linux-kernel - Re: [PATCH 0/5] VFS: Directory level cache cleaning

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Message-ID: <52AFFBE3.8020507@ubuntukylin.com>
Date:	Tue, 17 Dec 2013 15:23:15 +0800
From:	Li Wang <liwang@...ntukylin.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	Cong Wang <xiyou.wangcong@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Sage Weil <sage@...tank.com>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Yunchuan Wen <yunchuanwen@...ntukylin.com>
Subject: Re: [PATCH 0/5] VFS: Directory level cache cleaning

If we do wanna equip fadvise() with directory level page cache cleaning,
this could be solved by invoking (inode_permission() || 
capable(CAP_SYS_ADMIN)) before manipulating the page cache of that inode.
We think the current extension to 'drop_caches' has a complete back
compatibility, the old semantics keep unchanged, and with add-on
features to do finer granularity cache cleaning should be also
desirable.

On 2013/12/17 11:58, Matthew Wilcox wrote:
> On Tue, Dec 17, 2013 at 11:08:16AM +0800, Li Wang wrote:
>> As far as we know, fadvise(DONTNEED) does not support metadata
>> cache cleaning. We think that is desirable under massive small files
>> situations. Another thing is that do people accept the behavior
>> of feeding a directory fd to fadvise will recusively clean all
>> page caches of files inside that directory?
>
> I think there's a really good permissions-related question here.
> If that's an acceptable interface, should one have to be CAP_SYS_ADMIN
> to issue the request?  What if some of the files below this directory
> are not owned by the user issuing the request?
>
>> On 2013/12/17 1:45, Cong Wang wrote:
>>> On Mon, Dec 16, 2013 at 7:00 AM, Li Wang <liwang@...ntukylin.com> wrote:
>>>> This patch extend the 'drop_caches' interface to
>>>> support directory level cache cleaning and has a complete
>>>> backward compatibility. '{1,2,3}' keeps the same semantics
>>>> as before. Besides, "{1,2,3}:DIRECTORY_PATH_NAME" is allowed
>>>> to recursively clean the caches under DIRECTORY_PATH_NAME.
>>>> For example, 'echo 1:/home/foo/jpg > /proc/sys/vm/drop_caches'
>>>> will clean the page caches of the files inside 'home/foo/jpg'.
>>>>
>>>
>>> This interface is ugly...
>>>
>>> And we already have a file-level drop cache, that is,
>>> fadvise(DONTNEED). Can you extend it if it can't
>>> handle a directory fd?
>>>
>> --
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