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Thu, 30 Oct 2025 21:48:19 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id DvAFESPdA2mQSQAAD6G6ig (envelope-from ); Thu, 30 Oct 2025 21:48:19 +0000 Message-ID: Date: Thu, 30 Oct 2025 22:48:18 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/3] mm: introduce VM_MAYBE_GUARD and make visible for guard regions Content-Language: en-US To: Lorenzo Stoakes Cc: Pedro Falcato , Andrew Morton , Jonathan Corbet , David Hildenbrand , "Liam R . Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jann Horn , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Andrei Vagin , Barry Song <21cnbao@gmail.com> References: <7de40603015dee82970f5d37332a6d5af7532063.1761756437.git.lorenzo.stoakes@oracle.com> <61ae955e-310d-488e-b350-59bb809f06e1@lucifer.local> <053f3a04-9195-4f8d-8959-42e0c3ba077b@lucifer.local> <72ee2324-d599-44b6-92ce-ed0afafed78f@suse.cz> <3ae457cd-6c18-4870-a617-7f937b107cb4@suse.cz> <88b72728-fa3f-4a70-9ea2-40ff50673047@lucifer.local> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: <88b72728-fa3f-4a70-9ea2-40ff50673047@lucifer.local> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 5187D8000A X-Stat-Signature: n84c6ctfrj1yztkfkb3ac6m1dm3y8tpg X-Rspam-User: X-HE-Tag: 1761860902-625946 X-HE-Meta: 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 QUz6xMFm LmV26+Ywfjdrmw08MJ5xHxwxk51Z+ijvVYRifPNcMBCv4TcAOdJotq6zwiJtPpoTpdlPOtBvbEYL4JB9b/yTLEiohud/ttpHjmnO9SgFSczua1UHGbwXbOf0g4N2p9+wdVd+pB1jYYdiRvyfdWEaUtS59jg7yjjNYEkBPctv0Ji8/9Bt7n+HTena41C6dIDgWToicDTfUiXwmPixBRjvAbEtybUiMBlFIgGwMp8PjXUsRdiSlYGiwJ4MRYTUk7eqxmpw9aff5VbmwgNbJNTRNfsNkn2QuIm1VVM42/T2lJCvm8tjxLNsixs8mhHpdp3MyrKFwjLbw/ARuI0y0OhEKQRJELQ== 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: List-Subscribe: List-Unsubscribe: On 10/30/25 20:47, Lorenzo Stoakes wrote: > On Thu, Oct 30, 2025 at 07:47:34PM +0100, Vlastimil Babka wrote: >> > >> > Could we use MADVISE_VMA_READ_LOCK mode (would be actually an improvement >> > over the current MADVISE_MMAP_READ_LOCK), together with the atomic flag >> > setting? I think the places that could race with us to cause RMW use vma >> > write lock so that would be excluded. Fork AFAICS unfortunately doesn't (for >> > the oldmm) and it probably would't make sense to start doing it. Maybe we >> > could think of something to deal with this special case... >> >> During discussion with Pedro off-list I realized fork takes mmap lock for >> write on the old mm, so if we kept taking mmap sem for read, then vma lock >> for read in addition (which should be cheap enough, also we'd only need it >> in case VM_MAYBE_GUARD is not yet set), and set the flag atomicaly, perhaps >> that would cover all non-bening races? >> >> > > We take VMA write lock in dup_mmap() on each mpnt (old VMA). Ah yes I thought it was the new one. > We take the VMA write lock (vma_start_write()) for each mpnt. > > We then vm_area_dup() the mpnt to the new VMA before calling: > > copy_page_range() > -> vma_needs_copy() > > Which is where the check is done. > > So we are holding the VMA write lock, so a VMA read lock should suffice no? Yeah, even better! > For belts + braces we could atomically read the flag in vma_needs_copy(), > though note it's intended VM_COPY_ON_FORK could have more than one flag. > > We could drop that for now and be explicit. Great!