From: Minchan Kim Subject: Re: [PATCH V2 0/7] Cleancache (was Transcendent Memory): overview Date: Thu, 3 Jun 2010 01:38:52 +0900 Message-ID: <20100602163827.GA5450@barrios-desktop> References: <20100528173510.GA12166@ca-server1.us.oracle.comAANLkTilV-4_QaNq5O0WSplDx1Oq7JvkgVrEiR1rgf1up@mail.gmail.com> <1d88619a-bb1e-493f-ad96-bf204b60938d@default> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Cc: chris.mason@oracle.com, viro@zeniv.linux.org.uk, akpm@linux-foundation.org, adilger@sun.com, tytso@mit.edu, mfasheh@suse.com, joel.becker@oracle.com, matthew@wil.cx, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, ocfs2-devel@oss.oracle.com, linux-mm@kvack.org, ngupta@vflare.org, jeremy@goop.org, JBeulich@novell.com, kurt.hackel@oracle.com, npiggin@suse.de, dave.mccracken@oracle.com, riel@redhat.com, avi@redhat.com, konrad.wilk@oracle.com To: Dan Magenheimer Return-path: Content-Disposition: inline In-Reply-To: <1d88619a-bb1e-493f-ad96-bf204b60938d@default> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org Hi, Dan.=20 On Wed, Jun 02, 2010 at 08:27:48AM -0700, Dan Magenheimer wrote: > Hi Minchan -- >=20 > > I think cleancache approach is cool. :) > > I have some suggestions and questions. >=20 > Thanks for your interest! >=20 > > > If a get_page is successful on a non-shared pool, the page is flush= ed > > (thus > > > making cleancache an "exclusive" cache). =A0On a shared pool, the p= age > >=20 > > Do you have any reason about force "exclusive" on a non-shared pool? > > To free memory on pesudo-RAM? > > I want to make it "inclusive" by some reason but unfortunately I can'= t > > say why I want it now. >=20 > The main reason is to free up memory in pseudo-RAM and to > avoid unnecessary cleancache_flush calls. If you want > inclusive, the page can be put immediately following > the get. If put-after-get for inclusive becomes common, > the interface could easily be extended to add a "get_no_flush" > call. Sounds good to me.=20 > =20 > > While you mentioned it's "exclusive", cleancache_get_page doesn't > > flush the page at below code. > > Is it a role of user who implement cleancache_ops->get_page? >=20 > Yes, the flush is done by the cleancache implementation. >=20 > > If backed device is ram(ie), Could we _move_ the pages from page cach= e > > to cleancache? > > I mean I don't want to copy page when get/put operation. we can just > > move page in case of backed device "ram". Is it possible? >=20 > By "move", do you mean changing the virtual mappings? Yes, > this could be done as long as the source and destination are > both directly addressable (that is, true physical RAM), but > requires TLB manipulation and has some complicated corner > cases. The copy semantics simplifies the implementation on > both the "frontend" and the "backend" and also allows the > backend to do fancy things on-the-fly like page compression > and page deduplication. Agree. But I don't mean it.=20 If I use brd as backend, i want to do it follwing as.=20 put_page : remove_from_page_cache(page); brd_insert_page(page); get_page : brd_lookup_page(page); add_to_page_cache(page); Of course, I know it's impossible without new metadata and modification o= f=20 page cache handling and it makes front and backend's good layered design.= =20 What I want is to remove copy overhead when backend is ram and it's also part of main memory(ie, we have page descriptor).=20 Do you have an idea? >=20 > > You send the patches which is core of cleancache but I don't see any > > use case. > > Could you send use case patches with this series? > > It could help understand cleancache's benefit. >=20 > Do you mean the Xen Transcendent Memory ("tmem") implementation? > If so, this is four files in the Xen source tree (common/tmem.c, > common/tmem_xen.c, include/xen/tmem.h, include/xen/tmem_xen.h). > There is also an html document in the Xen source tree, which can > be viewed here: > http://oss.oracle.com/projects/tmem/dist/documentation/internals/xen4-i= nternals-v01.html=20 >=20 > Or did you mean a cleancache_ops "backend"? For tmem, there > is one file linux/drivers/xen/tmem.c and it interfaces between > the cleancache_ops calls and Xen hypercalls. It should be in > a Xenlinux pv_ops tree soon, or I can email it sooner. I mean "backend". :)=20 >=20 > I am also eagerly awaiting Nitin Gupta's cleancache backend > and implementation to do in-kernel page cache compression. Do Nitin say he will make backend of cleancache for page cache compression?=20 It would be good feature.=20 I have a interest, too. :) Thanks, Dan.=20 >=20 > Thanks, > Dan --=20 Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org