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=-6.0 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT 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 C6053C4360F for ; Tue, 19 Mar 2019 14:29:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9640F2085A for ; Tue, 19 Mar 2019 14:29:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=shutemov-name.20150623.gappssmtp.com header.i=@shutemov-name.20150623.gappssmtp.com header.b="dsShfLT5" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727708AbfCSO3Z (ORCPT ); Tue, 19 Mar 2019 10:29:25 -0400 Received: from mail-pg1-f194.google.com ([209.85.215.194]:40366 "EHLO mail-pg1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726778AbfCSO3Y (ORCPT ); Tue, 19 Mar 2019 10:29:24 -0400 Received: by mail-pg1-f194.google.com with SMTP id u9so13964993pgo.7 for ; Tue, 19 Mar 2019 07:29:23 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=shutemov-name.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=9om5QJZsJKNhMP+qIVE/OgL0ptHNmMTB1Erc9MBg9Gs=; b=dsShfLT5ONbIyV9P7eRs1l/EGJTZUK1Wx/pyJu1mXTBZUOiNW01pRqIGgSS745AQT9 mslmk0ARTFjsgy9s2CboNPlVYLeO8EL9QcMNAYsevKih8GhtHOqnDwQyZAgaaI/OF8Aw CQoy1lJPmeNhmBUaubMRQrbkbch8eH0QjMBkegJ1VtEM6VOo14XwXOU5VvL0cA67RfTb TpKzZQPAZUyV5ffp9ClckR4KTBlT1H8qGxf3pZas30ng/5md12Hpwpm0qNIUcMMwyTWF MyOIOsgcpeVCIH5xM5xiq6DkkgSsvCObtTlelL2yD6TbavY8uMXoWVLs8ZCOXOZMiSnz JWwA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=9om5QJZsJKNhMP+qIVE/OgL0ptHNmMTB1Erc9MBg9Gs=; b=tAq6ZlxtIoc5WrTFXeA7YV0TbIs3hCQzmgHaG70hrlIyOpGQ2EIQj4tXisFJHx+FXT AB8CY7RzUL2VZtBtQC5uLz2C6zS7/qSRzjefhyQ0mgnW61Eu0rKLz2vUCnWg9FlQsrRA bUZHnnezF1IxRKt7F+89mOSHLG0hIBrpH54N1enNyIKPFaHz4sX5Dhg2N7ya9BlyCKlr CAofVDrw+EifrURZLh+Fy9ZhVC7J5HmtslIH6F+WGeiccdqHt6oyl0cVHttQIcZCfBlz cMDZSw5ViHPWH/Q4DsXGbseaFl8utBqQzbnSSCbaiydO9TLz0CsJivK5GA5rXNjK9wM2 EjpQ== X-Gm-Message-State: APjAAAVxinyJNxDJPUwebmmCsMr+OSi4kKzKswIspHGuZr+PLlNTV6cI sUeNGeGReoaVssZN0JJQBJxZDg== X-Google-Smtp-Source: APXvYqwVlnkNNeWECRvv0LgzsnPJkLdTZl0ZYqkVgXeRvM69+DJG+xllmrD9YvrSDgTnYy8wdGHdNw== X-Received: by 2002:a17:902:168:: with SMTP id 95mr2414703plb.212.1553005763360; Tue, 19 Mar 2019 07:29:23 -0700 (PDT) Received: from kshutemo-mobl1.localdomain ([134.134.139.83]) by smtp.gmail.com with ESMTPSA id b7sm28545812pff.136.2019.03.19.07.29.22 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 19 Mar 2019 07:29:22 -0700 (PDT) Received: by kshutemo-mobl1.localdomain (Postfix, from userid 1000) id C09CC3011DA; Tue, 19 Mar 2019 17:29:18 +0300 (+03) Date: Tue, 19 Mar 2019 17:29:18 +0300 From: "Kirill A. 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 , Andrea Arcangeli Subject: Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions Message-ID: <20190319142918.6a5vom55aeojapjp@kshutemo-mobl1> 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: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -- Kirill A. Shutemov