linux-kernel - Re: [PATCH 9/12] ksm: fix oom deadlock

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Message-ID: <20090825145832.GP14722@random.random>
Date:	Tue, 25 Aug 2009 16:58:32 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Izik Eidus <ieidus@...hat.com>, Rik van Riel <riel@...hat.com>,
	Chris Wright <chrisw@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 9/12] ksm: fix oom deadlock

On Mon, Aug 03, 2009 at 01:18:16PM +0100, Hugh Dickins wrote:
> tables which have been freed for reuse; and even do_anonymous_page
> and __do_fault need to check they're not being called by break_ksm
> to reinstate a pte after zap_pte_range has zapped that page table.

This deadlocks exit_mmap in an infinite loop when there's some region
locked. mlock calls gup and pretends to page fault successfully if
there's a vma existing on the region, but it doesn't page fault
anymore because of the mm_count being 0 already, so follow_page fails
and gup retries the page fault forever. And generally I don't like to
add those checks to page fault fast path.

Given we check mm_users == 0 (ksm_test_exit) after taking mmap_sem in
unmerge_and_remove_all_rmap_items, why do we actually need to care
that a page fault happens? We hold mmap_sem so we're guaranteed to see
mm_users == 0 and we won't ever break COW on that mm with mm_users ==
0 so I think those troublesome checks from page fault can be simply
removed.
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