Merge tag 'iomap-5.17-fixes-1' of git://git.kernel.org/pub/scm/fs/xfs/xfs-linux

Pull iomap fix from Darrick Wong:
 "A single bugfix for iomap.

  The fix should eliminate occasional complaints about stall warnings
  when a lot of writeback IO completes all at once and we have to then
  go clearing status on a large number of folios.

  Summary:

   - Limit the length of ioend chains in writeback so that we don't trip
     the softlockup watchdog and to limit long tail latency on clearing
     PageWriteback"

* tag 'iomap-5.17-fixes-1' of git://git.kernel.org/pub/scm/fs/xfs/xfs-linux:
  xfs, iomap: limit individual ioend chain lengths in writeback

1 files changed, 15 insertions, 1 deletions

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 2705f91bdd0d..9d6a67c7d227 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -136,7 +136,20 @@ done:
 	memalloc_nofs_restore(nofs_flag);
 }
 
-/* Finish all pending io completions. */
+/*
+ * Finish all pending IO completions that require transactional modifications.
+ *
+ * We try to merge physical and logically contiguous ioends before completion to
+ * minimise the number of transactions we need to perform during IO completion.
+ * Both unwritten extent conversion and COW remapping need to iterate and modify
+ * one physical extent at a time, so we gain nothing by merging physically
+ * discontiguous extents here.
+ *
+ * The ioend chain length that we can be processing here is largely unbound in
+ * length and we may have to perform significant amounts of work on each ioend
+ * to complete it. Hence we have to be careful about holding the CPU for too
+ * long in this loop.
+ */
 void
 xfs_end_io(
 	struct work_struct	*work)
@@ -157,6 +170,7 @@ xfs_end_io(
 		list_del_init(&ioend->io_list);
 		iomap_ioend_try_merge(ioend, &tmp);
 		xfs_end_ioend(ioend);
+		cond_resched();
 	}
 }