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b=So7uUvuo7yvNGlGAev/jav+ILcYZxcD823dwNtJ+9zhK0OPIsY4oNfiY+kIBt0efO d7VumIIdvHEmhGJv8g5rt6tKrqcMc/sUogUrLer8bIj+Kmb4ZwJzsG+Xdc/PdHd+tS HyMdmicF36r4oMc0kXYcSANCNtsADigNUhrBAck2DFL0jMWbgVLrxDgKQ9TwtTJi2L lmN38lP+TJuj0U+twWc4COawUP3WyKlwI9oUkUZsRU4wzbBXlX2ee3j5aafaCIYy1r LhDho/cfbL8juFIEK/wBqsI1hgFkpcAJEyhI2EQm6yeR6hDwfu1sbcpcjlrvloCvSh iWKZlic1DVs1g== Message-ID: Date: Mon, 11 May 2026 08:48:08 +0200 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: Dev Jain , 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> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 7545D40003 X-Stat-Signature: 1qoyg6i5agot7a8yt8e3tif6cn9u46iu X-Rspam-User: X-HE-Tag: 1778482096-318417 X-HE-Meta: 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 GyJzQVDi UIvlMBue5Ijssc0gH0QVXjN7DEmamBdlDp1xp6+hqU/UtLWkP+vJ8bLcowbrx9bDxxaJEf5b25XqREqv4tsC9x/YJ8bBn5Pq3AmxkZzPpapkTPZkKimeYLYV+8G7uJ/KxEeb+t/iFd7m6Z5By2OEPGurWYJZTF8M/pR6QeQ67Ni3WvHLOaAx9mnzOTNNstnu5Rq9Jxmz4qLy9FJX8JkEIs+umWc26ATgc0WaPL/wbtkQ8YNDU0+q6ZMdHUhsRxWzORhJsI03+o+CSmlMyA6aTOxAJy5qfo+9RF83nrvOpFMFzfUn1lUBfWdrK/jYsWGfdXpdD+UrM06/nYQc= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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? -- Cheers, David