Re: [PATCH] btrfs: do not leak -ERESTARTSYS from FITRIM to userspace
From: Qu Wenruo
Date: Wed Oct 28 2026 - 04:58:03 EST
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@xxxxxxxxxx> wrote:
Looking at how ext4 handles this scenario it seems to me they don't set
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.
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@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>