[XFS] Prevent free space oversubscription and xfssyncd looping.

The fix for recent ENOSPC deadlocks introduced certain limitations on
allocations. The fix could cause xfssyncd to loop endlessly if we did not
leave some space free for the allocator to work correctly. Basically, we
needed to ensure that we had at least 4 blocks free for an AG free list
and a block for the inode bmap btree at all times.

However, this did not take into account the fact that each AG has a free
list that needs 4 blocks. Hence any filesystem with more than one AG could
cause oversubscription of free space and make xfssyncd spin forever trying
to allocate space needed for AG freelists that was not available in the
AG.

The following patch reserves space for the free lists in all AGs plus the
inode bmap btree which prevents oversubscription. It also prevents those
blocks from being reported as free space (as they can never be used) and
makes the SMP in-core superblock accounting code and the reserved block
ioctl respect this requirement.

SGI-PV: 955674
SGI-Modid: xfs-linux-melb:xfs-kern:26894a
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: David Chatterton <chatz@sgi.com>

fs/xfs/xfs_alloc.h

@@ -43,6 +43,26 @@ typedef enum xfs_alloctype
 #define	XFS_ALLOC_FLAG_TRYLOCK	0x00000001  /* use trylock for buffer locking */
 #define	XFS_ALLOC_FLAG_FREEING	0x00000002  /* indicate caller is freeing extents*/
 
+/*
+ * In order to avoid ENOSPC-related deadlock caused by
+ * out-of-order locking of AGF buffer (PV 947395), we place
+ * constraints on the relationship among actual allocations for
+ * data blocks, freelist blocks, and potential file data bmap
+ * btree blocks. However, these restrictions may result in no
+ * actual space allocated for a delayed extent, for example, a data
+ * block in a certain AG is allocated but there is no additional
+ * block for the additional bmap btree block due to a split of the
+ * bmap btree of the file. The result of this may lead to an
+ * infinite loop in xfssyncd when the file gets flushed to disk and
+ * all delayed extents need to be actually allocated. To get around
+ * this, we explicitly set aside a few blocks which will not be
+ * reserved in delayed allocation. Considering the minimum number of
+ * needed freelist blocks is 4 fsbs _per AG_, a potential split of file's bmap
+ * btree requires 1 fsb, so we set the number of set-aside blocks
+ * to 4 + 4*agcount.
+ */
+#define XFS_ALLOC_SET_ASIDE(mp)  (4 + ((mp)->m_sb.sb_agcount * 4))
+
 /*
  * Argument structure for xfs_alloc routines.
  * This is turned into a structure to avoid having 20 arguments passed

fs/xfs/xfs_fsops.c

@@ -462,7 +462,7 @@ xfs_fs_counts(
 
 	xfs_icsb_sync_counters_lazy(mp);
 	s = XFS_SB_LOCK(mp);
-	cnt->freedata = mp->m_sb.sb_fdblocks;
+	cnt->freedata = mp->m_sb.sb_fdblocks - XFS_ALLOC_SET_ASIDE(mp);
 	cnt->freertx = mp->m_sb.sb_frextents;
 	cnt->freeino = mp->m_sb.sb_ifree;
 	cnt->allocino = mp->m_sb.sb_icount;
@@ -519,15 +519,19 @@ xfs_reserve_blocks(
 		}
 		mp->m_resblks = request;
 	} else {
+		__int64_t	free;
+
+		free =  mp->m_sb.sb_fdblocks - XFS_ALLOC_SET_ASIDE(mp);
 		delta = request - mp->m_resblks;
-		lcounter = mp->m_sb.sb_fdblocks - delta;
+		lcounter = free - delta;
 		if (lcounter < 0) {
 			/* We can't satisfy the request, just get what we can */
-			mp->m_resblks += mp->m_sb.sb_fdblocks;
-			mp->m_resblks_avail += mp->m_sb.sb_fdblocks;
-			mp->m_sb.sb_fdblocks = 0;
+			mp->m_resblks += free;
+			mp->m_resblks_avail += free;
+			mp->m_sb.sb_fdblocks = XFS_ALLOC_SET_ASIDE(mp);
 		} else {
-			mp->m_sb.sb_fdblocks = lcounter;
+			mp->m_sb.sb_fdblocks =
+				lcounter + XFS_ALLOC_SET_ASIDE(mp);
 			mp->m_resblks = request;
 			mp->m_resblks_avail += delta;
 		}

fs/xfs/xfs_mount.c

@@ -1243,24 +1243,6 @@ xfs_mod_sb(xfs_trans_t *tp, __int64_t fields)
 	xfs_trans_log_buf(tp, bp, first, last);
 }
 
-/*
- * In order to avoid ENOSPC-related deadlock caused by
- * out-of-order locking of AGF buffer (PV 947395), we place
- * constraints on the relationship among actual allocations for
- * data blocks, freelist blocks, and potential file data bmap
- * btree blocks. However, these restrictions may result in no
- * actual space allocated for a delayed extent, for example, a data
- * block in a certain AG is allocated but there is no additional
- * block for the additional bmap btree block due to a split of the
- * bmap btree of the file. The result of this may lead to an
- * infinite loop in xfssyncd when the file gets flushed to disk and
- * all delayed extents need to be actually allocated. To get around
- * this, we explicitly set aside a few blocks which will not be
- * reserved in delayed allocation. Considering the minimum number of
- * needed freelist blocks is 4 fsbs, a potential split of file's bmap
- * btree requires 1 fsb, so we set the number of set-aside blocks to 8.
-*/
-#define SET_ASIDE_BLOCKS 8
 
 /*
  * xfs_mod_incore_sb_unlocked() is a utility routine common used to apply
@@ -1306,7 +1288,8 @@ xfs_mod_incore_sb_unlocked(xfs_mount_t *mp, xfs_sb_field_t field,
 		return 0;
 	case XFS_SBS_FDBLOCKS:
 
-		lcounter = (long long)mp->m_sb.sb_fdblocks - SET_ASIDE_BLOCKS;
+		lcounter = (long long)
+			mp->m_sb.sb_fdblocks - XFS_ALLOC_SET_ASIDE(mp);
 		res_used = (long long)(mp->m_resblks - mp->m_resblks_avail);
 
 		if (delta > 0) {		/* Putting blocks back */
@@ -1340,7 +1323,7 @@ xfs_mod_incore_sb_unlocked(xfs_mount_t *mp, xfs_sb_field_t field,
 			}
 		}
 
-		mp->m_sb.sb_fdblocks = lcounter + SET_ASIDE_BLOCKS;
+		mp->m_sb.sb_fdblocks = lcounter + XFS_ALLOC_SET_ASIDE(mp);
 		return 0;
 	case XFS_SBS_FREXTENTS:
 		lcounter = (long long)mp->m_sb.sb_frextents;
@@ -2021,7 +2004,8 @@ xfs_icsb_sync_counters_lazy(
  * when we get near ENOSPC.
  */
 #define XFS_ICSB_INO_CNTR_REENABLE	64
-#define XFS_ICSB_FDBLK_CNTR_REENABLE	512
+#define XFS_ICSB_FDBLK_CNTR_REENABLE(mp) \
+		(512 + XFS_ALLOC_SET_ASIDE(mp))
 STATIC void
 xfs_icsb_balance_counter(
 	xfs_mount_t	*mp,
@@ -2055,7 +2039,7 @@ xfs_icsb_balance_counter(
 	case XFS_SBS_FDBLOCKS:
 		count = mp->m_sb.sb_fdblocks;
 		resid = do_div(count, weight);
-		if (count < XFS_ICSB_FDBLK_CNTR_REENABLE)
+		if (count < XFS_ICSB_FDBLK_CNTR_REENABLE(mp))
 			goto out;
 		break;
 	default:
@@ -2110,11 +2094,11 @@ xfs_icsb_modify_counters_int(
 	case XFS_SBS_FDBLOCKS:
 		BUG_ON((mp->m_resblks - mp->m_resblks_avail) != 0);
 
-		lcounter = icsbp->icsb_fdblocks;
+		lcounter = icsbp->icsb_fdblocks - XFS_ALLOC_SET_ASIDE(mp);
 		lcounter += delta;
 		if (unlikely(lcounter < 0))
 			goto slow_path;
-		icsbp->icsb_fdblocks = lcounter;
+		icsbp->icsb_fdblocks = lcounter + XFS_ALLOC_SET_ASIDE(mp);
 		break;
 	default:
 		BUG();

fs/xfs/xfs_vfsops.c

@@ -811,7 +811,8 @@ xfs_statvfs(
 	statp->f_bsize = sbp->sb_blocksize;
 	lsize = sbp->sb_logstart ? sbp->sb_logblocks : 0;
 	statp->f_blocks = sbp->sb_dblocks - lsize;
-	statp->f_bfree = statp->f_bavail = sbp->sb_fdblocks;
+	statp->f_bfree = statp->f_bavail =
+				sbp->sb_fdblocks - XFS_ALLOC_SET_ASIDE(mp);
 	fakeinos = statp->f_bfree << sbp->sb_inopblog;
 #if XFS_BIG_INUMS
 	fakeinos += mp->m_inoadd;