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[209.132.183.28]) by mx.google.com with ESMTPS id e73si1146783qkj.203.2019.03.19.13.45.21 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 19 Mar 2019 13:45:21 -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-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 05578F74AC; Tue, 19 Mar 2019 20:45:20 +0000 (UTC) Received: from redhat.com (ovpn-120-246.rdu2.redhat.com [10.10.120.246]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A402D60C7F; Tue, 19 Mar 2019 20:45:15 +0000 (UTC) Date: Tue, 19 Mar 2019 16:45:13 -0400 From: Jerome Glisse To: Tom Talpey Cc: Ira Weiny , Jan Kara , "Kirill A. Shutemov" , john.hubbard@gmail.com, Andrew Morton , linux-mm@kvack.org, Al Viro , Christian Benvenuti , Christoph Hellwig , Christopher Lameter , Dan Williams , Dave Chinner , Dennis Dalessandro , Doug Ledford , Jason Gunthorpe , Matthew Wilcox , Michal Hocko , Mike Rapoport , Mike Marciniszyn , Ralph Campbell , LKML , linux-fsdevel@vger.kernel.org, John Hubbard , Andrea Arcangeli Subject: Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions Message-ID: <20190319204512.GB3096@redhat.com> References: <20190308213633.28978-1-jhubbard@nvidia.com> <20190308213633.28978-2-jhubbard@nvidia.com> <20190319120417.yzormwjhaeuu7jpp@kshutemo-mobl1> <20190319134724.GB3437@redhat.com> <20190319141416.GA3879@redhat.com> <20190319142918.6a5vom55aeojapjp@kshutemo-mobl1> <20190319153644.GB26099@quack2.suse.cz> <20190319090322.GE7485@iweiny-DESK2.sc.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Tue, 19 Mar 2019 20:45:20 +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 Tue, Mar 19, 2019 at 03:43:44PM -0500, Tom Talpey wrote: > On 3/19/2019 4:03 AM, Ira Weiny wrote: > > On Tue, Mar 19, 2019 at 04:36:44PM +0100, Jan Kara wrote: > > > On Tue 19-03-19 17:29:18, Kirill A. Shutemov wrote: > > > > On Tue, Mar 19, 2019 at 10:14:16AM -0400, Jerome Glisse wrote: > > > > > On Tue, Mar 19, 2019 at 09:47:24AM -0400, Jerome Glisse wrote: > > > > > > On Tue, Mar 19, 2019 at 03:04:17PM +0300, Kirill A. Shutemov wrote: > > > > > > > On Fri, Mar 08, 2019 at 01:36:33PM -0800, john.hubbard@gmail.com wrote: > > > > > > > > From: John Hubbard > > > > > > > > > > > > [...] > > > > > > > > > > > > > > diff --git a/mm/gup.c b/mm/gup.c > > > > > > > > index f84e22685aaa..37085b8163b1 100644 > > > > > > > > --- a/mm/gup.c > > > > > > > > +++ b/mm/gup.c > > > > > > > > @@ -28,6 +28,88 @@ struct follow_page_context { > > > > > > > > unsigned int page_mask; > > > > > > > > }; > > > > > > > > +typedef int (*set_dirty_func_t)(struct page *page); > > > > > > > > + > > > > > > > > +static void __put_user_pages_dirty(struct page **pages, > > > > > > > > + unsigned long npages, > > > > > > > > + set_dirty_func_t sdf) > > > > > > > > +{ > > > > > > > > + unsigned long index; > > > > > > > > + > > > > > > > > + for (index = 0; index < npages; index++) { > > > > > > > > + struct page *page = compound_head(pages[index]); > > > > > > > > + > > > > > > > > + if (!PageDirty(page)) > > > > > > > > + sdf(page); > > > > > > > > > > > > > > How is this safe? What prevents the page to be cleared under you? > > > > > > > > > > > > > > If it's safe to race clear_page_dirty*() it has to be stated explicitly > > > > > > > with a reason why. It's not very clear to me as it is. > > > > > > > > > > > > The PageDirty() optimization above is fine to race with clear the > > > > > > page flag as it means it is racing after a page_mkclean() and the > > > > > > GUP user is done with the page so page is about to be write back > > > > > > ie if (!PageDirty(page)) see the page as dirty and skip the sdf() > > > > > > call while a split second after TestClearPageDirty() happens then > > > > > > it means the racing clear is about to write back the page so all > > > > > > is fine (the page was dirty and it is being clear for write back). > > > > > > > > > > > > If it does call the sdf() while racing with write back then we > > > > > > just redirtied the page just like clear_page_dirty_for_io() would > > > > > > do if page_mkclean() failed so nothing harmful will come of that > > > > > > neither. Page stays dirty despite write back it just means that > > > > > > the page might be write back twice in a row. > > > > > > > > > > Forgot to mention one thing, we had a discussion with Andrea and Jan > > > > > about set_page_dirty() and Andrea had the good idea of maybe doing > > > > > the set_page_dirty() at GUP time (when GUP with write) not when the > > > > > GUP user calls put_page(). We can do that by setting the dirty bit > > > > > in the pte for instance. They are few bonus of doing things that way: > > > > > - amortize the cost of calling set_page_dirty() (ie one call for > > > > > GUP and page_mkclean() > > > > > - it is always safe to do so at GUP time (ie the pte has write > > > > > permission and thus the page is in correct state) > > > > > - safe from truncate race > > > > > - no need to ever lock the page > > > > > > > > > > Extra bonus from my point of view, it simplify thing for my generic > > > > > page protection patchset (KSM for file back page). > > > > > > > > > > So maybe we should explore that ? It would also be a lot less code. > > > > > > > > Yes, please. It sounds more sensible to me to dirty the page on get, not > > > > on put. > > > > > > I fully agree this is a desirable final state of affairs. > > > > I'm glad to see this presented because it has crossed my mind more than once > > that effectively a GUP pinned page should be considered "dirty" at all times > > until the pin is removed. This is especially true in the RDMA case. > > But, what if the RDMA registration is readonly? That's not uncommon, and > marking dirty unconditonally would add needless overhead to such pages. Yes and this is only when FOLL_WRITE is set ie when you are doing GUP and asking for write. Doing GUP and asking for read is always safe. Cheers, Jérôme