Re: [PATCH] btrfs: keep drop_progress when resuming snapshot deletion

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2026/11/4 06:59, Peter Lamberg via B4 Relay 写道:
> From: Peter Lamberg <kernel@pelam.fi>
> 
> When btrfs_drop_snapshot() resumes a partially dropped subvolume, it
> copies the saved resume key from the root item into wc->update_progress
> only. wc is kzalloc'ed, so wc->drop_progress and wc->drop_level stay
> zero. The main loop writes both back into the root item on every
> iteration, and wc->drop_progress is only updated once this run drops a
> reference itself (DROP_REFERENCE stage).
> 
> If the child at the resume slot is shared and the walk is still in
> UPDATE_BACKREF when a checkpoint happens (an unmount or commit before
> the first drop), the root item gets drop_progress (0 0 0) and
> drop_level 0 while the root item still exists. The next mount then
> restarts the drop from the left edge of the tree. Children that the
> earlier run already finished now have refs == 1, held by the surviving
> snapshot only, so the restarted walk treats them as exclusively owned
> and tries to free the dropped tree's reference again:
> 
>    unable to find ref byte nr ... parent 0 root N owner L offset 0
>    BTRFS: error ... in __btrfs_free_extent: errno=-117 Filesystem corrupted
> 
> and the transaction aborts.
> 
> Before the commit below, the root item's drop_progress was left alone
> while in UPDATE_BACKREF. Restore that behaviour by seeding
> wc->drop_progress and wc->drop_level from the root item on resume, so a
> checkpoint before the first drop rewrites the same resume point instead
> of zeroing it.
> 
> Fixes: aea6f028d01d ("btrfs: save drop_progress if we drop refs at all")
> Cc: stable@vger.kernel.org # 5.1+
> Assisted-by: LLM
> Signed-off-by: Peter Lamberg <kernel@pelam.fi>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
> Test notes (not for the commit log):
> 
> Reproducer: subvolume S1 with a level-3 tree, read-only snapshot S2,
> delete S1, unmount after the first root child is dropped, mount and
> unmount right away, then let the drop finish. fstests test sent
> separately (btrfs/355).
> 
> Reproducers and logs: https://pelam.fi/btrfs-drop-progress/
> 
> Tested on Ubuntu 26.10, kernel 7.3.0-9-generic (v7.3-rc6 based), with
> btrfs.ko rebuilt from the matching Ubuntu source, 3 runs per module of
> the same sequence on a loop image with dm-delay plus btrfs check
> --readonly:
> 
>    module              drop key zeroed  abort  btrfs check  snapshot S2
>    stock               3/3              3/3    errors 3/3   EIO 3/3
>    built, unpatched    3/3              3/3    errors 3/3   EIO 3/3
>    built, patched      0/3              0/3    clean 3/3    intact 3/3
> 
> Unpatched and patched were built from the same tree with the same
> toolchain. With the patch the drop key kept the resume point over 20
> mount/unmount cycles and the following mount finished the drop.
> 
> fstests -g quick (btrfs): 765 of 766 pass with and without the patch;
> generic/574 (fs-verity, 1K block size) fails in both runs, unrelated
> to this change.
> 
> btrfs/355 (no dm-delay, checks the drop key after each resume): fails
> 3/3 on stock and 3/3 unpatched, passes 3/3 with the patch, ~15 s per
> run; same result with the devices on tmpfs.
> 
> The patch applies to v7.0.14 through v7.4-rc1; v7.4-rc1 still has the
> unpatched resume code. Upstream tags were built (v7.3.1, W=1 clean on
> extent-tree.o) but not run.
> ---
> 
> ---
>   fs/btrfs/extent-tree.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 9b0a5c6b96f3..41a8b25c6e0f 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -6333,9 +6333,12 @@ int btrfs_drop_snapshot(struct btrfs_root *root, bool update_ref, bool for_reloc
>   		btrfs_disk_key_to_cpu(&key, &root_item->drop_progress);
>   		memcpy(&wc->update_progress, &key,
>   		       sizeof(wc->update_progress));
> +		/* keep the resume point until this run drops something itself */
> +		memcpy(&wc->drop_progress, &key, sizeof(wc->drop_progress));
>   
>   		level = btrfs_root_drop_level(root_item);
>   		BUG_ON(level == 0);
> +		wc->drop_level = level;
>   		path->lowest_level = level;
>   		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
>   		path->lowest_level = 0;