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Tue, 25 Feb 2025 15:54:23 +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 Vp/CDK/nvWcfMwAAD6G6ig (envelope-from ); Tue, 25 Feb 2025 15:54:23 +0000 Message-ID: Date: Tue, 25 Feb 2025 16:54:22 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem mappings Content-Language: en-US To: Lorenzo Stoakes , David Hildenbrand Cc: Andrew Morton , Suren Baghdasaryan , "Liam R . Howlett" , Matthew Wilcox , "Paul E . McKenney" , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Shuah Khan , linux-kselftest@vger.kernel.org, linux-api@vger.kernel.org, John Hubbard , Juan Yescas , Kalesh Singh References: <8d643393-ddc0-490d-8fad-ad0b2720afb1@lucifer.local> <37b606be-f1ef-4abf-83ff-c1f34567568e@redhat.com> <0db666da-10d3-4b2c-9b33-781fb265343f@redhat.com> <62c0ba1c-7724-4033-b1de-d62a59751ca5@lucifer.local> <6eb33b5d-3040-4637-b627-48f8f78e4e28@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+fMgqZkBQJkBREIBQkRadznAAoJECJPp+fMgqZkNxIQ ALZRqwdUGzqL2aeSavbum/VF/+td+nZfuH0xeWiO2w8mG0+nPd5j9ujYeHcUP1edE7uQrjOC Gs9sm8+W1xYnbClMJTsXiAV88D2btFUdU1mCXURAL9wWZ8Jsmz5ZH2V6AUszvNezsS/VIT87 AmTtj31TLDGwdxaZTSYLwAOOOtyqafOEq+gJB30RxTRE3h3G1zpO7OM9K6ysLdAlwAGYWgJJ V4JqGsQ/lyEtxxFpUCjb5Pztp7cQxhlkil0oBYHkudiG8j1U3DG8iC6rnB4yJaLphKx57NuQ PIY0Bccg+r9gIQ4XeSK2PQhdXdy3UWBr913ZQ9AI2usid3s5vabo4iBvpJNFLgUmxFnr73SJ KsRh/2OBsg1XXF/wRQGBO9vRuJUAbnaIVcmGOUogdBVS9Sun/Sy4GNA++KtFZK95U7J417/J Hub2xV6Ehc7UGW6fIvIQmzJ3zaTEfuriU1P8ayfddrAgZb25JnOW7L1zdYL8rXiezOyYZ8Fm ZyXjzWdO0RpxcUEp6GsJr11Bc4F3aae9OZtwtLL/jxc7y6pUugB00PodgnQ6CMcfR/HjXlae h2VS3zl9+tQWHu6s1R58t5BuMS2FNA58wU/IazImc/ZQA+slDBfhRDGYlExjg19UXWe/gMcl De3P1kxYPgZdGE2eZpRLIbt+rYnqQKy8UxlszsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZAUSmwUJDK5EZgAKCRAiT6fnzIKmZOJGEACOKABgo9wJXsbWhGWYO7mD 8R8mUyJHqbvaz+yTLnvRwfe/VwafFfDMx5GYVYzMY9TWpA8psFTKTUIIQmx2scYsRBUwm5VI EurRWKqENcDRjyo+ol59j0FViYysjQQeobXBDDE31t5SBg++veI6tXfpco/UiKEsDswL1WAr tEAZaruo7254TyH+gydURl2wJuzo/aZ7Y7PpqaODbYv727Dvm5eX64HCyyAH0s6sOCyGF5/p eIhrOn24oBf67KtdAN3H9JoFNUVTYJc1VJU3R1JtVdgwEdr+NEciEfYl0O19VpLE/PZxP4wX PWnhf5WjdoNI1Xec+RcJ5p/pSel0jnvBX8L2cmniYnmI883NhtGZsEWj++wyKiS4NranDFlA HdDM3b4lUth1pTtABKQ1YuTvehj7EfoWD3bv9kuGZGPrAeFNiHPdOT7DaXKeHpW9homgtBxj 8aX/UkSvEGJKUEbFL9cVa5tzyialGkSiZJNkWgeHe+jEcfRT6pJZOJidSCdzvJpbdJmm+eED w9XOLH1IIWh7RURU7G1iOfEfmImFeC3cbbS73LQEFGe1urxvIH5K/7vX+FkNcr9ujwWuPE9b 1C2o4i/yZPLXIVy387EjA6GZMqvQUFuSTs/GeBcv0NjIQi8867H3uLjz+mQy63fAitsDwLmR EP+ylKVEKb0Q2A== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 35BC340023 X-Stat-Signature: r8jk34db1ikd6bt5si1qgtq6w9moumyg X-Rspam-User: X-HE-Tag: 1740498864-538009 X-HE-Meta: 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 xbg8zek2 JPZZSICFdCkaKc0/rz4sDiCadwLVSOPGxJOv3X0cQDex+yWGRTEmGwfZ1vuPEx25sWXs/XUGdtKhaUYGKwksPJ4XFL7KT17Vo13R9M0F+0bjpGozA8gVN3a+JBb2FsScoUpab3F4U3mRHgCaVv073Fdkixw+XGkKTTBJdl4tPX0q+fkQgrD9mfcHhrNGaparj0lhvXS/awiyYn8os1j37Tq2OhHj/U//S9PCyJyl8tfmpA8kwfsLs5cJyiGOSB8BcFjmyCCKYONKDVw8mYjrjxGASgwikmPZUAkFs X-Bogosity: Ham, tests=bogofilter, spamicity=0.000273, 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 2/18/25 18:28, Lorenzo Stoakes wrote: > On Tue, Feb 18, 2025 at 06:25:35PM +0100, David Hildenbrand wrote: >> >> > > > >> > > > It fails because it tries to 'touch' the memory, but 'touching' guard >> > > > region memory causes a segfault. This kind of breaks the idea of >> > > > mlock()'ing guard regions. >> > > > >> > > > I think adding workarounds to make this possible in any way is not really >> > > > worth it (and would probably be pretty gross). >> > > > >> > > > We already document that 'mlock()ing lightweight guard regions will fail' >> > > > as per man page so this is all in line with that. >> > > >> > > Right, and I claim that supporting VM_LOCKONFAULT might likely be as easy as >> > > allowing install/remove of guard regions when that flag is set. >> > >> > We already allow this flag! VM_LOCKED and VM_HUGETLB are the only flags we >> > disallow. >> >> >> See mlock2(); >> >> SYSCALL_DEFINE3(mlock2, unsigned long, start, size_t, len, int, flags) >> { >> vm_flags_t vm_flags = VM_LOCKED; >> >> if (flags & ~MLOCK_ONFAULT) >> return -EINVAL; >> >> if (flags & MLOCK_ONFAULT) >> vm_flags |= VM_LOCKONFAULT; >> >> return do_mlock(start, len, vm_flags); >> } >> >> >> VM_LOCKONFAULT always as VM_LOCKED set as well. > > OK cool, that makes sense. > > As with much kernel stuff, I knew this in the past. Then I forgot. Then I knew > again, then... :P if only somebody would write it down in a book... > > Yeah then that makes sense to check explicitly for (VM_LOCKED | VM_LOCKONFAULT) > in any MADV_GUARD_INSTALL_LOCKED variant as obviously this would be passively > excluded right now. Sorry for the late reply. So AFAIU from your conversations, guards can't be compatible with VM_LOCKED, which means e.g. any attempts of glibc to use guards for stacks will soon discover that mlockall() users exist and are broken by this, and the attempts will fail? That's a bummer. As for compatibility with VM_LOCKONFAULT, do we need a new MADV_GUARD_INSTALL_LOCKED or can we say MADV_GUARD_INSTALL is new enough that it can be just retrofitted (like you retrofit file backed mappings)? AFAIU the only risk would be breaking somebody that already relies on a failure for VM_LOCKONFAULT, and it's unlikely there's such a somebody now.