btrfs: subpage: add overview comments

This patch adds an overview how btrfs subpage support works:

- limitations
- behavior
- basic implementation points

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2021-03-25 15:14:45 +08:00 committed by David Sterba
parent 5a2c60752a
commit 894d137818

View File

@ -4,6 +4,64 @@
#include "ctree.h"
#include "subpage.h"
/*
* Subpage (sectorsize < PAGE_SIZE) support overview:
*
* Limitations:
*
* - Only support 64K page size for now
* This is to make metadata handling easier, as 64K page would ensure
* all nodesize would fit inside one page, thus we don't need to handle
* cases where a tree block crosses several pages.
*
* - Only metadata read-write for now
* The data read-write part is in development.
*
* - Metadata can't cross 64K page boundary
* btrfs-progs and kernel have done that for a while, thus only ancient
* filesystems could have such problem. For such case, do a graceful
* rejection.
*
* Special behavior:
*
* - Metadata
* Metadata read is fully supported.
* Meaning when reading one tree block will only trigger the read for the
* needed range, other unrelated range in the same page will not be touched.
*
* Metadata write support is partial.
* The writeback is still for the full page, but we will only submit
* the dirty extent buffers in the page.
*
* This means, if we have a metadata page like this:
*
* Page offset
* 0 16K 32K 48K 64K
* |/////////| |///////////|
* \- Tree block A \- Tree block B
*
* Even if we just want to writeback tree block A, we will also writeback
* tree block B if it's also dirty.
*
* This may cause extra metadata writeback which results more COW.
*
* Implementation:
*
* - Common
* Both metadata and data will use a new structure, btrfs_subpage, to
* record the status of each sector inside a page. This provides the extra
* granularity needed.
*
* - Metadata
* Since we have multiple tree blocks inside one page, we can't rely on page
* locking anymore, or we will have greatly reduced concurrency or even
* deadlocks (hold one tree lock while trying to lock another tree lock in
* the same page).
*
* Thus for metadata locking, subpage support relies on io_tree locking only.
* This means a slightly higher tree locking latency.
*/
int btrfs_attach_subpage(const struct btrfs_fs_info *fs_info,
struct page *page, enum btrfs_subpage_type type)
{