From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt1-f198.google.com (mail-qt1-f198.google.com [209.85.160.198]) by kanga.kvack.org (Postfix) with ESMTP id 8FF878E0002 for ; Tue, 15 Jan 2019 21:23:19 -0500 (EST) Received: by mail-qt1-f198.google.com with SMTP id w15so4332269qtk.19 for ; Tue, 15 Jan 2019 18:23:19 -0800 (PST) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id g11si6918592qth.320.2019.01.15.18.23.18 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 15 Jan 2019 18:23:18 -0800 (PST) Date: Tue, 15 Jan 2019 21:23:12 -0500 From: Jerome Glisse Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions Message-ID: <20190116022312.GJ3696@redhat.com> References: <20190114145447.GJ13316@quack2.suse.cz> <20190114172124.GA3702@redhat.com> <20190115080759.GC29524@quack2.suse.cz> <20190115171557.GB3696@redhat.com> <752839e6-6cb3-a6aa-94cb-63d3d4265934@nvidia.com> <20190115221205.GD3696@redhat.com> <99110c19-3168-f6a9-fbde-0a0e57f67279@nvidia.com> <20190116015610.GH3696@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Dan Williams Cc: John Hubbard , Jan Kara , Matthew Wilcox , Dave Chinner , John Hubbard , Andrew Morton , Linux MM , tom@talpey.com, Al Viro , benve@cisco.com, Christoph Hellwig , Christopher Lameter , "Dalessandro, Dennis" , Doug Ledford , Jason Gunthorpe , Michal Hocko , Mike Marciniszyn , rcampbell@nvidia.com, Linux Kernel Mailing List , linux-fsdevel On Tue, Jan 15, 2019 at 06:01:09PM -0800, Dan Williams wrote: > On Tue, Jan 15, 2019 at 5:56 PM Jerome Glisse wrote: > > On Tue, Jan 15, 2019 at 04:44:41PM -0800, John Hubbard wrote: > [..] > > To make it clear. > > > > Lock code: > > GUP() > > ... > > lock_page(page); > > if (PageWriteback(page)) { > > unlock_page(page); > > wait_stable_page(page); > > goto retry; > > } > > atomic_add(page->refcount, PAGE_PIN_BIAS); > > unlock_page(page); > > > > test_set_page_writeback() > > bool pinned = false; > > ... > > pinned = page_is_pin(page); // could be after TestSetPageWriteback > > TestSetPageWriteback(page); > > ... > > return pinned; > > > > Memory barrier: > > GUP() > > ... > > atomic_add(page->refcount, PAGE_PIN_BIAS); > > smp_mb(); > > if (PageWriteback(page)) { > > atomic_add(page->refcount, -PAGE_PIN_BIAS); > > wait_stable_page(page); > > goto retry; > > } > > > > test_set_page_writeback() > > bool pinned = false; > > ... > > TestSetPageWriteback(page); > > smp_wmb(); > > pinned = page_is_pin(page); > > ... > > return pinned; > > > > > > One is not more complex than the other. One can contend, the other > > will _never_ contend. > > The complexity is in the validation of lockless algorithms. It's > easier to reason about locks than barriers for the long term > maintainability of this code. I'm with Jan and John on wanting to > explore lock_page() before a barrier-based scheme. How is the above hard to validate ? Either GUP see racing test_set_page_writeback because it test write back after incrementing the refcount, or test_set_page_writeback sees GUP because it checks for pin after setting the write back bits. So if GUP see !PageWriteback() then test_set_page_writeback see page_pin(page) as true. If test_set_page_writeback sees page_pin(page) as false then GUP did see PageWriteback() as true. You _never_ have !PageWriteback() in GUP and !page_pin() in test_set_page_writeback() if they are both racing. This is an impossible scenario because of memory barrier. Cheers, Jérôme