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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Pankaj Raghav , Hannes Reinecke , Hugh Dickins , Yang Shi , Kiryl Shutsemau , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hengbin Zhang , stable@vger.kernel.org Subject: Re: [PATCH mm-hotfixes 2/2] mm/huge_memory: fix huge_zero_pfn race Message-ID: References: <20260728-fix-refcounted-huge-zero-v1-0-3f261f5447b4@kernel.org> <20260728-fix-refcounted-huge-zero-v1-2-3f261f5447b4@kernel.org> <25d560df-6d7e-462a-a987-60a8695793ba@kernel.org> <39b527d7-9c6e-4798-be9d-b834c8a3d2a0@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <39b527d7-9c6e-4798-be9d-b834c8a3d2a0@kernel.org> X-Stat-Signature: tws1jgkt1rhpcy7sq968x75q5ak3jwtu X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: B2EC5A0003 X-Rspam-User: X-HE-Tag: 1785416978-28059 X-HE-Meta: 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 kpvNASAi YvcDNn/029gnLQuQeCSis0CJ68P9WKSvpVoQIXDJcdf1L4/z4qjS4GASJzi/W6tGrv9XGjzZXf+92Sl8xS224OgNF0FmYx/WnBgvDrXxj4qLo4KMKtgApgtd7yEHADY+sO1U6cJlO3mN++a4vxdw7uYpVJUIKvH2e9IISG+NZJb4Y+JrvnzvJSENjE8ObJHO2yiYEmzTq93S1sGCTPKkgVMWqcwB7AVOyi6dimiJzuxwIxg6Rxe1Yc+x3D7wc2dueyIrz/y619tKLP4RKeOpzuTmIWB5frV0zHU0SU6S3d3FCiL8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jul 30, 2026 at 02:10:23PM +0200, David Hildenbrand (Arm) wrote: > On 7/30/26 11:55, Lorenzo Stoakes (ARM) wrote: > > On Thu, Jul 30, 2026 at 11:22:09AM +0200, David Hildenbrand (Arm) wrote: > >> [...] > >> > >>> So invariants are required - huge_zero_refcount MUST: > >>> > >>> * Only be set in the huge_zero_lock critical section to ensure > >>> serialisation of huge_zero_pfn, huge_zero_folio and huge_zero_refcount > >>> writes. > >>> > >>> * Be set non-zero only AFTER huge_zero_[pfn, folio] are set to valid values > >>> so installation of the huge zero folio on read page fault ensures > >>> concurrent is_huge_zero_*() calls correctly identify the huge zero folio. > >>> > >>> * Be set zero only BEFORE huge_zero_[pfn, folio] are set to NULL and ~0UL > >>> respectively, and atomically. > >>> > >>> Establish these by: > >>> > >>> * Only updating huge_zero_refcount in the huge_zero_lock critical section > >>> in get_huge_zero_folio() and shrink_huge_zero_folio_scan(). > >> > >> That is imprecise. huge_zero_refcount is updated (incremented) outside of > >> huge_zero_lock in get_huge_zero_folio(). > > > > You're right. > > > > -> > > > > * Only setting huge_zero_refcount to zero or an absolute value in the > > huge_zero_lock critical section in get_huge_zero_folio() and > > shrink_huge_zero_folio_scan() and updating atomically elsewhere. > > > > Seems better? > > Yes, I guess the transition from/to 0 are the problematic bits. Everything else > can just be lockless. Yeah exactly. Also setting an absolute value in general is potentially problematic. > > -- > Cheers, > > David Cheers, Lorenzo