xfs: optimise xfs_iext_destroy

When unmounting XFS, we call:

xfs_inode_free => xfs_idestroy_fork => xfs_iext_destroy

This goes over the whole indirection array and calls
xfs_iext_irec_remove for each one of the erps (from the last one to
the first one). As a result, we keep shrinking (reallocating
actually) the indirection array until we shrink out all of its
elements. When we have files with huge numbers of extents, umount
takes 30-80 sec, depending on the amount of files that XFS loaded
and the amount of indirection entries of each file. The unmount
stack looks like:

[<ffffffffc0b6d200>] xfs_iext_realloc_indirect+0x40/0x60 [xfs]
[<ffffffffc0b6cd8e>] xfs_iext_irec_remove+0xee/0xf0 [xfs]
[<ffffffffc0b6cdcd>] xfs_iext_destroy+0x3d/0xb0 [xfs]
[<ffffffffc0b6cef6>] xfs_idestroy_fork+0xb6/0xf0 [xfs]
[<ffffffffc0b87002>] xfs_inode_free+0xb2/0xc0 [xfs]
[<ffffffffc0b87260>] xfs_reclaim_inode+0x250/0x340 [xfs]
[<ffffffffc0b87583>] xfs_reclaim_inodes_ag+0x233/0x370 [xfs]
[<ffffffffc0b8823d>] xfs_reclaim_inodes+0x1d/0x20 [xfs]
[<ffffffffc0b96feb>] xfs_unmountfs+0x7b/0x1a0 [xfs]
[<ffffffffc0b98e4d>] xfs_fs_put_super+0x2d/0x70 [xfs]
[<ffffffff811e9e36>] generic_shutdown_super+0x76/0x100
[<ffffffff811ea207>] kill_block_super+0x27/0x70
[<ffffffff811ea519>] deactivate_locked_super+0x49/0x60
[<ffffffff811eaaee>] deactivate_super+0x4e/0x70
[<ffffffff81207593>] cleanup_mnt+0x43/0x90
[<ffffffff81207632>] __cleanup_mnt+0x12/0x20
[<ffffffff8108f8e7>] task_work_run+0xa7/0xe0
[<ffffffff81014ff7>] do_notify_resume+0x97/0xb0
[<ffffffff81717c6f>] int_signal+0x12/0x17

Further, this reallocation prevents us from freeing the extent list
from a RCU callback as allocation can block. Hence if the extent
list is in indirect format, optimise the freeing of the extent list
to only use kmem_free calls by freeing entire extent buffer pages at
a time, rather than extent by extent.

[dchinner: simplified freeing loop based on Christoph's suggestion]
Signed-off-by: Alex Lyakas <alex@zadarastorage.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/libxfs/xfs_inode_fork.c

@@ -1496,6 +1496,24 @@ xfs_iext_indirect_to_direct(
 	}
 }
 
+/*
+ * Remove all records from the indirection array.
+ */
+STATIC void
+xfs_iext_irec_remove_all(
+	struct xfs_ifork *ifp)
+{
+	int		nlists;
+	int		i;
+
+	ASSERT(ifp->if_flags & XFS_IFEXTIREC);
+	nlists = ifp->if_real_bytes / XFS_IEXT_BUFSZ;
+	for (i = 0; i < nlists; i++)
+		kmem_free(ifp->if_u1.if_ext_irec[i].er_extbuf);
+	kmem_free(ifp->if_u1.if_ext_irec);
+	ifp->if_flags &= ~XFS_IFEXTIREC;
+}
+
 /*
  * Free incore file extents.
  */
@@ -1504,14 +1522,7 @@ xfs_iext_destroy(
 	xfs_ifork_t	*ifp)		/* inode fork pointer */
 {
 	if (ifp->if_flags & XFS_IFEXTIREC) {
-		int	erp_idx;
-		int	nlists;
-
-		nlists = ifp->if_real_bytes / XFS_IEXT_BUFSZ;
-		for (erp_idx = nlists - 1; erp_idx >= 0 ; erp_idx--) {
-			xfs_iext_irec_remove(ifp, erp_idx);
-		}
-		ifp->if_flags &= ~XFS_IFEXTIREC;
+		xfs_iext_irec_remove_all(ifp);
 	} else if (ifp->if_real_bytes) {
 		kmem_free(ifp->if_u1.if_extents);
 	} else if (ifp->if_bytes) {