xfs: fix locking in xchk_nlinks_collect_dir

On a filesystem with parent pointers, xchk_nlinks_collect_dir walks both
the directory entries (data fork) and the parent pointers (attr fork) to
determine the correct link count. Unfortunately I forgot to update the
lock mode logic to handle the case of a directory whose attr fork is in
btree format and has not yet been loaded *and* whose data fork doesn't
need loading.

This leads to a bunch of assertions from xfs/286 in xfs_iread_extents
because we only took ILOCK_SHARED, not ILOCK_EXCL. You'd need the rare
happenstance of a directory with a large number of non-pptr extended
attributes set and enough memory pressure to cause the directory to be
evicted and partially reloaded from disk.

I /think/ this only started in 6.18-rc1 because I've started seeing OOM
errors with the maple tree slab using 70% of memory, and this didn't
happen in 6.17. Yay dynamic systems!

Cc: [e-mail] # v6.10
Fixes: 77ede5f44b0d86 ("xfs: walk directory parent pointers to determine backref count")
Signed-off-by: Darrick J. Wong <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Carlos Maiolino <[e-mail]>

fs/xfs/scrub/nlinks.c

···
 	return error;
 }
 
+static uint
+xchk_nlinks_ilock_dir(
+	struct xfs_inode	*ip)
+{
+	uint			lock_mode = XFS_ILOCK_SHARED;
+
+	/*
+	 * We're going to scan the directory entries, so we must be ready to
+	 * pull the data fork mappings into memory if they aren't already.
+	 */
+	if (xfs_need_iread_extents(&ip->i_df))
+		lock_mode = XFS_ILOCK_EXCL;
+
+	/*
+	 * We're going to scan the parent pointers, so we must be ready to
+	 * pull the attr fork mappings into memory if they aren't already.
+	 */
+	if (xfs_has_parent(ip->i_mount) && xfs_inode_has_attr_fork(ip) &&
+	    xfs_need_iread_extents(&ip->i_af))
+		lock_mode = XFS_ILOCK_EXCL;
+
+	/*
+	 * Take the IOLOCK so that other threads cannot start a directory
+	 * update while we're scanning.
+	 */
+	lock_mode |= XFS_IOLOCK_SHARED;
+	xfs_ilock(ip, lock_mode);
+	return lock_mode;
+}
+
 /* Walk a directory to bump the observed link counts of the children. */
 STATIC int
 xchk_nlinks_collect_dir(
···
 		return 0;
 
 	/* Prevent anyone from changing this directory while we walk it. */
-	xfs_ilock(dp, XFS_IOLOCK_SHARED);
-	lock_mode = xfs_ilock_data_map_shared(dp);
+	lock_mode = xchk_nlinks_ilock_dir(dp);
 
 	/*
 	 * The dotdot entry of an unlinked directory still points to the last
···
 	xchk_iscan_abort(&xnc->collect_iscan);
 out_unlock:
 	xfs_iunlock(dp, lock_mode);
-	xfs_iunlock(dp, XFS_IOLOCK_SHARED);
 	return error;
 }