xfs: limit extent length for allocation to AG size

Delayed allocation extents can be larger than AGs, so when trying to
convert a large range we may scan every AG inside
xfs_bmap_alloc_nullfb() trying to find an AG with a size larger than
an AG. We should stop when we find the first AG with a maximum
possible allocation size. This causes excessive CPU usage when there
are lots of AGs.

The same problem occurs when doing preallocation of a range larger
than an AG.

Fix the problem by limiting real allocation lengths to the maximum
that an AG can support. This means if we have empty AGs, we'll stop
the search at the first of them. If there are no empty AGs, we'll
still scan them all, but that is a different problem....

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>

1 files changed, 10 insertions, 8 deletions

diff --git a/fs/xfs/xfs_bmap.c b/fs/xfs/xfs_bmap.c
index 4111cd3966c..f3a3768189b 100644
--- a/fs/xfs/xfs_bmap.c
+++ b/fs/xfs/xfs_bmap.c
@@ -2430,7 +2430,7 @@ xfs_bmap_btalloc_nullfb(
                 startag = ag = 0;
 
         pag = xfs_perag_get(mp, ag);
-        while (*blen < ap->alen) {
+        while (*blen < args->maxlen) {
                 if (!pag->pagf_init) {
                         error = xfs_alloc_pagf_init(mp, args->tp, ag,
                                                     XFS_ALLOC_FLAG_TRYLOCK);
@@ -2452,7 +2452,7 @@ xfs_bmap_btalloc_nullfb(
                         notinit = 1;
 
                 if (xfs_inode_is_filestream(ap->ip)) {
-                        if (*blen >= ap->alen)
+                        if (*blen >= args->maxlen)
                                 break;
 
                         if (ap->userdata) {
@@ -2498,14 +2498,14 @@ xfs_bmap_btalloc_nullfb(
          * If the best seen length is less than the request
          * length, use the best as the minimum.
          */
-        else if (*blen < ap->alen)
+        else if (*blen < args->maxlen)
                 args->minlen = *blen;
         /*
-         * Otherwise we've seen an extent as big as alen,
+         * Otherwise we've seen an extent as big as maxlen,
          * use that as the minimum.
          */
         else
-                args->minlen = ap->alen;
+                args->minlen = args->maxlen;
 
         /*
          * set the failure fallback case to look in the selected
@@ -2573,7 +2573,9 @@ xfs_bmap_btalloc(
         args.tp = ap->tp;
         args.mp = mp;
         args.fsbno = ap->rval;
-        args.maxlen = MIN(ap->alen, mp->m_sb.sb_agblocks);
+
+        /* Trim the allocation back to the maximum an AG can fit. */
+        args.maxlen = MIN(ap->alen, XFS_ALLOC_AG_MAX_USABLE(mp));
         args.firstblock = ap->firstblock;
         blen = 0;
         if (nullfb) {
@@ -2621,7 +2623,7 @@ xfs_bmap_btalloc(
                         /*
                          * Adjust for alignment
                          */
-                        if (blen > args.alignment && blen <= ap->alen)
+                        if (blen > args.alignment && blen <= args.maxlen)
                                 args.minlen = blen - args.alignment;
                         args.minalignslop = 0;
                 } else {
@@ -2640,7 +2642,7 @@ xfs_bmap_btalloc(
                          * of minlen+alignment+slop doesn't go up
                          * between the calls.
                          */
-                        if (blen > mp->m_dalign && blen <= ap->alen)
+                        if (blen > mp->m_dalign && blen <= args.maxlen)
                                 nextminlen = blen - mp->m_dalign;
                         else
                                 nextminlen = args.minlen;