Re: spurious -ENOSPC on XFS

From: Dave Chinner
Date: Tue Jan 20 2009 - 18:24:48 EST

On Tue, Jan 20, 2009 at 02:38:27PM -0500, Mikulas Patocka wrote:
> On Sun, 18 Jan 2009, Christoph Hellwig wrote:
> > On Tue, Jan 13, 2009 at 11:28:58PM -0500, Mikulas Patocka wrote:
> > > The result must not depend on magic timer values. If it does, you end up
> > > with undebbugable nondeterministic failures.
> > >
> > > Why don't you change that 500ms wait to "wait until the flush finishes"?
> > > That would be correct.
> >
> > Yes, this probably would better. Could I motivate you to come up with
> > a patch for that?
> >
> Hi
> I looked at the source and found out that it uses sync_blockdev for
> syncing --- but sync_blockdev writes only metadata buffers, it doesn't
> touch inodes and pages and doesn't resolve delayed allocations. So it
> really doesn't sync anything.

Ah, bugger. Thanks for finding this.

> I replaced it with correct syncing of all inodes. With this patch it
> passes my testcase (no more spurious -ENOSPCs), but it still isn't
> correct, there is that 500ms delay --- if the machine was so overloaded
> that it couldn't sync withing 500ms, you still get spurious -ENOSPC.

That's VFS level data syncing - there may be other XFS level stuff
that can be dones as well (e.g. cleanup/truncate of unlinked inodes)
that will release space.

> There are notions about possible deadlocks (the syncer may lock against
> the process that is waiting for the sync to finish), that's why removing
> that 500ms delay isn't that easy as it seems. I don't have XFS knowledge
> to check for the deadlocks, it should be done by XFS developers. Also,
> when you resolve the deadlocks and drop the timeout, replace WB_SYNC_NONE
> with WB_SYNC_ALL in this patch.

Right, so you need to use internal xfs sync functions that don't
have these problems. That is:

error = xfs_sync_inodes(ip->i_mount, SYNC_DELWRI|SYNC_WAIT);

will do a blocking flush of all the inodes without deadlocks occurring.
Then you can remove the 500ms wait.


Dave Chinner
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