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

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Date:	Thu, 10 May 2007 08:24:39 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Hugh Dickins <hugh@...itas.com>
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

Hugh Dickins wrote:
> 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.)

Well we could bring back the truncate_count, but I think that sucks
because that's moving work into the page fault handler in order to
avoid a bit of work when truncating mapped files.


>>>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).

Of course :P

-- 
SUSE Labs, Novell Inc.
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