2 files changed, 26 insertions, 8 deletions

diff --git a/fs/xfs/xfs_alloc.h b/fs/xfs/xfs_alloc.h
index 0ab56b32c7eb..d0b3bc72005b 100644
--- a/fs/xfs/xfs_alloc.h
+++ b/fs/xfs/xfs_alloc.h
@@ -75,6 +75,22 @@ typedef unsigned int xfs_alloctype_t;
 #define XFS_ALLOC_SET_ASIDE(mp)  (4 + ((mp)->m_sb.sb_agcount * 4))
 
 /*
+ * When deciding how much space to allocate out of an AG, we limit the
+ * allocation maximum size to the size the AG. However, we cannot use all the
+ * blocks in the AG - some are permanently used by metadata. These
+ * blocks are generally:
+ *      - the AG superblock, AGF, AGI and AGFL
+ *      - the AGF (bno and cnt) and AGI btree root blocks
+ *      - 4 blocks on the AGFL according to XFS_ALLOC_SET_ASIDE() limits
+ *
+ * The AG headers are sector sized, so the amount of space they take up is
+ * dependent on filesystem geometry. The others are all single blocks.
+ */
+#define XFS_ALLOC_AG_MAX_USABLE(mp)     \
+        ((mp)->m_sb.sb_agblocks - XFS_BB_TO_FSB(mp, XFS_FSS_TO_BB(mp, 4)) - 7)
+
+
+/*
  * Argument structure for xfs_alloc routines.
  * This is turned into a structure to avoid having 20 arguments passed
  * down several levels of the stack.
diff --git a/fs/xfs/xfs_bmap.c b/fs/xfs/xfs_bmap.c
index 4111cd3966c7..f3a3768189bb 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;