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[195.135.220.15]) by mx.google.com with ESMTPS id ck14si8605381ejb.191.2019.04.16.09.52.07 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Apr 2019 09:52:07 -0700 (PDT) Received-SPF: pass (google.com: domain of jack@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jack@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=jack@suse.cz 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 3D821B077; Tue, 16 Apr 2019 16:52:07 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id D663A1E15B4; Tue, 16 Apr 2019 18:52:06 +0200 (CEST) Date: Tue, 16 Apr 2019 18:52:06 +0200 From: Jan Kara To: Jerome Glisse Cc: Jan Kara , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-mm@kvack.org, John Hubbard , Dan Williams , Alexander Viro , Johannes Thumshirn , Christoph Hellwig , Jens Axboe , Ming Lei , Dave Chinner , Jason Gunthorpe , Matthew Wilcox Subject: Re: [PATCH v1 10/15] block: add gup flag to bio_add_page()/bio_add_pc_page()/__bio_add_page() Message-ID: <20190416165206.GC17148@quack2.suse.cz> References: <20190411210834.4105-1-jglisse@redhat.com> <20190411210834.4105-11-jglisse@redhat.com> <20190415145952.GE13684@quack2.suse.cz> <20190416002203.GA3158@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190416002203.GA3158@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon 15-04-19 20:22:04, Jerome Glisse wrote: > On Mon, Apr 15, 2019 at 04:59:52PM +0200, Jan Kara wrote: > > Hi Jerome! > > > > On Thu 11-04-19 17:08:29, jglisse@redhat.com wrote: > > > From: Jérôme Glisse > > > > > > We want to keep track of how we got a reference on page added to bio_vec > > > ie wether the page was reference through GUP (get_user_page*) or not. So > > > add a flag to bio_add_page()/bio_add_pc_page()/__bio_add_page() to that > > > effect. > > > > Thanks for writing this patch set! Looking through patches like this one, > > I'm a bit concerned. With so many bio_add_page() callers it's difficult to > > get things right and not regress in the future. I'm wondering whether the > > things won't be less error-prone if we required that all page reference > > from bio are gup-like (not necessarily taken by GUP, if creator of the bio > > gets to struct page he needs via some other means (e.g. page cache lookup), > > he could just use get_gup_pin() helper we'd provide). After all, a page > > reference in bio means that the page is pinned for the duration of IO and > > can be DMAed to/from so it even makes some sense to track the reference > > like that. Then bio_put() would just unconditionally do put_user_page() and > > we won't have to propagate the information in the bio. > > > > Do you think this would be workable and easier? > > Thinking again on this, i can drop that patch and just add a new > bio_add_page_from_gup() and then it would be much more obvious that > only very few places need to use that new version and they are mostly > obvious places. It is usualy GUP then right away add the pages to bio > or bvec. Yes, that's another option. Probably second preferred by me after my own proposal ;) > We can probably add documentation around GUP explaining that if you > want to build a bio or bvec from GUP you must pay attention to which > function you use. Yes, although we both know how careful people are in reading documentation... > Also pages going in a bio are not necessarily written too, they can > be use as source (writting to block) or as destination (reading from > block). So having all of them with refcount bias as GUP would muddy > the water somemore between pages we can no longer clean (ie GUPed) > and those that are just being use in regular read or write operation. Why would the difference matter here? Honza -- Jan Kara SUSE Labs, CR