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();
|