From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C2E05C10F14 for ; Thu, 18 Apr 2019 15:30:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 922B120869 for ; Thu, 18 Apr 2019 15:30:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389430AbfDRPay (ORCPT ); Thu, 18 Apr 2019 11:30:54 -0400 Received: from mx2.suse.de ([195.135.220.15]:36616 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1731317AbfDRPay (ORCPT ); Thu, 18 Apr 2019 11:30:54 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id B2FC1B11D; Thu, 18 Apr 2019 15:30:50 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 4B1B01E15AE; Thu, 18 Apr 2019 17:30:47 +0200 (CEST) Date: Thu, 18 Apr 2019 17:30:47 +0200 From: Jan Kara To: Jerome Glisse Cc: Jan Kara , Dan Williams , Kent Overstreet , Boaz Harrosh , Linux Kernel Mailing List , linux-fsdevel , linux-block@vger.kernel.org, Linux MM , John Hubbard , Alexander Viro , Johannes Thumshirn , Christoph Hellwig , Jens Axboe , Ming Lei , Jason Gunthorpe , Matthew Wilcox , Steve French , linux-cifs@vger.kernel.org, samba-technical@lists.samba.org, Yan Zheng , Sage Weil , Ilya Dryomov , Alex Elder , ceph-devel@vger.kernel.org, Eric Van Hensbergen , Latchesar Ionkov , Mike Marshall , Martin Brandenburg , devel@lists.orangefs.org, Dominique Martinet , v9fs-developer@lists.sourceforge.net, Coly Li , linux-bcache@vger.kernel.org, Ernesto =?iso-8859-1?Q?A=2E_Fern=E1ndez?= Subject: Re: [PATCH v1 00/15] Keep track of GUPed pages in fs and block Message-ID: <20190418153047.GN28541@quack2.suse.cz> References: <20190411210834.4105-1-jglisse@redhat.com> <2c124cc4-b97e-ee28-2926-305bc6bc74bd@plexistor.com> <20190416185922.GA12818@kmo-pixel> <20190416194936.GD21526@redhat.com> <20190417222858.GA4146@redhat.com> <20190418104205.GA28541@quack2.suse.cz> <20190418142729.GB3288@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190418142729.GB3288@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 18-04-19 10:27:29, Jerome Glisse wrote: > On Thu, Apr 18, 2019 at 12:42:05PM +0200, Jan Kara wrote: > > On Wed 17-04-19 18:28:58, Jerome Glisse wrote: > > > On Wed, Apr 17, 2019 at 02:53:28PM -0700, Dan Williams wrote: > > > > On Tue, Apr 16, 2019 at 12:50 PM Jerome Glisse wrote: > > > > > > > > > > On Tue, Apr 16, 2019 at 12:12:27PM -0700, Dan Williams wrote: > > > > > > On Tue, Apr 16, 2019 at 11:59 AM Kent Overstreet > > > > > > wrote: > > > > > > > > > > > > > > On Tue, Apr 16, 2019 at 09:35:04PM +0300, Boaz Harrosh wrote: > > > > > > > > On Thu, Apr 11, 2019 at 05:08:19PM -0400, jglisse@redhat.com wrote: > > > > > > > > > From: Jérôme Glisse > > > > > > > > > > > > > > > > > > This patchset depends on various small fixes [1] and also on patchset > > > > > > > > > which introduce put_user_page*() [2] and thus is 5.3 material as those > > > > > > > > > pre-requisite will get in 5.2 at best. Nonetheless i am posting it now > > > > > > > > > so that it can get review and comments on how and what should be done > > > > > > > > > to test things. > > > > > > > > > > > > > > > > > > For various reasons [2] [3] we want to track page reference through GUP > > > > > > > > > differently than "regular" page reference. Thus we need to keep track > > > > > > > > > of how we got a page within the block and fs layer. To do so this patch- > > > > > > > > > set change the bio_bvec struct to store a pfn and flags instead of a > > > > > > > > > direct pointer to a page. This way we can flag page that are coming from > > > > > > > > > GUP. > > > > > > > > > > > > > > > > > > This patchset is divided as follow: > > > > > > > > > - First part of the patchset is just small cleanup i believe they > > > > > > > > > can go in as his assuming people are ok with them. > > > > > > > > > > > > > > > > > > > > > > > > > - Second part convert bio_vec->bv_page to bio_vec->bv_pfn this is > > > > > > > > > done in multi-step, first we replace all direct dereference of > > > > > > > > > the field by call to inline helper, then we introduce macro for > > > > > > > > > bio_bvec that are initialized on the stack. Finaly we change the > > > > > > > > > bv_page field to bv_pfn. > > > > > > > > > > > > > > > > Why do we need a bv_pfn. Why not just use the lowest bit of the page-ptr > > > > > > > > as a flag (pointer always aligned to 64 bytes in our case). > > > > > > > > > > > > > > > > So yes we need an inline helper for reference of the page but is it not clearer > > > > > > > > that we assume a page* and not any kind of pfn ? > > > > > > > > It will not be the first place using low bits of a pointer for flags. > > > > > > > > > > > > > > > > That said. Why we need it at all? I mean why not have it as a bio flag. If it exist > > > > > > > > at all that a user has a GUP and none-GUP pages to IO at the same request he/she > > > > > > > > can just submit them as two separate BIOs (chained at the block layer). > > > > > > > > > > > > > > > > Many users just submit one page bios and let elevator merge them any way. > > > > > > > > > > > > > > Let's please not add additional flags and weirdness to struct bio - "if this > > > > > > > flag is set interpret one way, if not interpret another" - or eventually bios > > > > > > > will be as bad as skbuffs. I would much prefer just changing bv_page to bv_pfn. > > > > > > > > > > > > This all reminds of the failed attempt to teach the block layer to > > > > > > operate without pages: > > > > > > > > > > > > https://lore.kernel.org/lkml/20150316201640.33102.33761.stgit@dwillia2-desk3.amr.corp.intel.com/ > > > > > > > > > > > > > > > > > > > > Question though - why do we need a flag for whether a page is a GUP page or not? > > > > > > > Couldn't the needed information just be determined by what range the pfn is not > > > > > > > (i.e. whether or not it has a struct page associated with it)? > > > > > > > > > > > > That amounts to a pfn_valid() check which is a bit heavier than if we > > > > > > can store a flag in the bv_pfn entry directly. > > > > > > > > > > > > I'd say create a new PFN_* flag, and make bv_pfn a 'pfn_t' rather than > > > > > > an 'unsigned long'. > > > > > > > > > > > > That said, I'm still in favor of Jan's proposal to just make the > > > > > > bv_page semantics uniform. Otherwise we're complicating this core > > > > > > infrastructure for some yet to be implemented GPU memory management > > > > > > capabilities with yet to be determined value. Circle back when that > > > > > > value is clear, but in the meantime fix the GUP bug. > > > > > > > > > > This has nothing to do with GPU, what make you think so ? Here i am > > > > > trying to solve GUP and to keep the value of knowing wether a page > > > > > has been GUP or not. I argue that if we bias every page in every bio > > > > > then we loose that information and thus the value. > > > > > > > > > > I gave the page protection mechanisms as an example that would be > > > > > impacted but it is not the only one. Knowing if a page has been GUP > > > > > can be useful for memory reclaimation, compaction, NUMA balancing, > > > > > > > > Right, this is what I was reacting to in your pushback to Jan's > > > > proposal. You're claiming value for not doing the simple thing for > > > > some future "may be useful in these contexts". To my knowledge those > > > > things are not broken today. You're asking for the complexity to be > > > > carried today for some future benefit, and I'm asking for the > > > > simplicity to be maintained as much as possible today and let the > > > > value of future changes stand on their own to push for more complexity > > > > later. > > > > > > > > Effectively don't use this bug fix to push complexity for a future > > > > agenda where the value has yet to be quantified. > > > > > > Except that this solution (biasing everyone in bio) would _more complex_ > > > it is only conceptualy appealing. The changes are on the other hand much > > > deeper and much riskier but you decided to ignore that and focus on some- > > > thing i was just giving as an example. > > > > Yeah, after going and reading several places like fs/iomap.c, fs/mpage.c, > > drivers/md/dm-io.c I agree with you. The places that are not doing direct > > IO usually just don't hold any page reference that could be directly > > attributed to the bio (and they don't drop it when bio finishes). They > > rather use other means (like PageLocked, PageWriteback) to make sure the > > page stays alive so mandating gup-pin reference for all pages attached to a > > bio would require a lot of reworking of places that are not related to our > > problem and currently work just fine. So I withdraw my suggestion. Nice in > > theory, too much work in practice ;). > > Have you seem Boaz proposal ? I have started it and it does not look to > bad (but you knwo taste and color :)) You can take a peek: > > https://cgit.freedesktop.org/~glisse/linux/log/?h=gup-bio-v2 > > I need to finish that and run fstests on bunch of different fs before > posting. Dunno if i will have enough time to do that before LSF/MM. Yes, I've seen it. I just wasn't sure how the result will look like. What you have in your tree looks pretty clean so far. BTW (I know I'm repeating myself ;) what if we made iov_iter_get_pages() & iov_iter_get_pages_alloc() always return gup-pin reference? That would get rid of the need for two ioend handlers for each call site... Honza -- Jan Kara SUSE Labs, CR