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[209.132.183.28]) by mx.google.com with ESMTPS id v28si1087627qvc.26.2019.04.16.16.34.13 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Apr 2019 16:34:13 -0700 (PDT) Received-SPF: pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jglisse@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 4B7183199363; Tue, 16 Apr 2019 23:34:12 +0000 (UTC) Received: from redhat.com (unknown [10.20.6.236]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BA40E60C61; Tue, 16 Apr 2019 23:34:04 +0000 (UTC) Date: Tue, 16 Apr 2019 19:34:03 -0400 From: Jerome Glisse To: Boaz Harrosh Cc: Boaz Harrosh , Dan Williams , Kent Overstreet , Linux Kernel Mailing List , linux-fsdevel , linux-block@vger.kernel.org, Linux MM , John Hubbard , Jan Kara , Alexander Viro , Johannes Thumshirn , Christoph Hellwig , Jens Axboe , Ming Lei , Jason Gunthorpe , Matthew Wilcox , Steve French , linux-cifs@vger.kernel.org, Yan Zheng , Sage Weil , Ilya Dryomov , Alex Elder , ceph-devel@vger.kernel.org, Eric Van Hensbergen , Latchesar Ionkov , Mike Marshall , Martin Brandenburg , 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: <20190416233402.GC22465@redhat.com> References: <20190411210834.4105-1-jglisse@redhat.com> <2c124cc4-b97e-ee28-2926-305bc6bc74bd@plexistor.com> <20190416185922.GA12818@kmo-pixel> <20190416195735.GE21526@redhat.com> <41e2d7e1-104b-a006-2824-015ca8c76cc8@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <41e2d7e1-104b-a006-2824-015ca8c76cc8@gmail.com> User-Agent: Mutt/1.11.3 (2019-02-01) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Tue, 16 Apr 2019 23:34:12 +0000 (UTC) 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 Wed, Apr 17, 2019 at 01:09:22AM +0300, Boaz Harrosh wrote: > On 16/04/19 22:57, Jerome Glisse wrote: > <> > > > > A very long thread on this: > > > > https://lkml.org/lkml/2018/12/3/1128 > > > > especialy all the reply to this first one > > > > There is also: > > > > https://lkml.org/lkml/2019/3/26/1395 > > https://lwn.net/Articles/753027/ > > > > OK I have re-read this patchset and a little bit of the threads above (not all) > > As I understand the long term plan is to keep two separate ref-counts one > for GUP-ref and one for the regular page-state/ownership ref. > Currently looking at page-ref we do not know if we have a GUP currently held. > With the new plan we can (Still not sure what's the full plan with this new info) > > But if you make it such as the first GUP-ref also takes a page_ref and the > last GUp-dec also does put_page. Then the all of these becomes a matter of > matching every call to get_user_pages or iov_iter_get_pages() with a new > put_user_pages or iov_iter_put_pages(). So sorry forgot to answer that part. So idea is to do: GUP() { ... - page_ref_inc(page); + page_ref_add(page, GUP_BIAS); ... } with GUP_BIAS = 1024 or something big but not too big to avoid risk of overflow by GUP. Then put_user_page() just ref_sub instead of ref_dec the same amount. We can have false GUP positive if a page is map so many time or reference so many time that its refcount reach the GUP_BIAS value but considering such page as GUPed should not be too harmful (not more harmful than what we do with GUPed page). So we want to call put_user_page() for GUPed page and only GUPed page so that we keep the reference count properly balance. Cheers, Jérôme