linux-kernel - Re: [RFC Patch] fs: implement per-file drop caches

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Date:	Thu, 31 May 2012 02:30:22 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Cong Wang <amwang@...hat.com>
CC:	Pádraig Brady <P@...igBrady.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <matthew@....cx>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, kosaki.motohiro@...il.com
Subject: Re: [RFC Patch] fs: implement per-file drop caches

(5/31/12 2:20 AM), Cong Wang wrote:
> On Wed, 2012-05-30 at 16:14 +0100, Pádraig Brady wrote:
>> On 05/30/2012 02:38 PM, Cong Wang wrote:
>>> This is a draft patch of implementing per-file drop caches.
>>>
>>> It introduces a new fcntl command  F_DROP_CACHES to drop
>>> file caches of a specific file. The reason is that currently
>>> we only have a system-wide drop caches interface, it could
>>> cause system-wide performance down if we drop all page caches
>>> when we actually want to drop the caches of some huge file.
>>
>> This is useful functionality.
>> Though isn't it already provided with POSIX_FADV_DONTNEED?
>
> Thanks for teaching this!
>
> However, from the source code of madvise_dontneed() it looks like it is
> using a totally different way to drop page caches, that is to invalidate
> the page mapping, and trigger a re-mapping of the file pages after a
> page fault. So, yeah, this could probably drop the page caches too (I am
> not so sure, haven't checked the code in details), but with my patch, it
> flushes the page caches directly, what's more, it can also prune
> dcache/icache of the file.

madvise should work. I don't think we need duplicate interface. Moreomover
madvise(2) is cleaner than fcntl(2).

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