From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8205BC43381 for ; Tue, 19 Mar 2019 14:15:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E0E6213F2 for ; Tue, 19 Mar 2019 14:15:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726778AbfCSOPi (ORCPT ); Tue, 19 Mar 2019 10:15:38 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49934 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726579AbfCSOPi (ORCPT ); Tue, 19 Mar 2019 10:15:38 -0400 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 866CC40F44; Tue, 19 Mar 2019 14:15:37 +0000 (UTC) Received: from redhat.com (unknown [10.20.6.236]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3A17218BBA; Tue, 19 Mar 2019 14:15:35 +0000 (UTC) Date: Tue, 19 Mar 2019 10:15:33 -0400 From: Jerome Glisse To: "Kirill A. Shutemov" 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: <20190319141533.GB3879@redhat.com> References: <20190308213633.28978-1-jhubbard@nvidia.com> <20190308213633.28978-2-jhubbard@nvidia.com> <20190319120417.yzormwjhaeuu7jpp@kshutemo-mobl1> <20190319134724.GB3437@redhat.com> <20190319140623.tblqyb4dcjabjn3o@kshutemo-mobl1> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190319140623.tblqyb4dcjabjn3o@kshutemo-mobl1> User-Agent: Mutt/1.10.0 (2018-05-17) 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 [10.5.110.30]); Tue, 19 Mar 2019 14:15:38 +0000 (UTC) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Mar 19, 2019 at 05:06:23PM +0300, Kirill A. Shutemov 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. > > Fair enough. Should we get it into a comment here? Yes definitly also i just sent an email with an alternative that is slightly better from my POV as it simplify my life for other things :) Cheers, Jérôme