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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=epGTUN05kehIbYfaoSskXQTPl16giKeapCElsPK+msLNWWa83OCWnMf5P2VSFnDLB oTGvj8cVFncOae/jEbjCnJpkuVnO7ELjYAh1ibBVy54OlKFJC/DXnIa0PGS6ELUihA oVcub2cB++Nrclu4uAYeqHJ/NdI3M3MtuZw5cqJs= Message-ID: <06029485-9e85-4d2d-a324-abba918eecf3@arm.com> Date: Mon, 11 May 2026 13:48:39 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/9] mm/rmap: initialize nr_pages to 1 at loop start in try_to_unmap_one To: "David Hildenbrand (Arm)" , akpm@linux-foundation.org, ljs@kernel.org, hughd@google.com, chrisl@kernel.org, kasong@tencent.com Cc: riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, qi.zheng@linux.dev, shakeel.butt@linux.dev, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, rppt@kernel.org, surenb@google.com, mhocko@suse.com, baolin.wang@linux.alibaba.com, shikemeng@huaweicloud.com, nphamcs@gmail.com, bhe@redhat.com, youngjun.park@lge.com, pfalcato@suse.de, ryan.roberts@arm.com, anshuman.khandual@arm.com References: <20260506094504.2588857-1-dev.jain@arm.com> <20260506094504.2588857-2-dev.jain@arm.com> Content-Language: en-US From: Dev Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 3148820007 X-Stat-Signature: n5ceqdqu5w3on6iq75nuk9tnhbzm3ssz X-HE-Tag: 1778487534-111708 X-HE-Meta: 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 Ih1GNB1G w4eJoTFFo/u86QmU5DRwmgdONRfkGf5mp8kKZUlwS+Sa2ziqht/zeK2JXH4xWjoe6Gh0OsSbwvz1dEqGZMJZR81cNpLSH1L7kh7R6nctPtJ2YjMJnD534Vp8vGXzFxvh4UcqQ0bXFSmog/P8zDVibZPpwtPkPlq3ei2cet/kTeqqsI7qWdZpiYuMwIPZiqFweAsOcAMmfKMgXNX6pXrwDVpzl+FWDOyMAPSncXC5/5Ca7m84V80akmuA23IXnC2UsaYoYg4STW12GNkkDEMT/J36BT9U2QKNXQG044h849TI++uek5XV4x+z4Nw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 11/05/26 12:18 pm, David Hildenbrand (Arm) wrote: > On 5/6/26 11:44, Dev Jain wrote: >> Initialize nr_pages to 1 at the start of each loop iteration, like >> folio_referenced_one() does. >> >> Without this, nr_pages computed by a previous folio_unmap_pte_batch() call >> can be reused on a later iteration that does not run >> folio_unmap_pte_batch() again. >> >> I don’t think this is causing a bug today, but it is fragile. >> >> A real bug would require this sequence within the same try_to_unmap_one() >> call: >> >> 1. Hit the pte_present(pteval) branch and set nr_pages > 1. >> 2. Later hit the else branch and do pte_clear() for device-exclusive PTE, >> and execute rest of the code with nr_pages > 1. > > Right, for hugetlb folios it should always stay at 1. > >> >> Executing the above would imply a lazyfree folio is mapped by a mix of >> present PTEs and device-exclusive PTEs. > > Why lazyfree? We use nr_pages also for > > folio_remove_rmap_ptes(folio, subpage, nr_pages, vma); > > and > > folio_put_refs(folio, nr_pages); > > Given that make_device_exclusive() operates on individual PTEs, wouldn't it be > possible to trigger that? At the point of this patch, batching is supported for lazyfree and file folios. make_device_exclusive does not operate on file folios. > >