linux-kernel - Re: [PATCH RFC] fadvise: move active pages to inactive list with POSIX_FADV

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Message-ID: <4E084AA7.2030701@draigBrady.com>
Date:	Mon, 27 Jun 2011 10:17:27 +0100
From:	Pádraig Brady <P@...igBrady.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	andrea@...terlinux.com, akpm@...ux-foundation.org,
	minchan.kim@...il.com, riel@...hat.com, peterz@...radead.org,
	hannes@...xchg.org, kamezawa.hiroyu@...fujitsu.com,
	aarcange@...hat.com, hughd@...gle.com, jamesjer@...terlinux.com,
	marcus@...ehost.com, matt@...ehost.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] fadvise: move active pages to inactive list with
 POSIX_FADV_DONTNEED

On 27/06/11 06:38, KOSAKI Motohiro wrote:
>> Hmm, What if you do want to evict it from the cache for testing purposes?
>> Perhaps this functionality should be associated with POSIX_FADV_NOREUSE?
>> dd has been recently modified to support invalidating the cache for a file,
>> and it uses POSIX_FADV_DONTNEED for that.
>> http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=5f311553
> 
> This change don't break dd. dd don't have a special privilege of file cache
> dropping if it's also used by other processes.
> 
> if you want to drop a cache forcely (maybe for testing), you need to use
> /proc/sys/vm/drop_caches. It's ok to ignore other processes activity because
> it's privilege operation.

Well the function and privileges are separate things.
I think we've agreed that the new functionality is
best associated with POSIX_FADV_NOREUSE,
and the existing functionality with POSIX_FADV_DONTNEED.

BTW, I don't think privileges are currently enforced
as I got root to cache a file here with:
  # (time md5sum; sleep 100) < big.file
And a normal user was able to uncache with:
  $ dd iflag=nocache if=big.file count=0
Anyway as said, this is a separate "issue".

cheers,
Pádraig.
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