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Shutemov" To: Jerome Glisse Cc: 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 , 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 Subject: Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions Message-ID: <20190319140623.tblqyb4dcjabjn3o@kshutemo-mobl1> References: <20190308213633.28978-1-jhubbard@nvidia.com> <20190308213633.28978-2-jhubbard@nvidia.com> <20190319120417.yzormwjhaeuu7jpp@kshutemo-mobl1> <20190319134724.GB3437@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190319134724.GB3437@redhat.com> User-Agent: NeoMutt/20180716 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 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. Fair enough. Should we get it into a comment here? > > > +void put_user_pages(struct page **pages, unsigned long npages) > > > +{ > > > + unsigned long index; > > > + > > > + for (index = 0; index < npages; index++) > > > + put_user_page(pages[index]); > > > > I believe there's an room for improvement for compound pages. > > > > If there's multiple consequential pages in the array that belong to the > > same compound page we can get away with a single atomic operation to > > handle them all. > > Yes maybe just add a comment with that for now and leave this kind of > optimization to latter ? Sounds good to me. -- Kirill A. Shutemov