linux-kernel - Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon

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Date:	Sat, 10 Apr 2010 16:24:01 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Borislav Petkov <bp@...en8.de>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com
Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas
 of a mergeable VMA

On 04/10/2010 04:05 PM, Linus Torvalds wrote:

> And vma_adjust is the one place that does that anon_vma_merge(), which is
> apart from the actual unmapping sequence the only other place that
> actually free's anon_vmas. So there are reasons to be very suspicious of
> that code.

It frees anon_vma_chain structures, but not actual anon_vmas.

Walking the anon_vma (from rmap) requires the anon_vma->lock,
which is taken in anon_vma_merge whenever a chain is unlinked.

> And I think that code can actually lose an anon_vma chain. It's totally
> screwing up the "import anonvma" case: when it does
>
>                          if (anon_vma_clone(importer, vma)) {
>                                  return -ENOMEM;
>                          }
>                          importer->anon_vma = anon_vma;
>
> we can actually have "importer == vma", but "anon_vma = next->anon_vma".

A few lines up from that code, we have:

         if (vma->anon_vma && (insert || importer || start != 
vma->vm_start))
                 anon_vma = vma->anon_vma;

So anon_vma should always be vma->anon_vma.

If we have already imported an anon_vma, we will not
do so twice, because of the !importer->anon_vma check.

What am I overlooking?

> In which case we actually end up with an _empty_ chain (because importer
> didn't have a chain to begin with!) but "importer->anon_vma" points to an
> anon_vma.

If we import a chain, from vma to importer, importer->anon_vma
will be equal to vma->anon_vma.

I do not see how 'importer' could get a state different from 'vma'.

> Also, the conditional nesting makes no sense (the whole anon_vma_clone()
> only makes sense if importer is set, and it is only ever set _inside_ the
> earlier if-statement, so the whole code should be moved inside there), nor
> does some of the comments.

No argument there, vma_adjust is very hard to read and it took
me a few days to convince myself that my changes kept things
equivalent to how they were before.

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