Re: [PATCH] btrfs: do not leak -ERESTARTSYS from FITRIM to userspace

[Qu Wenruo <wqu@suse.com>]

在 2026/10/28 19:21, Luca Stefani 写道:
> On Wed, 28 Oct 2026 at 07:57, Zhiwei Liang via B4 Relay
> <devnull+zhiwei.liang.zliang.me@kernel.org> wrote:
>>
>> From: Zhiwei Liang <zhiwei.liang@zliang.me>
>>
>> When a suspend starts during FITRIM, fstrim can fail with:
>>
>>    fstrim: /: FITRIM ioctl failed: Unknown error 512
>>
>> On a Fedora 44 laptop with kernel 7.2.7, 3 of the last 21 weekly
>> fstrim.service runs were in progress when an automatic suspend started.
>> All three failed this way and left the filesystem untrimmed.
>>
>> Since commit 69313850dce3 ("btrfs: add cancellation points to trim
>> loops"), the trim loops return -ERESTARTSYS when
>> btrfs_trim_interrupted() sees freezing(current), and FITRIM returns it
>> to userspace. Since commit bfb670b9183b ("btrfs: handle user interrupt
>> properly in btrfs_trim_fs()"), btrfs_trim_fs() returns it right away.
>>
>> -ERESTARTSYS is only valid while a signal is pending, because only the
>> signal handling on syscall exit turns it into a restart. But
>> freeze_processes() first sets pm_freezing, which makes freezing() true
>> for every user task. Then it walks the task list and sends each task a
>> fake signal. The trim loops check btrfs_trim_interrupted() after every
>> extent. If a check falls in this window, the loop returns -ERESTARTSYS
>> with no signal pending. Nothing restarts the syscall. On that laptop,
>> with a few thousand threads, freezing user space took 2 to 5 ms in each
>> of the three suspends above, and all three runs hit this window.
>>
>> Make btrfs_trim_interrupted() treat a freeze as an interruption only
>> once the fake signal has arrived, i.e. when
>> freezing(current) && signal_pending(current). freeze_processes() sends
>> that signal in the same call. The trim loop stops at the next check
>> after it, so suspend is not delayed.
> Looking at how ext4 handles this scenario it seems to me they don't set
> the error code to -ERESTARTSYS and (from a quick glance) return the
> bytes discarded to the caller.
> And like before they continue the discard (which will skip/fail again) until
> there's nothing to be done.
> 
> I don't know what the intention of btrfs: handle user interrupt
> properly in btrfs_trim_fs()
> was, maybe we should just swallow the ERESTARTSYS case in those scenarios.

I agree on that btrfs is the exception, we should follow ext4/xfs, not 
rely on some tricky signal handling behavior for suspension.

I think the tokens of the author should be spent on unifying the btrfs 
fstrim suspension behavior instead.


>>
>> The restarted syscall re-reads struct fstrim_range from user memory.
>> When the device phase is interrupted, btrfs_trim_fs() has already stored
>> the trimmed byte count in range->len, and btrfs_ioctl_fitrim() copies
>> the struct back unconditionally. The restart is then asked to trim
>> [start, start + trimmed) instead of the requested range, or fails with
>> -EINVAL when nothing was counted yet. Skip the copy_to_user() for
>> -ERESTARTSYS. In all other cases the partial count is still copied back.
>>
>> Tested in a VM with pm_test=freezer. Without the patch, 18 of 20 freezes
>> during a trim leaked errno 512, and 10 of 10 restarts after a
>> device-phase interruption failed with -EINVAL. With the patch, all 30
>> runs restarted and trimmed the full amount.
>>
>> Fixes: 69313850dce3 ("btrfs: add cancellation points to trim loops")
>> CC: stable@vger.kernel.org # 5.15+
>> Signed-off-by: Zhiwei Liang <zhiwei.liang@zliang.me>
>> ---
>>   fs/btrfs/free-space-cache.h | 3 ++-
>>   fs/btrfs/ioctl.c            | 7 +++++++
>>   2 files changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/btrfs/free-space-cache.h b/fs/btrfs/free-space-cache.h
>> index e22443598b..cba2f5fd6b 100644
>> --- a/fs/btrfs/free-space-cache.h
>> +++ b/fs/btrfs/free-space-cache.h
>> @@ -58,7 +58,8 @@ static inline bool btrfs_free_space_trimming_bitmap(
>>
>>   static inline bool btrfs_trim_interrupted(void)
>>   {
>> -       return fatal_signal_pending(current) || freezing(current);
>> +       return fatal_signal_pending(current) ||
>> +              (freezing(current) && signal_pending(current));
>>   }
>>
>>   /*
>> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
>> index ac070c745b..b02c59cbb0 100644
>> --- a/fs/btrfs/ioctl.c
>> +++ b/fs/btrfs/ioctl.c
>> @@ -529,6 +529,13 @@ static noinline int btrfs_ioctl_fitrim(struct btrfs_fs_info *fs_info,
>>          range.minlen = max(range.minlen, minlen);
>>          ret = btrfs_trim_fs(fs_info, &range);
>>
>> +       /*
>> +        * A restart re-reads the range from user space, so do not clobber it
>> +        * with the partial len the rerun would otherwise take as its input.
>> +        */
>> +       if (ret == -ERESTARTSYS)
>> +               return ret;
>> +
>>          if (copy_to_user(arg, &range, sizeof(range)))
>>                  return -EFAULT;
>>
>>
>> ---
>> base-commit: e17e64baf63f7c85bec209ed73f14c1b2a7e39fb
>> change-id: 20261028-btrfs-fitrim-freezer-d4cba16e7e13
>>
>> Best regards,
>> --
>> Zhiwei Liang <zhiwei.liang@zliang.me>
>>
>>