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Tue, 28 Oct 2025 11:48:57 +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 wJN7FqmtAGlAegAAD6G6ig (envelope-from ); Tue, 28 Oct 2025 11:48:57 +0000 Date: Tue, 28 Oct 2025 11:48:51 +0000 From: Pedro Falcato To: Dev Jain Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, David Hildenbrand , Andrew Morton , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Barry Song , "open list:MEMORY MAPPING" Subject: Re: [PATCH] mm/mremap: Honour writable bit in mremap pte batching Message-ID: References: <20251028063952.90313-1-dev.jain@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251028063952.90313-1-dev.jain@arm.com> X-Rspam-User: X-Stat-Signature: wbfaxm8ojafxua8b6xn9w6b5maifcj9d X-Rspamd-Queue-Id: F062EC0008 X-Rspamd-Server: rspam09 X-HE-Tag: 1761652139-789436 X-HE-Meta: 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 YZ+tM5Dy 3iZ+dblocAygcQy/IHT1rSUHdcaGuFbruTdkIPupswLEkGI0rZEB2MGtLtFN1gOCymfDTSDDLXrVwV8cGnTbhPskRKtFAFVIyz1YWHK2aWGF5X0fqFr9Ls1Wh5Oau0ssQGG8QaFaac1gIzraMbGC5rHyVP94dP9M6m9EMewCj14ILk/pU0bj/xuh9cy+RAOD5PqBP71PWUJ5Gmdz/5Wc/qsvqb9H+B6/DYco9dodXuSXQKuvC4H+5k6nn6ckP6fCk+ox6yDlx7dYNcljKs5eeAeaHUSdZdXtAsEC83JMSCpaseE5htC3+i+no2AE5D2s8UoJJexReB0vzIDrHOK01LIe60uKHVQ2zL/hwn49SCcGqVrrmpJeRb8Sbl1jFouZWBb6fIs9boMjDRbSKtweie8zfkg== 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 Tue, Oct 28, 2025 at 12:09:52PM +0530, Dev Jain wrote: > Currently mremap folio pte batch ignores the writable bit during figuring > out a set of similar ptes mapping the same folio. Suppose that the first > pte of the batch is writable while the others are not - set_ptes will > end up setting the writable bit on the other ptes, which is a violation > of mremap semantics. Therefore, use FPB_RESPECT_WRITE to check the writable > bit while determining the pte batch. > Hmm, it seems to be like we're doing the wrong thing by default here? I must admit I haven't followed the contpte work as much as I would've liked, but it doesn't make much sense to me why FPB_RESPECT_WRITE would be an option you have to explicitly pass, and where folio_pte_batch (the "simple" interface) doesn't Just Do The Right Thing for naive callers. Auditing all callers: - khugepaged clears a variable number of ptes - memory.c clears a variable number of ptes - mempolicy.c grabs folios for migrations - mlock.c steps over nr_ptes - 1 ptes, speeding up traversal - mremap is borked since we're remapping nr_ptes ptes - rmap.c TTU unmaps nr_ptes ptes for a given folio so while the vast majority of callers don't seem to care, it would make sense that folio_pte_batch() works conservatively by default, and folio_pte_batch_flags() would allow for further batching (or maybe we would add a separate folio_pte_batch_clear() or folio_pte_batch_greedy() or whatnot). > Cc: stable@vger.kernel.org #6.17 > Fixes: f822a9a81a31 ("mm: optimize mremap() by PTE batching") > Reported-by: David Hildenbrand > Debugged-by: David Hildenbrand > Signed-off-by: Dev Jain But the solution itself looks okay to me. so, fwiw: Acked-by: Pedro Falcato -- Pedro