xfs: drop xfarray sortinfo folio on error

Chandan Babu reports the following livelock in xfs/708:

 run fstests xfs/708 at 2024-05-04 15:35:29
 XFS (loop16): EXPERIMENTAL online scrub feature in use. Use at your own risk!
 XFS (loop5): Mounting V5 Filesystem e96086f0-a2f9-4424-a1d5-c75d53d823be
 XFS (loop5): Ending clean mount
 XFS (loop5): Quotacheck needed: Please wait.
 XFS (loop5): Quotacheck: Done.
 XFS (loop5): EXPERIMENTAL online scrub feature in use. Use at your own risk!
 INFO: task xfs_io:143725 blocked for more than 122 seconds.
       Not tainted 6.9.0-rc4+ #1
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 task:xfs_io          state:D stack:0     pid:143725 tgid:143725 ppid:117661 flags:0x00004006
 Call Trace:
  <TASK>
  __schedule+0x69c/0x17a0
  schedule+0x74/0x1b0
  io_schedule+0xc4/0x140
  folio_wait_bit_common+0x254/0x650
  shmem_undo_range+0x9d5/0xb40
  shmem_evict_inode+0x322/0x8f0
  evict+0x24e/0x560
  __dentry_kill+0x17d/0x4d0
  dput+0x263/0x430
  __fput+0x2fc/0xaa0
  task_work_run+0x132/0x210
  get_signal+0x1a8/0x1910
  arch_do_signal_or_restart+0x7b/0x2f0
  syscall_exit_to_user_mode+0x1c2/0x200
  do_syscall_64+0x72/0x170
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

The shmem code is trying to drop all the folios attached to a shmem
file and gets stuck on a locked folio after a bnobt repair.  It looks
like the process has a signal pending, so I started looking for places
where we lock an xfile folio and then deal with a fatal signal.

I found a bug in xfarray_sort_scan via code inspection.  This function
is called to set up the scanning phase of a quicksort operation, which
may involve grabbing a locked xfile folio.  If we exit the function with
an error code, the caller does not call xfarray_sort_scan_done to put
the xfile folio.  If _sort_scan returns an error code while si->folio is
set, we leak the reference and never unlock the folio.

Therefore, change xfarray_sort to call _scan_done on exit.  This is safe
to call multiple times because it sets si->folio to NULL and ignores a
NULL si->folio.  Also change _sort_scan to use an intermediate variable
so that we never pollute si->folio with an errptr.

Fixes: 232ea05 ("xfs: enable sorting of xfile-backed arrays")
Reported-by: Chandan Babu R <chandanbabu@kernel.org>
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

Author: Darrick J. Wong

fs/xfs/scrub/xfarray.c

@@ -822,12 +822,14 @@ xfarray_sort_scan(
 
 	/* Grab the first folio that backs this array element. */
 	if (!si->folio) {
+		struct folio	*folio;
 		loff_t		next_pos;
 
-		si->folio = xfile_get_folio(si->array->xfile, idx_pos,
+		folio = xfile_get_folio(si->array->xfile, idx_pos,
 				si->array->obj_size, XFILE_ALLOC);
-		if (IS_ERR(si->folio))
-			return PTR_ERR(si->folio);
+		if (IS_ERR(folio))
+			return PTR_ERR(folio);
+		si->folio = folio;
 
 		si->first_folio_idx = xfarray_idx(si->array,
 				folio_pos(si->folio) + si->array->obj_size - 1);
@@ -1048,6 +1050,7 @@ xfarray_sort(
 
 out_free:
 	trace_xfarray_sort_stats(si, error);
+	xfarray_sort_scan_done(si);
 	kvfree(si);
 	return error;
 }