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[150.101.137.141]) by mx.google.com with ESMTP id q20si106277plr.136.2019.03.19.14.23.51 for ; Tue, 19 Mar 2019 14:23:52 -0700 (PDT) Received-SPF: neutral (google.com: 150.101.137.141 is neither permitted nor denied by best guess record for domain of david@fromorbit.com) client-ip=150.101.137.141; Authentication-Results: mx.google.com; spf=neutral (google.com: 150.101.137.141 is neither permitted nor denied by best guess record for domain of david@fromorbit.com) smtp.mailfrom=david@fromorbit.com Received: from ppp59-167-129-252.static.internode.on.net (HELO dastard) ([59.167.129.252]) by ipmail03.adl2.internode.on.net with ESMTP; 20 Mar 2019 07:53:49 +1030 Received: from dave by dastard with local (Exim 4.80) (envelope-from ) id 1h6MD4-0003Pg-M8; Wed, 20 Mar 2019 08:23:46 +1100 Date: Wed, 20 Mar 2019 08:23:46 +1100 From: Dave Chinner To: Jerome Glisse Cc: "Kirill A. Shutemov" , john.hubbard@gmail.com, Andrew Morton , linux-mm@kvack.org, Al Viro , Christian Benvenuti , Christoph Hellwig , Christopher Lameter , Dan Williams , Dennis Dalessandro , Doug Ledford , Ira Weiny , Jan Kara , Jason Gunthorpe , Matthew Wilcox , Michal Hocko , Mike Rapoport , Mike Marciniszyn , Ralph Campbell , Tom Talpey , 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: <20190319212346.GA26298@dastard> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190319141416.GA3879@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) 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 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 I seem to have missed this conversation, so please excuse me for asking a stupid question: if it's a file backed page, what prevents background writeback from cleaning the dirty page ~30s into a long term pin? i.e. I don't see anything in this proposal that prevents the page from being cleaned by writeback and putting us straight back into the situation where a long term RDMA is writing to a clean page.... Cheers, Dave. -- Dave Chinner david@fromorbit.com