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

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Message-ID: <4FC7C1CD.7020701@gmail.com>
Date:	Thu, 31 May 2012 15:09:01 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Cong Wang <amwang@...hat.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	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
Subject: Re: [RFC Patch] fs: implement per-file drop caches

(5/31/12 8:11 AM), Cong Wang wrote:
> On Thu, 2012-05-31 at 02:30 -0400, KOSAKI Motohiro wrote:
>> (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).
>>
>
> I think madvise(DONTNEED) attacks the problem in a different approach,
> it munmaps the file mapping and by the way drops the page caches, my
> approach is to drop the page caches directly similar to what sysctl
> drop_caches.
>
> What about private file mapping? Could madvise(DONTNEED) drop the page
> caches too even when the other process is doing the same private file
> mapping? At least my patch could do this.

Right. But a process can makes another mappings if a process have enough
permission. and if it doesn't, a process shouldn't be able to drop a shared
cache.


> I am not sure if fcntl() is a good interface either, this is why the
> patch is marked as RFC. :-D

But, if you can find certain usecase, I'm not against anymore.


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