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Thu, 6 Nov 2025 11:31:30 +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 S23WFRKHDGmQEgAAD6G6ig (envelope-from ); Thu, 06 Nov 2025 11:31:30 +0000 Message-ID: Date: Thu, 6 Nov 2025 12:31:29 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/5] mm: add atomic VMA flags, use VM_MAYBE_GUARD as such Content-Language: en-US To: Lorenzo Stoakes , Andrew Morton Cc: Jonathan Corbet , David Hildenbrand , "Liam R . Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jann Horn , Pedro Falcato , 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 References: <94935cf140e3279c234b39e0d976c4718c547c73.1762422915.git.lorenzo.stoakes@oracle.com> 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: <94935cf140e3279c234b39e0d976c4718c547c73.1762422915.git.lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 46CDD160014 X-Stat-Signature: j7pbr75aqb5xprz4jtmkafay71tdm5sh X-Rspam-User: X-HE-Tag: 1762428693-529087 X-HE-Meta: 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 31NEiFFq 9cPMwPxflwDjlhPtNVgtQZU6JviJQxFtTiZjPZmy0g594XOYvX3r5HOKAvJZyECNT9ZXjxu0nc6xEaunFdbMEOE3kZ3mKpqZHn3/b4pdZOSTBP2jS1eak/mYolsWG/xUhXhvnn6TRYzGG4ZkAwHGyhyK6/RfC+x+Jk2ECdHEcHG5iRmDAnLyryQlHszD6vT3wGPyWNxEQNtdxI+239I8QaNguMd9YNF7ueYg7Av+/e2O8Qv0MqJp+iGHmrzcSmmhKyFUbyesStHr/Fg6/UjsbZhIuB5n5NImaFyEKnPN2fd6IgcUY88VFLhewy7oahdllJCq/BsEsmdhR/rqiCUsfFjy5da0ZWX9xZ+SjNG31Caab1Dg= 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 11/6/25 11:46, Lorenzo Stoakes wrote: > This patch adds the ability to atomically set VMA flags with only the mmap > read/VMA read lock held. > > As this could be hugely problematic for VMA flags in general given that all > other accesses are non-atomic and serialised by the mmap/VMA locks, we > implement this with a strict allow-list - that is, only designated flags > are allowed to do this. > > We make VM_MAYBE_GUARD one of these flags, and then set it under the mmap > read flag upon guard region installation. > > The places where this flag is used currently and matter are: > > * VMA merge - performed under mmap/VMA write lock, therefore excluding > racing writes. > > * /proc/$pid/smaps - can race the write, however this isn't meaningful as > the flag write is performed at the point of the guard region being > established, and thus an smaps reader can't reasonably expect to avoid > races. Due to atomicity, a reader will observe either the flag being set > or not. Therefore consistency will be maintained. > > In all other cases the flag being set is irrelevant and atomicity > guarantees other flags will be read correctly. Could we maybe also spell out that we rely on the read mmap/VMA lock to exclude with writers that have write lock and then use non-atomic updates to update completely different flags than VM_MAYBE_GUARD? Those non-atomic updates could cause RMW races when only our side uses an atomic update, but the trick is that the read lock excludes with the write lock. > We additionally update madvise_guard_install() to ensure that > anon_vma_prepare() is set for anonymous VMAs to maintain consistency with > the assumption that any anonymous VMA with page tables will have an > anon_vma set, and any with an anon_vma unset will not have page tables > established. Could we more obviously say that we did anon_vma_prepare() unconditionally before this patch to trigger the page table copying in fork, but it's not needed anymore because fork now checks also VM_MAYBE_GUARD that we're setting here. Maybe it would be even more obvious to move that vma_needs_copy() hunk from previous patch to this one, but doesn't matter that much. Also we could mention that this patch alone will prevent merging of VMAs in some situations, but that's addressed next. I don't think it's such a bisect hazard to need reordering or combining changes, just mention perhaps. > Signed-off-by: Lorenzo Stoakes Otherwise LGTM. Reviewed-by: Vlastimil Babka