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b=mZIJVbXM4XB1a0Tz1IPi2hV9yfHM9XZLuK3X4kSgmZKoLw9I0IrLNWZDHe+a9xJq9w2YW+ TR77jarbUdO9STAA== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 5ABED134F4; Sun, 15 Nov 2026 21:59:40 +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 408tCkwr+mr9bgAAD6G6ig (envelope-from ); Sun, 15 Nov 2026 21:59:40 +0000 Message-ID: <743f60f7-d611-4d73-a2d1-d5873bcdda36@suse.de> Date: Sun, 15 Nov 2026 22:59:39 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 4/9] mm: keep writable entries writable on protection changes To: "David Hildenbrand (Arm)" , kvm@vger.kernel.org, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, linux-riscv@lists.infradead.org, seanjc@google.com, pbonzini@redhat.com, akpm@linux-foundation.org, peterx@redhat.com, ljs@kernel.org, imbrenda@linux.ibm.com, borntraeger@linux.ibm.com, frankja@linux.ibm.com, Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Vlastimil Babka , Jann Horn , Pedro Falcato References: <20261112142039.907090-2-clopez@suse.de> <20261112142039.907090-6-clopez@suse.de> <603cd8e0-c2c5-4c1e-85c4-d0a5ceda8d4c@kernel.org> From: =?UTF-8?Q?Carlos_L=C3=B3pez?= Content-Language: en-US In-Reply-To: <603cd8e0-c2c5-4c1e-85c4-d0a5ceda8d4c@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 1ED7B1F449 X-Spam-Level: X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Action: no action X-Spam-Flag: NO X-Spamd-Result: default: False [-13.50 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; XM_UA_NO_VERSION(0.01)[]; FROM_HAS_DN(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; URIBL_BLOCKED(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:dkim]; ARC_NA(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RCPT_COUNT_TWELVE(0.00)[27]; MIME_TRACE(0.00)[0:+]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_TRACE(0.00)[suse.de:+]; DNSWL_BLOCKED(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; R_RATELIMIT(0.00)[to_ip_from(RL3zg8z7sygsujbub1yh4z3pr1)]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:dkim] X-Spam-Score: -13.50 On 11/13/26 7:56 PM, David Hildenbrand (Arm) wrote: > On 11/12/26 15:20, Carlos López wrote: >> change_protection() applies vma->vm_page_prot, which lacks the write bit >> for CoW and write-notified mappings. MM_CP_TRY_CHANGE_WRITABLE makes >> entries writable again if a write fault would only set the write bit. >> In shared mappings, it relies on the dirty bit for this, as a clean >> entry might still need a write fault to notify the FS. >> >> However, this misses clean entries that were already writable. For >> example, a uffd-wp unprotect makes shmem PTEs read-only if they were >> mapped by a read fault before uffd-wp was registered, so the next write >> to each of them faults. > > See my other reply, adding such optimizations without any motivation why we > should even care is not really what we want. > > Drop it from your series and keep the MM changes to a bare minimum please. > > Or is there a real dependency for your work? > Yes there is, but I clearly did not explain it properly. The whole point of the series is to avoid zaps when no permission bits are removed by a protection change, e.g. mprotect(PROT_READ | PROT_WRITE) on a read-only VMA. My proposal is for change_protection() to signal this is the case to MMU notifiers with a new flag. However, change_protection() sometimes write-protects entries that were writable, which makes the flag wrong, so this patch fixes that. I'll be sure to explain this properly.