linux-kernel - Re: [PATCH 06/15] readahead: replace ra->mmap_miss with ra->ra

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Date:	Thu, 25 Feb 2010 10:52:28 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Andi Kleen <andi@...stfloor.org>,
	Steven Whitehouse <swhiteho@...hat.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Clemens Ladisch <clemens@...isch.de>,
	Olivier Galibert <galibert@...ox.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Matt Mackall <mpm@...enic.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/15] readahead: replace ra->mmap_miss with ra->ra_flags

On 02/23/2010 10:10 PM, Wu Fengguang wrote:
> Introduce a readahead flags field and embed the existing mmap_miss in it
> (mainly to save space).
>
> It also changes the mmap_miss upper bound from LONG_MAX to 4096.
> This is to help adapt properly for changing mmap access patterns.
>
> It will be possible to lose the flags in race conditions, however the
> impact should be limited.  For the race to happen, there must be two
> threads sharing the same file descriptor to be in page fault or
> readahead at the same time.
>
> Note that it has always been racy for "page faults" at the same time.
>
> And if ever the race happen, we'll lose one mmap_miss++ or mmap_miss--.
> Which may change some concrete readahead behavior, but won't really
> impact overall I/O performance.
>
> CC: Nick Piggin<npiggin@...e.de>
> CC: Andi Kleen<andi@...stfloor.org>
> CC: Steven Whitehouse<swhiteho@...hat.com>
> Signed-off-by: Wu Fengguang<fengguang.wu@...el.com>

Acked-by: Rik van Riel <riel@...hat.com>
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