* [PATCH] btrfs: check snapshot_force_cow earlier in can_nocow_file_extent
@ 2026-02-16 22:16 jkchen
2026-02-17 11:42 ` Filipe Manana
0 siblings, 1 reply; 3+ messages in thread
From: jkchen @ 2026-02-16 22:16 UTC (permalink / raw)
To: Chris Mason, Josef Bacik, David Sterba; +Cc: linux-btrfs, linux-kernel, jkchen
When a snapshot is being created, the atomic counter snapshot_force_cow
is incremented to force incoming writes to fallback to COW. This is a
critical mechanism to protect the consistency of the snapshot being taken.
Currently, can_nocow_file_extent() checks this counter only after
performing several checks, most notably the expensive cross-reference
check via btrfs_cross_ref_exist(). btrfs_cross_ref_exist() releases the
path and performs a search in the extent tree or backref cache, which
involves btree traversals and locking overhead.
This patch moves the snapshot_force_cow check to the very beginning of
can_nocow_file_extent().
This reordering is safe and beneficial because:
1. args->writeback_path is invariant for the duration of the call (set
by caller run_delalloc_nocow).
2. is_freespace_inode is a static property of the inode.
3. The state of snapshot_force_cow is driven by the btrfs_mksnapshot
process. Checking it earlier does not change the outcome of the NOCOW
decision, but effectively prunes the expensive code path when a
fallback to COW is inevitable.
By failing fast when a snapshot is pending, we avoid the unnecessary
overhead of btrfs_cross_ref_exist() and other extent item checks in the
scenario where NOCOW is already known to be impossible.
Signed-off-by: jkchen <jkchen@ugreen.com>
---
fs/btrfs/inode.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 845164485..3549031f3 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1921,6 +1921,11 @@ static int can_nocow_file_extent(struct btrfs_path *path,
int ret = 0;
bool nowait = path->nowait;
+ /* If there are pending snapshots for this root, we must COW. */
+ if (args->writeback_path && !is_freespace_inode &&
+ atomic_read(&root->snapshot_force_cow))
+ goto out;
+
fi = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_file_extent_item);
extent_type = btrfs_file_extent_type(leaf, fi);
@@ -1982,11 +1987,6 @@ static int can_nocow_file_extent(struct btrfs_path *path,
path = NULL;
}
- /* If there are pending snapshots for this root, we must COW. */
- if (args->writeback_path && !is_freespace_inode &&
- atomic_read(&root->snapshot_force_cow))
- goto out;
-
args->file_extent.num_bytes = min(args->end + 1, extent_end) - args->start;
args->file_extent.offset += args->start - key->offset;
io_start = args->file_extent.disk_bytenr + args->file_extent.offset;
--
2.43.0
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] btrfs: check snapshot_force_cow earlier in can_nocow_file_extent
2026-02-16 22:16 [PATCH] btrfs: check snapshot_force_cow earlier in can_nocow_file_extent jkchen
@ 2026-02-17 11:42 ` Filipe Manana
[not found] ` <CAJFOPmtqt_u_JUQyntOjr+0W9X_CgcAXCkx8gArR9-L57Et5TA@mail.gmail.com>
0 siblings, 1 reply; 3+ messages in thread
From: Filipe Manana @ 2026-02-17 11:42 UTC (permalink / raw)
To: jkchen
Cc: Chris Mason, Josef Bacik, David Sterba, linux-btrfs,
linux-kernel, jkchen
On Mon, Feb 16, 2026 at 10:17 PM jkchen <jk.chen1095@gmail.com> wrote:
>
> When a snapshot is being created, the atomic counter snapshot_force_cow
> is incremented to force incoming writes to fallback to COW. This is a
> critical mechanism to protect the consistency of the snapshot being taken.
>
> Currently, can_nocow_file_extent() checks this counter only after
> performing several checks, most notably the expensive cross-reference
> check via btrfs_cross_ref_exist(). btrfs_cross_ref_exist() releases the
> path and performs a search in the extent tree or backref cache, which
> involves btree traversals and locking overhead.
>
> This patch moves the snapshot_force_cow check to the very beginning of
> can_nocow_file_extent().
>
> This reordering is safe and beneficial because:
> 1. args->writeback_path is invariant for the duration of the call (set
> by caller run_delalloc_nocow).
> 2. is_freespace_inode is a static property of the inode.
> 3. The state of snapshot_force_cow is driven by the btrfs_mksnapshot
> process. Checking it earlier does not change the outcome of the NOCOW
> decision, but effectively prunes the expensive code path when a
> fallback to COW is inevitable.
>
> By failing fast when a snapshot is pending, we avoid the unnecessary
> overhead of btrfs_cross_ref_exist() and other extent item checks in the
> scenario where NOCOW is already known to be impossible.
>
> Signed-off-by: jkchen <jkchen@ugreen.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Looks good, I'll add it to the github for-next branch, thanks.
> ---
> fs/btrfs/inode.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 845164485..3549031f3 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -1921,6 +1921,11 @@ static int can_nocow_file_extent(struct btrfs_path *path,
> int ret = 0;
> bool nowait = path->nowait;
>
> + /* If there are pending snapshots for this root, we must COW. */
> + if (args->writeback_path && !is_freespace_inode &&
> + atomic_read(&root->snapshot_force_cow))
> + goto out;
> +
> fi = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_file_extent_item);
> extent_type = btrfs_file_extent_type(leaf, fi);
>
> @@ -1982,11 +1987,6 @@ static int can_nocow_file_extent(struct btrfs_path *path,
> path = NULL;
> }
>
> - /* If there are pending snapshots for this root, we must COW. */
> - if (args->writeback_path && !is_freespace_inode &&
> - atomic_read(&root->snapshot_force_cow))
> - goto out;
> -
> args->file_extent.num_bytes = min(args->end + 1, extent_end) - args->start;
> args->file_extent.offset += args->start - key->offset;
> io_start = args->file_extent.disk_bytenr + args->file_extent.offset;
> --
> 2.43.0
>
>
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2026-02-17 17:56 UTC | newest]
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2026-02-16 22:16 [PATCH] btrfs: check snapshot_force_cow earlier in can_nocow_file_extent jkchen
2026-02-17 11:42 ` Filipe Manana
[not found] ` <CAJFOPmtqt_u_JUQyntOjr+0W9X_CgcAXCkx8gArR9-L57Et5TA@mail.gmail.com>
2026-02-17 17:56 ` Filipe Manana
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