linux-kernel - Re: [PATCH] mm: don't return 0 too early from find_get

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Date:	Thu, 24 Feb 2011 13:40:13 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Salman Qazi <sqazi@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: don't return 0 too early from find_get_pages()

On Wed, 2011-02-23 at 21:35 -0800, Hugh Dickins wrote:
> Callers of find_get_pages(), or its wrapper pagevec_lookup() - notably
> truncate_inode_pages_range() - stop looking further when it returns 0.
> 
> But if an interrupt comes just after its radix_tree_gang_lookup_slot(),
> especially if we have preemptible RCU enabled, isn't it conceivable
> that all 14 pages returned could be removed from the page cache by
> shrink_page_list(), before find_get_pages() gets to process them?  So
> causing it to return 0 although there may be plenty more pages beyond.
> 
> Make find_get_pages() and find_get_pages_tag() check for this unlikely
> case, and restart should it occur; but callers of find_get_pages_contig()
> have no such expectation, it's okay for that to return 0 early.
> 
> I have not seen this in practice, just worried by the possibility.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
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