From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vivek Goyal Subject: Re: [RFC] Storing cgroup id in page->private (Was: Re: [RFC] [PATCH 0/6] Provide cgroup isolation for buffered writes.) Date: Thu, 10 Mar 2011 16:38:32 -0500 Message-ID: <20110310213832.GK29464@redhat.com> References: <1299619256-12661-1-git-send-email-teravest@google.com> <20110309142237.6ab82523.kamezawa.hiroyu@jp.fujitsu.com> <20110310181529.GF29464@redhat.com> <20110310191115.GG29464@redhat.com> <20110310194106.GH29464@redhat.com> <1299791640-sup-1874@think> <3EC7D30A-B8F7-416B-8B1D-A19350C57D82@dilger.ca> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Chris Mason , Justin TerAvest , KAMEZAWA Hiroyuki , m-ikeda , jaxboe , linux-kernel , ryov , taka , "righi.andrea" , guijianfeng , balbir , ctalbott , nauman , mrubin , linux-fsdevel To: Andreas Dilger Return-path: Content-Disposition: inline In-Reply-To: <3EC7D30A-B8F7-416B-8B1D-A19350C57D82@dilger.ca> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Thu, Mar 10, 2011 at 02:24:07PM -0700, Andreas Dilger wrote: > On 2011-03-10, at 2:15 PM, Chris Mason wrote: > > Excerpts from Vivek Goyal's message of 2011-03-10 14:41:06 -0500: > >> On Thu, Mar 10, 2011 at 02:11:15PM -0500, Vivek Goyal wrote: > >>>>> I think the person who dirtied the page can store the information in > >>>>> page->private (assuming buffer heads were not generated) and if flusher > >>>>> thread later ends up generating buffer heads and ends up modifying > >>>>> page->private, this can be copied in buffer heads? > >>>> > >>>> This scares me a bit. > >>>> > >>>> As I understand it, fs/ code expects total ownership of page->private. > >>>> This adds a responsibility for every user to copy the data through and > >>>> store it in the buffer head (or anything else). btrfs seems to do > >>>> something entirely different in some cases and store a different kind > >>>> of value. > >>> > >>> If filesystems are using page->private for some other purpose also, then > >>> I guess we have issues. > >>> > >>> I am ccing linux-fsdevel to have some feedback on the idea of trying > >>> to store cgroup id of page dirtying thread in page->private and/or buffer > >>> head for tracking which group originally dirtied the page in IO controller > >>> during writeback. > >> > >> A quick "grep" showed that btrfs, ceph and logfs are using page->private > >> for other purposes also. > >> > >> I was under the impression that either page->private is null or it > >> points to buffer heads for the writeback case. So storing the info > >> directly in either buffer head directly or first in page->private and > >> then transferring it to buffer heads would have helped. > > > > Right, btrfs has its own uses for page->private, and we expect to own > > it. With a proper callback, the FS could store the extra information you > > need in out own structs. > > There is no requirement that page->private ever points to a buffer_head, and Lustre clients use it for its own tracking structure (never touching buffer_heads at all). Any assumption about what a filesystem is storing in page->private in other parts of the code is just broken. Andreas, As Chris mentioned, will providing callbacks so that filesystem can save/restore page->private be reasonable? Thanks Vivek