Re: [PATCH 04/10] xfs: remove an unsafe retry in xfs_bmbt_alloc_block

From: Brian Foster <bfoster@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH 04/10] xfs: remove an unsafe retry in xfs_bmbt_alloc_block
Date: Mon, 17 Apr 2017 10:19:23 -0400	[thread overview]
Message-ID: <20170417141923.GB41659@bfoster.bfoster> (raw)
In-Reply-To: <20170414074658.GA24766@lst.de>

On Fri, Apr 14, 2017 at 09:46:58AM +0200, Christoph Hellwig wrote:
> On Thu, Apr 13, 2017 at 02:30:06PM -0400, Brian Foster wrote:
> > I'm not quite following why this retry is unsafe as noted in the patch
> > title.. do you mean "unnecessary?" AFAICT, the firstblock == NULLFSBLOCK
> > case means we can issue this first allocation from any AG.
> 
> Yes.
> 
> > If no AG can
> > allocate a block while satisfying minleft, then we can still safely
> > allocate from any AG provided any subsequent allocations occur in
> > increasing AG order (i.e., by setting dop_low), right?
> 
> Yes.  But minleft is set exactly because we require this number of
> blocks to be left after the current allocation.  If we could only
> allocate the current allocation, but not satisfy minleft we risk
> shutting the file system during subsequent allocations instead of
> just returning ENOSPC now.
> 

I don't see anything about setting minleft here that says the allocation
is required to come from one AG as opposed to that simply being
preferred.

Also, I think we risk shutdown if this allocation fails at all,
regardless of the firstblock state, because the transaction is likely
already dirty. I have by no means audited all of the possible contexts
that lead here, but a quick tracepoint check shows the transcation as
dirty when punching holes. I'm also guessing this is why we currently
try so hard to allocate here.

> > Also, if this is unnecessary, what exactly verifies that all of the
> > reserved blocks are available within the same AG?
> 
> xfs_alloc_space_available verifies that ->total blocks are available
> in the current AG.  Callers of the allocator need to set it to the
> correct value currently, although I have more xfs_bmapi changes in
> the pipe to get this right automatically - but those aren't 4.12
> material.

Not all bmbt block allocations are tied to extent allocations. This is
the firstblock == NULLFSBLOCK case after all, which I take it means an
allocation hasn't yet occurred. IOW, what about other potentially
record-inserting operations like hole punch, extent conversion, etc.?

Brian

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