Re: 2.6.22-rc5-yesterdaygit with VM debug: BUG in mm/rmap.c:66: anon_vma_link ?

[Nick Piggin <nickpiggin@yahoo.com.au>]

Hugh Dickins wrote:
> On Mon, 25 Jun 2007, Petr Vandrovec wrote:
>
> > Hello,
> >  to catch some memory corruption bug in our code I've modified malloc to do
> > mmap + mprotect - which has unfortunate effect that it creates thousands and
> > thousands of VMAs.  Everything works (though rather slowly on kernel with
> > CONFIG_VM_DEBUG) until application does fork() - kernel crashes on fork()
> > because copy_process()'s anon_vma_link complains that it could not find anon
> > vma after walking through 100000 elements of anon list - which seems strange,
> > as I did not touch system wide limit (which is 65536 vmas), and mprotect()s
> > started failing after creating 65536 vmas, as expected.
> >
> > Full output of test program and full kernel dmesg are at
> > http://buk.vc.cvut.cz/linux/rmap.
>
>
> Thanks for finding that, Petr.  Patch below just solves the problem
> by removing validate_anon_vma; but in the past both Nick and Andrea
> have been less eager to delete old debug code than I am, so it would
> be rude to put this patch in without an Ack from at least one of them
> - they may prefer to tinker with the limit instead, but removing the
> whole function is my preference.
>
> You were puzzled by the numbers.  What happens is that the parent
> builds up to 65536 vmas, and from that point on is not allowed to
> split vmas any more, so the mprotects fail as you expected and
> observed.  But further mmaps succeed, up to your own 131072 limit,
> because each added area can simply extend the last vma.
>
> All the vmas of interest here (i.e. not the executable, libs, stack
> etc.), for better or worse, share the same anon_vma: so that if
> mprotect were later used to undo the difference between neighbouring
> vmas, they could be merged together - assigning different anon_vmas
> would obstruct that merge (but yes, we've a guessed tradeoff there).
>
> So the parent has around 65500 vmas all linked to the same anon_vma;
> and in the course of its fork, links the child's dup vmas one by one
> to that same anon_vma, until it hits the validate_anon_vma's 100000
> BUG_ON.  It's very much the nature of the anon_vma, to be shared
> between parent and child: anon pages may be shared between both.
>
> If we raised the 100000 limit to 2*sysctl_max_map_count, then your
> program would be safe (setting aside changes to that max_map_count),
> but another program in which the child also forked would then BUG.
>
>
>
> [PATCH] kill validate_anon_vma to avoid mapcount BUG

Fine by me. I had been meaning to get rid of that so DEBUG_VM is more
useful to be turned on in betas or even production kernels.

--
SUSE Labs, Novell Inc.
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