From 894d137818723ae4bc4df36c2c19d5ae5ddd8c78 Mon Sep 17 00:00:00 2001 From: Qu Wenruo Date: Thu, 25 Mar 2021 15:14:45 +0800 Subject: [PATCH] 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 Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/subpage.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 58 insertions(+) diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c index df1461c..2d19089 100644 --- a/fs/btrfs/subpage.c +++ b/fs/btrfs/subpage.c @@ -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) { -- 2.7.4