linux-kernel - Re: 2.6.22 -mm merge plans -- vm bugfixes

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Date:	Wed, 9 May 2007 16:38:16 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrea Arcangeli <andrea@...e.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: 2.6.22 -mm merge plans -- vm bugfixes

On Thu, 10 May 2007, Nick Piggin wrote:
> > 
> > The filesystem (or page cache) allows pages beyond i_size to come
> > in there?  That wasn't a problem before, was it?  But now it is?
> 
> The filesystem still doesn't, but if i_size is updated after the page
> is returned, we can have a problem that was previously taken care of
> with the truncate_count but now isn't.

But... I thought the page lock was now taking care of that in your
scheme?  truncate_inode_pages has to wait for the page lock, then
it finds the page is mapped and... ahh, it finds the copiee page
is not mapped, so doesn't do its own little unmap_mapping_range,
and the copied page squeaks through.  Drat.

I really think the truncate_count solution worked better, for
truncation anyway.  There may be persuasive reasons you need the
page lock for invalidation: I gave up on trying to understand the
required behaviour(s) for invalidation.

So, bring back (the original use of, not my tree marker use of)
truncate_count?  Hmm, you probably don't want to do that, because
there was some pleasure in removing the strange barriers associated
with it.

A second unmap_mapping_range is just one line of code - but it sure
feels like a defeat to me, calling the whole exercise into question.
(But then, you'd be right to say my perfectionism made it impossible
for me to come up with any solution to the invalidation issues.)

> > Suspect you'd need a barrier of some kind between the i_size_write and
> > the mapping_mapped test?
> 
> The unmap_mapping_range that runs after the truncate_inode_pages should
> run in the correct order, I believe.

Yes, if there's going to be that backup call, the first won't really
need a barrier.

> > But that's a change we could have made at
> > any time if we'd bothered, it's not really the issue here.
> 
> I don't see how you could, because you need to increment truncate_count.

Though indeed we did so, I don't see that we needed to increment
truncate_count in that case (nobody could be coming through
do_no_page on that file, when there are no mappings of it).

> But I believe this is fixing the issue, even if it does so in a peripheral
> manner, because it avoids the added cost for unmapped files.

It's a small improvement to your common case, I agree.

Hugh
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