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,USER_AGENT_NEOMUTT autolearn=ham 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 E36C3C43381 for ; Wed, 20 Mar 2019 09:28:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A0A6721841 for ; Wed, 20 Mar 2019 09:28:44 +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="EfTOEFcH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726686AbfCTJ2n (ORCPT ); Wed, 20 Mar 2019 05:28:43 -0400 Received: from mail-pf1-f195.google.com ([209.85.210.195]:36450 "EHLO mail-pf1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726504AbfCTJ2n (ORCPT ); Wed, 20 Mar 2019 05:28:43 -0400 Received: by mail-pf1-f195.google.com with SMTP id p10so1512051pff.3 for ; Wed, 20 Mar 2019 02:28:42 -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=PMKD8S5T9yArnDmdhpqN5MR0f4dumLtOhH5vwozX4IQ=; b=EfTOEFcH5cnj4doeFG9zxcqsWx8ydGaJhsKvypWgceFUIkZFyY9CLnRWTuxRLp1+ZS 52khN2+/SXpeLN+uzhC8amB1KBLKmuL+A7F7NA3Tz5/o4x0mtKx0BnZ0QqPlzR43PN9R 8f84HPNQDXtodyytZx/1A+4+Dcge4L78IVeSvcTfDRLBT+LvzT/nUNPjtzTh+drtfdg2 B5Ycd2HGe8iPDKPew7MwVqh9UdCQ/rv8b/VBFDdh+Q6X5uPMpK1dY1DTc82hXg+lgkvC hK+ug40jMozhArURfK4G7FT7DBI4sFcFMUUfwPnKWrv3o0Omq/FksIP5NnnfzsyxYpoe 7XMQ== 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=PMKD8S5T9yArnDmdhpqN5MR0f4dumLtOhH5vwozX4IQ=; b=hs8W2pBK43djKZPw9bH8ZcmQVQN/BaitqBFIeFUn7fimRCEkzD/QFnvICUQNdNx4+X ap+Iz5/llqoQ2GdCHugezHSX6bLC40AfAXyv8UhHrnYcZUbOYD/CaPyKQxUMNx4d1hD3 aeyiHEAR+/yKM61cm4tXP3mmDOS9fsVFblukzjqQs+dKn06n6GDqOWsA5L8svL6smDXP kjUkeBP5sO3ch+zWp49F83PUH1HUaBxXlqfCQ2KQ1Vd0+Zkn3/6Ca1vhfjLhOI+nhBFE m+wHKVkigr7TGKhxE9BjcNog9+O8nGAI3ZZicyy7gu/rkC9svQNirfPmGHEwTb+jQ2AQ 3Nmg== X-Gm-Message-State: APjAAAXNIbvIndBuKGOIVoT1ViZX3UMxyef03OElNx0eB0R6P6/kkHNV WMkuljAPgEArLdYhTD7AL5rNIA== X-Google-Smtp-Source: APXvYqxSqLim0h0p/+px6G2tqdWX71LosiVLrLstelYvy1G6zCFngK8HljidHMKL3LjJ+hzG22N7Gw== X-Received: by 2002:a17:902:b481:: with SMTP id y1mr3762966plr.338.1553074121866; Wed, 20 Mar 2019 02:28:41 -0700 (PDT) Received: from kshutemo-mobl1.localdomain ([134.134.139.82]) by smtp.gmail.com with ESMTPSA id e123sm2034061pfe.35.2019.03.20.02.28.40 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 20 Mar 2019 02:28:40 -0700 (PDT) Received: by kshutemo-mobl1.localdomain (Postfix, from userid 1000) id 783A13011DB; Wed, 20 Mar 2019 12:28:36 +0300 (+03) Date: Wed, 20 Mar 2019 12:28:36 +0300 From: "Kirill A. Shutemov" To: John Hubbard Cc: Jerome Glisse , 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 Subject: Re: [PATCH v4 1/1] mm: introduce put_user_page*(), placeholder versions Message-ID: <20190320092836.rbc3fscxxpibgt3m@kshutemo-mobl1> 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> <6aa32cca-d97a-a3e5-b998-c67d0a6cc52a@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6aa32cca-d97a-a3e5-b998-c67d0a6cc52a@nvidia.com> User-Agent: NeoMutt/20180716 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 01:01:01PM -0700, John Hubbard wrote: > On 3/19/19 7:06 AM, 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? > > How's this read to you? I reworded and slightly expanded Jerome's > description: > > diff --git a/mm/gup.c b/mm/gup.c > index d1df7b8ba973..86397ae23922 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -61,6 +61,24 @@ static void __put_user_pages_dirty(struct page **pages, > for (index = 0; index < npages; index++) { > struct page *page = compound_head(pages[index]); > > + /* > + * Checking PageDirty at this point may race with > + * clear_page_dirty_for_io(), but that's OK. Two key cases: > + * > + * 1) This code sees the page as already dirty, so it skips > + * the call to sdf(). That could happen because > + * clear_page_dirty_for_io() called page_mkclean(), > + * followed by set_page_dirty(). However, now the page is > + * going to get written back, which meets the original > + * intention of setting it dirty, so all is well: > + * clear_page_dirty_for_io() goes on to call > + * TestClearPageDirty(), and write the page back. > + * > + * 2) This code sees the page as clean, so it calls sdf(). > + * The page stays dirty, despite being written back, so it > + * gets written back again in the next writeback cycle. > + * This is harmless. > + */ > if (!PageDirty(page)) > sdf(page); Looks good to me. Other nit: effectively the same type of callback called 'spd' in set_page_dirty(). Should we rename 'sdf' to 'sdp' here too? > >>>> +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. > > > > Here's a comment for that: > > @@ -127,6 +145,11 @@ void put_user_pages(struct page **pages, unsigned long npages) > { > unsigned long index; > > + /* > + * TODO: this can be optimized for huge pages: if a series of pages is > + * physically contiguous and part of the same compound page, then a Comound pages are always physically contiguous. I initially ment that the optimization makes sense if they are next to each other in 'pages' array. > + * single operation to the head page should suffice. > + */ > for (index = 0; index < npages; index++) > put_user_page(pages[index]); > } > > > thanks, > -- > John Hubbard > NVIDIA -- Kirill A. Shutemov