linux-kernel - Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon

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Date:	Fri,  9 Apr 2010 11:19:52 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma()

Hi

> > - Then checks page_mapped() without having any apparent defence
> >   against page_mapped() becoming untrue one nanosecond later.
> > 
> > - Checks page_mapped() inside the rcu_read_locked() section for
> >   inscrutable reasons.
> 
> Right, I think the page_mapped() stuff is just an early bail out.

FWIW, not only early bail out.
page_remove_rmap() don't have "page->mapping = NULL". page_remove_rmap's
comment says

        /*
         * It would be tidy to reset the PageAnon mapping here,
         * but that might overwrite a racing page_add_anon_rmap
         * which increments mapcount after us but sets mapping
         * before us: so leave the reset to free_hot_cold_page,
         * and remember that it's only reliable while mapped.
         * Leaving it set also helps swapoff to reinstate ptes
         * faster for those pages still in swapcache.
         */

So, If following scenario happen, we can't dereference page->mapping
(iow, can't call spin_lock(&anon_vma->lock)). It might be pointed 
invalid address.


   CPU0                                CPU1
===============================================================
page->mappings become -1
anon_vma_unlink()
-- grace period --
                                      page_lock_anon_vma
                                      page_mapped()
                                      spin_lock(&anon_vma->lock);



Of cource, this statement doesn't mean I'm against your patch at all.
I like it.



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