From: Qu Wenruo wqu@suse.com
[ Upstream commit 54df8b80cc63aa0f22c4590cad11542731ed43ff ]
[BUG] When a scrub failed immediately without any byte scrubbed, the returned btrfs_scrub_progress::last_physical will always be 0, even if there is a non-zero @start passed into btrfs_scrub_dev() for resume cases.
This will reset the progress and make later scrub resume start from the beginning.
[CAUSE] The function btrfs_scrub_dev() accepts a @progress parameter to copy its updated progress to the caller, there are cases where we either don't touch progress::last_physical at all or copy 0 into last_physical:
- last_physical not updated at all If some error happened before scrubbing any super block or chunk, we will not copy the progress, leaving the @last_physical untouched.
E.g. failed to allocate @sctx, scrubbing a missing device or even there is already a running scrub and so on.
All those cases won't touch @progress at all, resulting the last_physical untouched and will be left as 0 for most cases.
- Error out before scrubbing any bytes In those case we allocated @sctx, and sctx->stat.last_physical is all zero (initialized by kvzalloc()). Unfortunately some critical errors happened during scrub_enumerate_chunks() or scrub_supers() before any stripe is really scrubbed.
In that case although we will copy sctx->stat back to @progress, since no byte is really scrubbed, last_physical will be overwritten to 0.
[FIX] Make sure the parameter @progress always has its @last_physical member updated to @start parameter inside btrfs_scrub_dev().
At the very beginning of the function, set @progress->last_physical to @start, so that even if we error out without doing progress copying, last_physical is still at @start.
Then after we got @sctx allocated, set sctx->stat.last_physical to @start, this will make sure even if we didn't get any byte scrubbed, at the progress copying stage the @last_physical is not left as zero.
This should resolve the resume progress reset problem.
Signed-off-by: Qu Wenruo wqu@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Sasha Levin sashal@kernel.org ---
LLM Generated explanations, may be completely bogus:
## ANALYSIS SUMMARY
### 1. Commit Message Analysis
The commit describes a clear bug with well-structured [BUG], [CAUSE], and [FIX] sections: - **Bug**: When scrub fails early (before scrubbing any bytes), `last_physical` is returned as 0 instead of the `start` parameter, causing resume to restart from the beginning - **Root cause**: Either `last_physical` isn't touched at all (early error paths) or it's left as 0 (kvzalloc zeroes sctx->stat) - **Notable**: No explicit "Cc: stable@vger.kernel.org" or "Fixes:" tag, but has "Reviewed-by: David Sterba" (btrfs maintainer)
### 2. Code Change Analysis
The fix adds just **2 lines of code** (plus a comment):
```c /* Set the basic fallback @last_physical before we got a sctx. */ if (progress) progress->last_physical = start; ```
And after `sctx` allocation: ```c sctx->stat.last_physical = start; ```
**Technical mechanism**: The function `btrfs_scrub_dev()` takes a `start` parameter indicating where to begin (or resume) scrubbing. The `progress` struct is returned to userspace even on error (see `btrfs_ioctl_scrub()` comment: "Copy scrub args to user space even if btrfs_scrub_dev() returned an error...Later user space can...resume scrub from where it left off"). Without this fix, if scrub fails early, `last_physical` is 0, causing btrfs-progs to restart from the beginning.
### 3. Classification
- **Bug fix**: Yes - fixes incorrect initialization of a progress tracking field - **Not an exception category**: Regular bug fix, not device IDs/quirks/DT
### 4. Scope and Risk Assessment
- **Scope**: 2 lines in 1 function (`btrfs_scrub_dev()`) - **Risk**: **EXTREMELY LOW** - just initializing a value to `start` instead of leaving it as 0 - The fix is purely additive and defensive; it doesn't change the normal code path where scrubbing succeeds - If scrubbing proceeds normally, `last_physical` gets updated with actual progress anyway
### 5. User Impact
- **Who**: Users with large btrfs filesystems who use scrub resume - **Severity**: Moderate usability issue - scrub on multi-TB filesystems can take hours/days; restarting from 0 wastes significant time - **Real-world**: The ioctl is explicitly designed for resume functionality (per the code comment), and btrfs-progs relies on this
### 6. Stability Indicators
- ✅ **Reviewed-by**: David Sterba (btrfs maintainer) - ✅ **Signed-off-by**: David Sterba (btrfs maintainer) - The fix has been properly reviewed through the btrfs maintainer tree
### 7. Dependency Check
- No dependencies on other commits - The affected code structure (`btrfs_scrub_dev`, `btrfs_scrub_progress`, `start` parameter) has existed for many years and is present in all stable kernels
## Verdict
This commit is an excellent candidate for stable backport:
1. **Fixes a real bug**: Scrub resume is broken when scrub fails early 2. **Small and surgical**: Just 2 lines of initialization code 3. **Obviously correct**: Setting `last_physical = start` is clearly the right behavior 4. **Low risk**: Cannot break anything - just ensures proper initialization of a progress field 5. **Benefits stable users**: Large filesystem users who rely on scrub resume functionality 6. **Maintainer reviewed**: Reviewed and signed off by btrfs maintainer
The lack of explicit "Cc: stable" tag appears to be an oversight - the bug clearly affects all kernels with scrub resume functionality, and the fix is exactly the type of small, safe bugfix appropriate for stable trees.
**YES**
fs/btrfs/scrub.c | 5 +++++ 1 file changed, 5 insertions(+)
diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c index ba20d9286a340..cff67ed630196 100644 --- a/fs/btrfs/scrub.c +++ b/fs/btrfs/scrub.c @@ -3039,6 +3039,10 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start, unsigned int nofs_flag; bool need_commit = false;
+ /* Set the basic fallback @last_physical before we got a sctx. */ + if (progress) + progress->last_physical = start; + if (btrfs_fs_closing(fs_info)) return -EAGAIN;
@@ -3057,6 +3061,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start, sctx = scrub_setup_ctx(fs_info, is_dev_replace); if (IS_ERR(sctx)) return PTR_ERR(sctx); + sctx->stat.last_physical = start;
ret = scrub_workers_get(fs_info); if (ret)