xfs: kick extra large ioends to completion workqueue

We've had reports of soft lockup warnings in the iomap ioend
completion path due to very large bios and/or bio chains. Divert any
ioends with 256k or more pages to process to the workqueue so
completion occurs in non-atomic context and can reschedule to avoid
soft lockup warnings.

Signed-off-by: Brian Foster <bfoster@redhat.com>

fs/xfs/xfs_aops.c

@@ -30,6 +30,13 @@ XFS_WPC(struct iomap_writepage_ctx *ctx)
 	return container_of(ctx, struct xfs_writepage_ctx, ctx);
 }
 
+/*
+ * Kick extra large ioends off to the workqueue. Completion will process a lot
+ * of pages for a large bio or bio chain and a non-atomic context is required to
+ * reschedule and avoid soft lockup warnings.
+ */
+#define XFS_LARGE_IOEND	(262144 << PAGE_SHIFT)
+
 /*
  * Fast and loose check if this write could update the on-disk inode size.
  */
@@ -239,7 +246,8 @@ static inline bool xfs_ioend_needs_workqueue(struct iomap_ioend *ioend)
 {
 	return ioend->io_private ||
 		ioend->io_type == IOMAP_UNWRITTEN ||
-		(ioend->io_flags & IOMAP_F_SHARED);
+		(ioend->io_flags & IOMAP_F_SHARED) ||
+		(ioend->io_size >= XFS_LARGE_IOEND);
 }
 
 STATIC void