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Tue, 5 Nov 2024 09:17:32 +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 ncV5BqziKWfdOQAAD6G6ig (envelope-from ); Tue, 05 Nov 2024 09:17:32 +0000 Message-ID: <606ae81e-3339-41eb-88a9-21ed6289a948@suse.cz> Date: Tue, 5 Nov 2024 10:17:31 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: remove unnecessary page_table_lock on stack expansion To: Lorenzo Stoakes , Andrew Morton Cc: "Liam R . Howlett" , Jann Horn , Linus Torvalds , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20241101184627.131391-1-lorenzo.stoakes@oracle.com> Content-Language: en-US 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: <20241101184627.131391-1-lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 22EB5C000E X-Stat-Signature: g7y9zp7tccdgxipdp96xqqc5efb5jjwb X-HE-Tag: 1730798213-264010 X-HE-Meta: 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 Jpgn/48n xy/iLvtB+ggue90cviVcn84ESnsXop36DiHwIUS6ISjSQmm6vQbqjaDpG6sazKu0baf1emLtUiUI+JUchMCo/1tJ6dhYoFb4Pzyu/vGoGVdU20yJPZOqI3J14IhWmZ1g9QxOO9ON5c8rt+ive38Qv4pJi5kPRO9fTMekOmxyNoDjLuP4X9mbucq4vnay0+TWBm8+nG3Q5Y6kXWd6BZlb7H0Z7uTgN2K9g+2zTqyBewMAokUefd60dM/7ACk4qplgf04cSJMuUC6kGAxUZBr5c6pAp9OliHKXGAgMVgwa0ABLZebEjAI5Vu4niRheBPb9YXVH+cHEygVB82ZlOkDkbr4+mJqMl8qqtTSm/ X-Bogosity: Ham, tests=bogofilter, spamicity=0.002182, 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 11/1/24 19:46, Lorenzo Stoakes wrote: > Ever since commit 8d7071af8907 ("mm: always expand the stack with the mmap > write lock held") we have been expanding the stack with the mmap write lock > held. > > This is true in all code paths: > > get_arg_page() > -> expand_downwards() > setup_arg_pages() > -> expand_stack_locked() > -> expand_downwards() / expand_upwards() > lock_mm_and_find_vma() > -> expand_stack_locked() > -> expand_downwards() / expand_upwards() > create_elf_tables() > -> find_extend_vma_locked() > -> expand_stack_locked() > expand_stack() > -> vma_expand_down() > -> expand_downwards() > expand_stack() > -> vma_expand_up() > -> expand_upwards() > > Each of which acquire the mmap write lock before doing so. Despite this, we > maintain code that acquires a page table lock in the expand_upwards() and > expand_downwards() code, stating that we hold a shared mmap lock and thus > this is necessary. > > It is not, we do not have to worry about concurrent VMA expansions so we > can simply drop this, and update comments accordingly. > > We do not even need be concerned with racing page faults, as > vma_start_write() is invoked in both cases. > > Signed-off-by: Lorenzo Stoakes Acked-by: Vlastimil Babka