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Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the
 anon_vmas  of a mergeable VMA
From: Peter Zijlstra <peterz@infradead.org>
To: Minchan Kim <minchan.kim@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Borislav Petkov <bp@alien8.de>, Johannes Weiner <hannes@cmpxchg.org>,
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Date: Mon, 12 Apr 2010 17:33:25 +0200
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On Tue, 2010-04-13 at 00:17 +0900, Minchan Kim wrote:
> > 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?

Hmm, I think you're right. The race I was thinking of makes the
page_lock_anon_vma() RCU section overlap with that of the mmu_gather,
which ensures the thing is long enough, or hits the !_mapcount case.

I'm not sure there are other page->mapping users that are interesting.