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

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Date:	Tue, 13 Apr 2010 00:17:56 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 Mon, Apr 12, 2010 at 11:40 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sat, 2010-04-10 at 11:21 -0700, Linus Torvalds wrote:
>>
>
>> Ho humm.
>>
>> Maybe I'm crazy, but something started bothering me. And I started
>> wondering: when is the 'page->mapping' of an anonymous page actually
>> cleared?
>>
>> The thing is, the mapping of an anonymous page is actually cleared only
>> when the page is _freed_, in "free_hot_cold_page()".
>>
>> Now, let's think about that. And in particular, let's think about how that
>> relates to the freeing of the 'anon_vma' that the page->mapping points to.
>>
>> The way the anon_vma is freed is when the mapping is torn down, and we do
>> roughly:
>>
>>       tlb = tlb_gather_mmu(mm,..)
>>       ..
>>       unmap_vmas(&tlb, vma ..
>>       ..
>>       free_pgtables()
>>       ..
>>       tlb_finish_mmu(tlb, start, end);
>>
>> and we actually unmap all the pages in "unmap_vmas()", and then _after_
>> unmapping all the pages we do the "unlink_anon_vmas(vma);" in
>> "free_pgtables()". Fine so far - the anon_vma stay around until after the
>> page has been happily unmapped.
>>
>> But "unmapped all the pages" is _not_ actually the same as "free'd all the
>> pages". The actual _freeing_ of the page happens generally in
>> tlb_finish_mmu(), because we can free the page only after we've flushed
>> any TLB entries.
>>
>> So what we have in that tlb_gather structure is a list of _pending_ pages
>> to be freed, while we already actually free'd the anon_vmas earlier!
>>
>> Now, the thing is, tlb_gather_mmu() begins a preempt-safe region (because
>> we use a per-cpu variable), but as far as I can tell it is _not_ an
>> RCU-safe region.
>>
>> So I think we might actually get a real RCU freeing event while this all
>> happens. So now the 'anon_vma' that 'page->mapping' points to has not just
>> been released back to the SLUB caches, the page itself might have been
>> released too.
>>
>> I dunno. Does the above sound at all sane? Or am I just raving?
>>
>> Something hacky like the above might fix it if I'm not just raving. I
>> really might be missing something here.
>
> Right, so unless you have CONFIG_TREE_PREEMPT_RCU=y, the preempt-disable
> == RCU read lock assumption does hold.

Indeed.

>
> But even with your patch it doesn't close all holes because while
> zap_pte_range() can remove the last mapcount of the page, the
> page_remove_tlb() et al. don't need to be the last use count of the
> page.
>
> Concurrent reclaim/gup/whatever could still have a count out on the page
> delaying the actual free beyond the tlb gather RCU section.

anon_vma lock is just valid in case of page_mapped.
if reclaim/gup/whatever want to use anon_vma, it should check with page_mapped.
And last put_page doesn't touch anon_vma for freeing the page so I
think it's not a problem. Do I miss something?

>
> This also complicates the whole page_lock_anon_vma() thing, so it would
> be nice to be able to remove this race and clear page->mapping in
> page_remove_rmap().
>

BTW, I totally agree with you.
Now anon_vma is very complicated.
SLAB_DESTROY_BY_RCU, vma merge, when page->mapping is cleared,
anon_vma_chain and so on.. :(


-- 
Kind regards,
Minchan Kim
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