Re: Infinite loop in xfssyncd on full file system

[David Chinner <dgc@sgi.com>]

On Thu, Aug 24, 2006 at 09:14:29AM +1000, David Chinner wrote:
> On Wed, Aug 23, 2006 at 11:00:43AM -0400, Stephane Doyon wrote:
> > On Wed, 23 Aug 2006, David Chinner wrote:
> > 
> > >On Wed, Aug 23, 2006 at 02:02:18PM +1000, David Chinner wrote:
> > >>On Tue, Aug 22, 2006 at 04:01:10PM -0400, Stephane Doyon wrote:
> > >>>I'm seeing what appears to be an infinite loop in xfssyncd. It is
> > >>>triggered when writing to a file system that is full or nearly full. I
> > >>>have pinpointed the change that introduced this problem: it's
> > >>>
> > >>>    "TAKE 947395 - Fixing potential deadlock in space allocation and
> > >>>    freeing due to ENOSPC"
> > >>>
> > >>>git commit d210a28cd851082cec9b282443f8cc0e6fc09830.
> 
> .....
> 
> > >>Now we know what patch introduces the problem, we know where to look.
> > >>Stay tuned...
> > >
> > >I've had a quick look at the above commit. I'm not yet certain that
> > >everything is correct in terms of the semantics laid down in the
> > >change or that enough blocks are reserved for btree splits , but I
> > 
> > I actually tried, naively, to bump up SET_ASIDE_BLOCKS from 8 to 32. I 
> > won't claim to understand half of what's going on but I wondered whether 
> > that might make the problem noticeably harder to reproduce at least, but 
> > it had no effect ;-).
> 
> That was going to be my next question. ;)
> 
> At least that rules out a small error in the block reservation decision,
> so I'm going to have  analyse all the code paths the mod introduced
> and work out what is going wrong.

You know, if you had of buumped it up just a bit higher, the problem might
have gone away. With a fielsystem that only has 8 AGs in it, if you bumped
it to 33, then problem would have disappeared....

What we have here is a small error in the block reservation code. Basically,
all the logic is correct except for one critical detail - while we need to
reserve 4 blocks for the AG freelist so a minimum allocation can succeed,
we need to reserve 4 blocks in _every AG_ so that when every AG is empty
we will fail with ENOSPC instead of trying to allocate a block when we
have an AG with less thaan 4 free blocks in it.

So, it's not 4 blocks filesystem wide we need to reserve, it's 4 blocks per AG
we need to reserve.

Stephane and Luciano, can you try the patch attæched below - it fixes the
100% repeatable test case (while [ 1 ]; dd to enospc; done) on my test
machine.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group


---
 fs/xfs/xfs_mount.c |   18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

Index: 2.6.x-xfs-new/fs/xfs/xfs_mount.c
===================================================================
--- 2.6.x-xfs-new.orig/fs/xfs/xfs_mount.c	2006-08-18 15:29:28.000000000 +1000
+++ 2.6.x-xfs-new/fs/xfs/xfs_mount.c	2006-08-28 17:11:18.496258662 +1000
@@ -1257,10 +1257,11 @@ xfs_mod_sb(xfs_trans_t *tp, __int64_t fi
  * 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
+ * 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_SET_ASIDE_BLOCKS(mp)  (4 + ((mp)->m_sb.sb_agcount * 4))
 
 /*
  * xfs_mod_incore_sb_unlocked() is a utility routine common used to apply
@@ -1306,7 +1307,8 @@ xfs_mod_incore_sb_unlocked(xfs_mount_t *
 		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_SET_ASIDE_BLOCKS(mp);
 		res_used = (long long)(mp->m_resblks - mp->m_resblks_avail);
 
 		if (delta > 0) {		/* Putting blocks back */
@@ -1340,7 +1342,7 @@ xfs_mod_incore_sb_unlocked(xfs_mount_t *
 			}
 		}
 
-		mp->m_sb.sb_fdblocks = lcounter + SET_ASIDE_BLOCKS;
+		mp->m_sb.sb_fdblocks = lcounter + XFS_SET_ASIDE_BLOCKS(mp);
 		return 0;
 	case XFS_SBS_FREXTENTS:
 		lcounter = (long long)mp->m_sb.sb_frextents;
@@ -2108,11 +2110,11 @@ again:
 	case XFS_SBS_FDBLOCKS:
 		BUG_ON((mp->m_resblks - mp->m_resblks_avail) != 0);
 
-		lcounter = icsbp->icsb_fdblocks;
+		lcounter = icsbp->icsb_fdblocks - XFS_SET_ASIDE_BLOCKS(mp);
 		lcounter += delta;
 		if (unlikely(lcounter < 0))
 			goto slow_path;
-		icsbp->icsb_fdblocks = lcounter;
+		icsbp->icsb_fdblocks = lcounter + XFS_SET_ASIDE_BLOCKS(mp);
 		break;
 	default:
 		BUG();