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mm: share PG_readahead and PG_reclaim

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Share the same page flag bit for PG_readahead and PG_reclaim.

One is used only on file reads, another is only for emergency writes.  One
is used mostly for fresh/young pages, another is for old pages.

Combinations of possible interactions are:

a) clear PG_reclaim => implicit clear of PG_readahead
	it will delay an asynchronous readahead into a synchronous one
	it actually does _good_ for readahead:
		the pages will be reclaimed soon, it's readahead thrashing!
		in this case, synchronous readahead makes more sense.

b) clear PG_readahead => implicit clear of PG_reclaim
	one(and only one) page will not be reclaimed in time
	it can be avoided by checking PageWriteback(page) in readahead first

c) set PG_reclaim => implicit set of PG_readahead
	will confuse readahead and make it restart the size rampup process
	it's a trivial problem, and can mostly be avoided by checking
	PageWriteback(page) first in readahead

d) set PG_readahead => implicit set of PG_reclaim
	PG_readahead will never be set on already cached pages.
	PG_reclaim will always be cleared on dirtying a page.
	so not a problem.

In summary,
	a)   we get better behavior
	b,d) possible interactions can be avoided
	c)   racy condition exists that might affect readahead, but the chance
	     is _really_ low, and the hurt on readahead is trivial.

Compound pages also use PG_reclaim, but for now they do not interact with
reclaim/readahead code.

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Fengguang Wu 2007-07-19 01:48:07 -07:00 committed by Linus Torvalds
parent d8983910a4
commit fe3cba17c4
4 changed files with 10 additions and 8 deletions
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4
include/linux/page-flags.h
View File

@ -83,7 +83,6 @@
#define PG_private 11 /* If pagecache, has fs-private data */
#define PG_writeback 12 /* Page is under writeback */
#define PG_readahead 13 /* Reminder to do async read-ahead */
#define PG_compound 14 /* Part of a compound page */
#define PG_swapcache 15 /* Swap page: swp_entry_t in private */
@ -91,6 +90,9 @@
#define PG_reclaim 17 /* To be reclaimed asap */
#define PG_buddy 19 /* Page is free, on buddy lists */
/* PG_readahead is only used for file reads; PG_reclaim is only for writes */
#define PG_readahead PG_reclaim /* Reminder to do async read-ahead */
/* PG_owner_priv_1 users should have descriptive aliases */
#define PG_checked PG_owner_priv_1 /* Used by some filesystems */
#define PG_pinned PG_owner_priv_1 /* Xen pinned pagetable */

1
mm/page-writeback.c
View File

@ -920,6 +920,7 @@ int clear_page_dirty_for_io(struct page *page)
BUG_ON(!PageLocked(page));
ClearPageReclaim(page);
if (mapping && mapping_cap_account_dirty(mapping)) {
/*
* Yes, Virginia, this is indeed insane.

7
mm/page_alloc.c
View File

@ -453,12 +453,6 @@ static inline int free_pages_check(struct page *page)
1 << PG_reserved |
1 << PG_buddy ))))
bad_page(page);
/*
* PageReclaim == PageTail. It is only an error
* for PageReclaim to be set if PageCompound is clear.
*/
if (unlikely(!PageCompound(page) && PageReclaim(page)))
bad_page(page);
if (PageDirty(page))
__ClearPageDirty(page);
/*
@ -602,7 +596,6 @@ static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
1 << PG_locked |
1 << PG_active |
1 << PG_dirty |
1 << PG_reclaim |
1 << PG_slab |
1 << PG_swapcache |
1 << PG_writeback |

6
mm/readahead.c
View File

@ -448,6 +448,12 @@ page_cache_readahead_ondemand(struct address_space *mapping,
return 0;
if (page) {
/*
* It can be PG_reclaim.
*/
if (PageWriteback(page))
return 0;
ClearPageReadahead(page);
/*