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 14:41:06 -0500 Message-ID: <20110310194106.GH29464@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> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: KAMEZAWA Hiroyuki , m-ikeda@ds.jp.nec.com, jaxboe@fusionio.com, linux-kernel@vger.kernel.org, ryov@valinux.co.jp, taka@valinux.co.jp, righi.andrea@gmail.com, guijianfeng@cn.fujitsu.com, balbir@linux.vnet.ibm.com, ctalbott@google.com, nauman@google.com, mrubin@google.com, linux-fsdevel@vger.kernel.org, Chris Mason To: Justin TerAvest Return-path: Content-Disposition: inline In-Reply-To: <20110310191115.GG29464@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Thu, Mar 10, 2011 at 02:11:15PM -0500, Vivek Goyal wrote: > On Thu, Mar 10, 2011 at 10:57:52AM -0800, Justin TerAvest wrote: > > On Thu, Mar 10, 2011 at 10:15 AM, Vivek Goyal w= rote: > > > On Thu, Mar 10, 2011 at 10:08:03AM -0800, Justin TerAvest wrote: > > > > > > [..] > > >> > I don't like to increase size of page_cgroup but I think you c= an record > > >> > information without increasing size of page_cgroup. > > >> > > > >> > A) As Andrea did, encode it to pc->flags. > > >> > =A0 But I'm afraid that there is a racy case because memory cg= roup uses some > > >> > =A0 test_and_set() bits. > > >> > B) I wonder why the information cannot be recorded in page->pr= ivate. > > >> > =A0 When page has buffers, you can record the information to b= uffer struct. > > >> > =A0 About swapio (if you take care of), you can record informa= tion to bio. > > >> > > >> Hi Kame, > > >> > > >> I'm concerned that by using something like buffer_heads stored i= n > > >> page->private, we will only be supported on some filesystems and= not > > >> others. In addition, I'm not sure if all filesystems attach buff= er > > >> heads at the same time; if page->private is modified in the flus= her > > >> thread, we might not be able to determine the thread that dirtie= d the > > >> page in the first place. > > > > > > I think the person who dirtied the page can store the information= in > > > page->private (assuming buffer heads were not generated) and if f= lusher > > > thread later ends up generating buffer heads and ends up modifyin= g > > > page->private, this can be copied in buffer heads? > >=20 > > This scares me a bit. > >=20 > > As I understand it, fs/ code expects total ownership of page->priva= te. > > 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 ki= nd > > of value. >=20 > If filesystems are using page->private for some other purpose also, t= hen > I guess we have issues.=20 >=20 > 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 bu= ffer > head for tracking which group originally dirtied the page in IO contr= oller > during writeback. A quick "grep" showed that btrfs, ceph and logfs are using page->privat= e for other purposes also. I was under the impression that either page->private is null or it=20 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.=20 Thanks Vivek