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b=KzJJPHAJIISUt1FdYf58bAeIYkOcE1zUWlLODgXPpZ6n6rAbaIvb9XzZ/frk9NDQr NhEuLaZtWyHIvNgDA9kqn+SHDn/nwwoiBzH8m8AUG1XEDBC+Bl3vQXuOMP9NyNT8TK 1GHQBbCu5hM2bu3P3pFrUuIM7p716NVAWcw2nr7tmWZqoPVCaF+5evQYvWoRIWlnrj yuohzEkfN9T1OO/tdBLZIjTLDNP41t1MW+VChRB6QEuQlYBVPV5B/BDTc8bN6Czaqr K//s6/0fpGmC+qtOz2RKiWfqAAIcxm0Wk8JALWOzlXEmoUQRdAGmzZRwa/jeF1mnhs mTnOlDeXqlCPQ== Message-ID: <3249f38f-b1e4-4483-b4c2-156e184175d5@kernel.org> Date: Mon, 18 May 2026 10:07:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] 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 Cc: riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, stable@vger.kernel.org References: <20260518063656.3721056-1-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: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 1BC1740005 X-Rspamd-Server: rspam04 X-Stat-Signature: b6j85gam6gm6bqkahpi9qzg7kb7euing X-HE-Tag: 1779091674-579440 X-HE-Meta: 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 WYAdfGjl U4etK6kc6A5cjKySnE/axymzLH4jd0NZIfNWoB+qhuAVvz7984AvaeahVumcymLYetMIiVcrZhUJljf15qWTNi/v1LczXT+mkzD+B96llIrO249/7lGetHDSl/AskKYkmgrJ1ERmzl1f0V4LbYN07x5UY2En7OtDCaB1L205VMI2zv2se7dBWdhp+gZIDtO1OKeCR6A4R6L8er2OZg4+WoIwNZ4AlfQFX+12zT4q8L31jZJK5fP/q8j6aRX/h70je3AYiLJIfzA+Og9aqfuI1IeXyNrKaFzhp01V0/daF6aiF1ItnQMDI/n9QIQqNGu2kOFzhG2jACNQD43hVeLjL1myBy1nUAj5BodkT1+/VjvjuuG5dLGUYw1urZ3FuB7F7mVSI44Hj0/nhE24= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/18/26 08:36, 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. > > mmap a 64K large folio with MAP_ANONYMOUS | MAP_DROPPABLE, then call > madvise(MADV_FREE), then make the last page device-exclusive via > HMM_DMIRROR_EXCLUSIVE. > > Trigger node reclaim through sysfs. Now, in try_to_unmap_one(), we will > first clear the first 15 out of 16 entries mapping the lazyfree folio. > This will set nr_pages to 15. In the next pvmw walk, this nr_pages gets > reused on a device-exclusive pte, thus potentially corrupting folio > refcount/mapcount. > > At the moment, I have a userspace program which can make the kernel spit > out a trace, but the blow up is in folio_referenced_one(), because there > are existing bugs in the interaction between device-private and rmap > (which too I am investigating). I did a one liner kernel change to avoid > going into folio_referenced_one(), and the kernel blows up at > folio_remove_rmap_ptes in try_to_unmap_one which is what I wanted. > > Note that the bug is there not since file folio batching but lazyfree folio > batching, since device-exclusive only works for anonymous folios. > > Userspace visible effect is simply kernel crashing somewhere due to > refcount/mapcount corruption. > > Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation") > Cc: stable@vger.kernel.org > Signed-off-by: Dev Jain > Acked-by: Barry Song > --- Acked-by: David Hildenbrand (Arm) Thanks! -- Cheers, David