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

[Luca Stefani]

On Wed, 28 Oct 2026 at 07:57, Zhiwei Liang via B4 Relay
<devnull+zhiwei.liang.zliang.me@xxxxxxxxxx> wrote:
>
> From: Zhiwei Liang <zhiwei.liang@xxxxxxxxx>
>
> 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.
>
> 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@xxxxxxxxxxxxxxx # 5.15+
> Signed-off-by: Zhiwei Liang <zhiwei.liang@xxxxxxxxx>
> ---
>  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@xxxxxxxxx>
>
>