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[70.53.122.15]) by smtp.gmail.com with ESMTPSA id f76sm11725245qke.19.2020.09.22.11.49.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Sep 2020 11:49:07 -0700 (PDT) Date: Tue, 22 Sep 2020 14:49:05 -0400 From: Peter Xu To: Oleg Nesterov Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jason Gunthorpe , Andrew Morton , Jan Kara , Michal Hocko , Kirill Tkhai , Kirill Shutemov , Hugh Dickins , Christoph Hellwig , Andrea Arcangeli , John Hubbard , Leon Romanovsky , Linus Torvalds , Jann Horn Subject: Re: [PATCH 3/5] mm: Rework return value for copy_one_pte() Message-ID: <20200922184905.GM19098@xz-x1> References: <20200921211744.24758-1-peterx@redhat.com> <20200921211744.24758-4-peterx@redhat.com> <20200922100840.GA11679@redhat.com> <20200922101815.GB11679@redhat.com> <20200922153612.GF19098@xz-x1> <20200922154845.GE11679@redhat.com> <20200922160330.GH19098@xz-x1> <20200922165354.GG11679@redhat.com> <20200922181306.GJ19098@xz-x1> <20200922182317.GH11679@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200922182317.GH11679@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 22, 2020 at 08:23:18PM +0200, Oleg Nesterov wrote: > On 09/22, Peter Xu wrote: > > > > On Tue, Sep 22, 2020 at 06:53:55PM +0200, Oleg Nesterov wrote: > > > On 09/22, Peter Xu wrote: > > > > > > > > On Tue, Sep 22, 2020 at 05:48:46PM +0200, Oleg Nesterov wrote: > > > > > > However since I didn't change this logic in this patch, it probably means this > > > > > > bug is also in the original code before this series... I'm thinking maybe I > > > > > > should prepare a standalone patch to clear the swp_entry_t and cc stable. > > > > > > > > > > Well, if copy_one_pte(src_pte) hits a swap entry and returns entry.val != 0, then > > > > > pte_none(*src_pte) is not possible after restart? This means that copy_one_pte() > > > > > will be called at least once. > > > > > > > > Note that we've released the page table locks, so afaict the old swp entry can > > > > be gone under us when we go back to the "do" loop... :) > > > > > > But how? > > > > > > I am just curious, I don't understand this code enough. > > > > Me neither. > > > > The point is I think we can't assume *src_pte will read the same if we have > > released the src_ptl in copy_pte_range(), > > This is clear. > > But I still think that !pte_none() -> pte_none() transition is not possible > under mmap_write_lock()... > > OK, let me repeat I don't understans these code paths enough, let me reword: > I don't see how this transition is possible. Though I guess I'll keep my wording, because I still think it's accurate to me. :) Can we e.g. punch a page hole without changing vmas? -- Peter Xu