis == NULL);
if (frontswap_test(sis, offset)) {
(*frontswap_ops.invalidate_page)(type, offset);
atomic_dec(&sis->frontswap_pages);
frontswap_clear(sis, offset);
inc_frontswap_invalidates();
}
}
EXPORT_SYMBOL(__frontswap_invalidate_page);
/*
* Invalidate all data from frontswap associated with all offsets for the
* specified swaptype.
*/
void __frontswap_invalidate_area(unsigned type)
{
struct swap_info_struct *sis = swap_info[type];
BUG_ON(sis == NULL);
if (sis->frontswap_map == NULL)
return;
(*frontswap_ops.invalidate_area)(type);
atomic_set(&sis->frontswap_pages, 0);
memset(sis->frontswap_map, 0, sis->max / sizeof(long));
}
EXPORT_SYMBOL(__frontswap_invalidate_area);
/*
* Frontswap, like a true swap device, may unnecessarily retain pages
* under certain circumstances; "shrink" frontswap is essentially a
* "partial swapoff" and works by calling try_to_unuse to attempt to
* unuse enough frontswap pages to attempt to -- subject to memory
* constraints -- reduce the number of pages in frontswap to the
* number given in the parameter target_pages.
*/
void frontswap_shrink(unsigned long target_pages)
{
struct swap_info_struct *si = NULL;
int si_frontswap_pages;
unsigned long total_pages = 0, total_pages_to_unuse;
unsigned long pages = 0, pages_to_unuse = 0;
int type;
bool locked = false;
/*
* we don't want to hold swap_lock while doing a very
* lengthy try_to_unuse, but swap_list may change
* so restart scan from swap_list.head each time
*/
spin_lock(&swap_lock);
locked = true;
total_pages = 0;
for (type = swap_list.head; type >= 0; type = si->next) {
si = swap_info[type];
total_pages += atomic_read(&si->frontswap_pages);
}
if (total_pages <= target_pages)
goto out;
total_pages_to_unuse = total_pages - target_pages;
for (type = swap_list.head; type >= 0; type = si->next) {
si = swap_info[type];
si_frontswap_pages = atomic_read(&si->frontswap_pages);
if (total_pages_to_unuse < si_frontswap_pages)
pages = pages_to_unuse = total_pages_to_unuse;
else {
pages = si_frontswap_pages;
pages_to_unuse = 0; /* unuse all */
}
/* ensure there is enough RAM to fetch pages from frontswap */
if (security_vm_enough_memory_mm(current->mm, pages))
continue;
vm_unacct_memory(pages);
break;
}
if (type < 0)
goto out;
locked = false;
spin_unlock(&swap_lock);
try_to_unuse(type, true, pages_to_unuse);
out:
if (locked)
spin_unlock(&swap_lock);
return;
}
EXPORT_SYMBOL(frontswap_shrink);
/*
* Count and return the number of frontswap pages across all
* swap devices. This is exported so that backend drivers can
* determine current usage without reading debugfs.
*/
unsigned long frontswap_curr_pages(void)
{
int type;
unsigned long totalpages = 0;
struct swap_info_struct *si = NULL;
spin_lock(&swap_lock);
for (type = swap_list.head; type >= 0; type = si->next) {
si = swap_info[type];
totalpages += atomic_read(&si->frontswap_pages);
}
spin_unlock(&swap_lock);
return totalpages;
}
EXPORT_SYMBOL(frontswap_curr_pages);
static int __init init_frontswap(void)
{
#ifdef CONFIG_DEBUG_FS
struct dentry *root = debugfs_create_dir("frontswap", NULL);
if (root == NULL)
return -ENXIO;
debugfs_create_u64("loads", S_IRUGO, root, &frontswap_loads);
debugfs_create_u64("succ_stores", S_IRUGO, root, &frontswap_succ_stores);
debugfs_create_u64("failed_stores", S_IRUGO, root,
&frontswap_failed_stores);
debugfs_create_u64("invalidates", S_IRUGO,
root, &frontswap_invalidates);
#endif
return 0;
}
module_init(init_frontswap);

12
mm/page_io.c
View File

@ -18,6 +18,7 @@
#include <linux/bio.h>
#include <linux/swapops.h>
#include <linux/writeback.h>
#include <linux/frontswap.h>
#include <asm/pgtable.h>
static struct bio *get_swap_bio(gfp_t gfp_flags,
@ -98,6 +99,12 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
unlock_page(page);
goto out;
}
if (frontswap_store(page) == 0) {
set_page_writeback(page);
unlock_page(page);
end_page_writeback(page);
goto out;
}
bio = get_swap_bio(GFP_NOIO, page, end_swap_bio_write);
if (bio == NULL) {
set_page_dirty(page);
@ -122,6 +129,11 @@ int swap_readpage(struct page *page)
VM_BUG_ON(!PageLocked(page));
VM_BUG_ON(PageUptodate(page));
if (frontswap_load(page) == 0) {
SetPageUptodate(page);
unlock_page(page);
goto out;
}
bio = get_swap_bio(GFP_KERNEL, page, end_swap_bio_read);
if (bio == NULL) {
unlock_page(page);

54
mm/swapfile.c
View File

@ -31,6 +31,8 @@
#include <linux/memcontrol.h>
#include <linux/poll.h>
#include <linux/oom.h>
#include <linux/frontswap.h>
#include <linux/swapfile.h>
#include <asm/pgtable.h>
#include <asm/tlbflush.h>
@ -42,7 +44,7 @@ static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
static void free_swap_count_continuations(struct swap_info_struct *);
static sector_t map_swap_entry(swp_entry_t, struct block_device**);
static DEFINE_SPINLOCK(swap_lock);
DEFINE_SPINLOCK(swap_lock);
static unsigned int nr_swapfiles;
long nr_swap_pages;
long total_swap_pages;
@ -53,9 +55,9 @@ static const char Unused_file[] = "Unused swap file entry ";
static const char Bad_offset[] = "Bad swap offset entry ";
static const char Unused_offset[] = "Unused swap offset entry ";
static struct swap_list_t swap_list = {-1, -1};
struct swap_list_t swap_list = {-1, -1};
static struct swap_info_struct *swap_info[MAX_SWAPFILES];
struct swap_info_struct *swap_info[MAX_SWAPFILES];
static DEFINE_MUTEX(swapon_mutex);
@ -556,6 +558,7 @@ static unsigned char swap_entry_free(struct swap_info_struct *p,
swap_list.next = p->type;
nr_swap_pages++;
p->inuse_pages--;
frontswap_invalidate_page(p->type, offset);
if ((p->flags & SWP_BLKDEV) &&
disk->fops->swap_slot_free_notify)
disk->fops->swap_slot_free_notify(p->bdev, offset);
@ -985,11 +988,12 @@ static int unuse_mm(struct mm_struct *mm,
}
/*
* Scan swap_map from current position to next entry still in use.
* Scan swap_map (or frontswap_map if frontswap parameter is true)
* from current position to next entry still in use.
* Recycle to start on reaching the end, returning 0 when empty.
*/
static unsigned int find_next_to_unuse(struct swap_info_struct *si,
unsigned int prev)
unsigned int prev, bool frontswap)
{
unsigned int max = si->max;
unsigned int i = prev;
@ -1015,6 +1019,12 @@ static unsigned int find_next_to_unuse(struct swap_info_struct *si,
prev = 0;
i = 1;
}
if (frontswap) {
if (frontswap_test(si, i))
break;
else
continue;
}
count = si->swap_map[i];
if (count && swap_count(count) != SWAP_MAP_BAD)
break;
@ -1026,8 +1036,12 @@ static unsigned int find_next_to_unuse(struct swap_info_struct *si,
* We completely avoid races by reading each swap page in advance,
* and then search for the process using it. All the necessary
* page table adjustments can then be made atomically.
*
* if the boolean frontswap is true, only unuse pages_to_unuse pages;
* pages_to_unuse==0 means all pages; ignored if frontswap is false
*/
static int try_to_unuse(unsigned int type)
int try_to_unuse(unsigned int type, bool frontswap,
unsigned long pages_to_unuse)
{
struct swap_info_struct *si = swap_info[type];
struct mm_struct *start_mm;
@ -1060,7 +1074,7 @@ static int try_to_unuse(unsigned int type)
* one pass through swap_map is enough, but not necessarily:
* there are races when an instance of an entry might be missed.
*/
while ((i = find_next_to_unuse(si, i)) != 0) {
while ((i = find_next_to_unuse(si, i, frontswap)) != 0) {
if (signal_pending(current)) {
retval = -EINTR;
break;
@ -1227,6 +1241,10 @@ static int try_to_unuse(unsigned int type)
* interactive performance.
*/
cond_resched();
if (frontswap && pages_to_unuse > 0) {
if (!--pages_to_unuse)
break;
}
}
mmput(start_mm);
@ -1486,7 +1504,8 @@ bad_bmap:
}
static void enable_swap_info(struct swap_info_struct *p, int prio,
unsigned char *swap_map)
unsigned char *swap_map,
unsigned long *frontswap_map)
{
int i, prev;
@ -1496,6 +1515,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
else
p->prio = --least_priority;
p->swap_map = swap_map;
frontswap_map_set(p, frontswap_map);
p->flags |= SWP_WRITEOK;
nr_swap_pages += p->pages;
total_swap_pages += p->pages;
@ -1512,6 +1532,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
swap_list.head = swap_list.next = p->type;
else
swap_info[prev]->next = p->type;
frontswap_init(p->type);
spin_unlock(&swap_lock);
}
@ -1585,7 +1606,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
spin_unlock(&swap_lock);
oom_score_adj = test_set_oom_score_adj(OOM_SCORE_ADJ_MAX);
err = try_to_unuse(type);
err = try_to_unuse(type, false, 0); /* force all pages to be unused */
compare_swap_oom_score_adj(OOM_SCORE_ADJ_MAX, oom_score_adj);
if (err) {
@ -1596,7 +1617,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
* sys_swapoff for this swap_info_struct at this point.
*/
/* re-insert swap space back into swap_list */
enable_swap_info(p, p->prio, p->swap_map);
enable_swap_info(p, p->prio, p->swap_map, frontswap_map_get(p));
goto out_dput;
}
@ -1622,9 +1643,11 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
swap_map = p->swap_map;
p->swap_map = NULL;
p->flags = 0;
frontswap_invalidate_area(type);
spin_unlock(&swap_lock);
mutex_unlock(&swapon_mutex);
vfree(swap_map);
vfree(frontswap_map_get(p));
/* Destroy swap account informatin */
swap_cgroup_swapoff(type);
@ -1988,6 +2011,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
sector_t span;
unsigned long maxpages;
unsigned char *swap_map = NULL;
unsigned long *frontswap_map = NULL;
struct page *page = NULL;
struct inode *inode = NULL;
@ -2071,6 +2095,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
error = nr_extents;
goto bad_swap;
}
/* frontswap enabled? set up bit-per-page map for frontswap */
if (frontswap_enabled)
frontswap_map = vzalloc(maxpages / sizeof(long));
if (p->bdev) {
if (blk_queue_nonrot(bdev_get_queue(p->bdev))) {
@ -2086,14 +2113,15 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
if (swap_flags & SWAP_FLAG_PREFER)
prio =
(swap_flags & SWAP_FLAG_PRIO_MASK) >> SWAP_FLAG_PRIO_SHIFT;
enable_swap_info(p, prio, swap_map);
enable_swap_info(p, prio, swap_map, frontswap_map);
printk(KERN_INFO "Adding %uk swap on %s. "
"Priority:%d extents:%d across:%lluk %s%s\n",
"Priority:%d extents:%d across:%lluk %s%s%s\n",
p->pages<<(PAGE_SHIFT-10), name, p->prio,
nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10),
(p->flags & SWP_SOLIDSTATE) ? "SS" : "",
(p->flags & SWP_DISCARDABLE) ? "D" : "");
(p->flags & SWP_DISCARDABLE) ? "D" : "",
(frontswap_map) ? "FS" : "");
mutex_unlock(&swapon_mutex);
atomic_inc(&proc_poll_event);