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[209.85.128.50]) by smtp.gmail.com with ESMTPSA id k21sm3902678edo.87.2021.12.17.13.50.35 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Dec 2021 13:50:35 -0800 (PST) Received: by mail-wm1-f50.google.com with SMTP id a203-20020a1c7fd4000000b003457874263aso2707820wmd.2 for ; Fri, 17 Dec 2021 13:50:35 -0800 (PST) X-Received: by 2002:a7b:cb17:: with SMTP id u23mr11341018wmj.155.1639777824159; Fri, 17 Dec 2021 13:50:24 -0800 (PST) MIME-Version: 1.0 References: <20211217113049.23850-1-david@redhat.com> <20211217113049.23850-7-david@redhat.com> <9c3ba92e-9e36-75a9-9572-a08694048c1d@redhat.com> <02cf4dcf-74e8-9cbd-ffbf-8888f18a9e8a@redhat.com> In-Reply-To: <02cf4dcf-74e8-9cbd-ffbf-8888f18a9e8a@redhat.com> From: Linus Torvalds Date: Fri, 17 Dec 2021 13:50:08 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb) To: David Hildenbrand Cc: Linux Kernel Mailing List , Andrew Morton , Hugh Dickins , David Rientjes , Shakeel Butt , John Hubbard , Jason Gunthorpe , Mike Kravetz , Mike Rapoport , Yang Shi , "Kirill A . Shutemov" , Matthew Wilcox , Vlastimil Babka , Jann Horn , Michal Hocko , Nadav Amit , Rik van Riel , Roman Gushchin , Andrea Arcangeli , Peter Xu , Donald Dutile , Christoph Hellwig , Oleg Nesterov , Jan Kara , Linux-MM , "open list:KERNEL SELFTEST FRAMEWORK" , "open list:DOCUMENTATION" Content-Type: text/plain; charset="UTF-8" Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=google header.b="LTaR9/gI"; spf=pass (imf02.hostedemail.com: domain of torvalds@linuxfoundation.org designates 209.85.208.46 as permitted sender) smtp.mailfrom=torvalds@linuxfoundation.org; dmarc=none X-Rspamd-Queue-Id: 986B68003F X-Stat-Signature: 9he74kpkqajbfpptnw6j1t6yigz87eiu X-Rspamd-Server: rspam04 X-HE-Tag: 1639777834-530664 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Dec 17, 2021 at 1:47 PM David Hildenbrand wrote: > > For now I have not heard a compelling argument why the mapcount is > dubious, I repeat: > > * mapcount can only increase due to fork() > * mapcount can decrease due to unmap / zap > > We can protect from the transtition == 1 -> >1 using the mmap_lock. > > For COW the mapcount is the only thing that matters *if we take GUP* out > of the equation. And that's exactly what we What do you have against just doing what we already do in other parts, that a/b thing? Which avoids the whole mmap_sem issue. That was a big issue for the rdma people, afaik. Linus