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b=YCApkB98bjF2ENMhASp4r3DdYbm3aea8c+dCZWnQdvq0xs/ZmvhNepadolKrRhcZ0 NIUtKoAfeTZs1fe+1qoortKcNpBKzu8gXjnNCnS7LXgkcg8fxYWPgs32AVHEFrzi+1 Zz0Q6kuoYVmvrETQQa4KDh78vSxYOTu77KCUI6HRbaatuNztRwtRUGCsbKc8iBryNm Ih1jTPpgwnjc0tWpuQwcwdI+6k4HnoZj5vXqb7zXP6SmIhG936m2w65Eulg6WVjBOY eHSipTtxkECUllX0VWdVf7w2NXMxwsQESm7LCqgc3hn+Qo8t9yhljWzw4D3LhSmfXb 9XRAsN8cPq/NA== Message-ID: Date: Tue, 18 Aug 2026 09:30:40 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm: Use a folio in the softleaf_is_device_private path To: Hongfu Li Cc: akpm@linux-foundation.org, liam@infradead.org, lihongfu@kylinos.cn, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, mhocko@suse.com, rppt@kernel.org, surenb@google.com, vbabka@kernel.org References: <20260818021326.5910-1-hongfu.li@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260818021326.5910-1-hongfu.li@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 7239B1A0008 X-Rspamd-Server: rspam07 X-Stat-Signature: qot74fibbc8ahuifehpw6qdqjxirndwq X-HE-Tag: 1787038248-996752 X-HE-Meta: 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 ENnxqCA9 +DvEJ0vnrg4WDbI6Gg9FMKsjEIDlOYE962odX7GAhtLoG979onlE77Yha+VA4UAIfkQovViYyqnvfaBAHA8tZpkoNc0pulD6M55v0TzlxdobOH0bP0Lcn9srT9nww0bQU3Ue8m9UE1JH+cGSK9BSNDyQD49jpzCuq3t5l8N+q5MW06N+lmcT2FGKOeSQ6G95osQzPVvFw+I9XZHHlin/pM9J9WAxW9NzXjwJQLQe5lhbIv/Zfshi4WHqjEF1cfMBWw9vx7CwXbSwtkcMH0hwHzi5Wi5rV3e+fn4ZQOboMRH4MeMtDGtwqCFuAeg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/18/26 04:13, Hongfu Li wrote: >>> So after migrate_to_ram() the folio might have been split or otherwise somehow >>> the page doesn't belong to the same locked, refcount-incremented folio it did >>> before? >> >> I suspect a split. >> >>> >>> That's kinda a footgun... but this documents it at least. >>> >>> I think a comment explaining how this can happen would be helpful though as this >>> doesn't seem intuitive. >> >> I think, conceptually, calling into something that consumes a page (vmf->page) >> always needs care when operating on folios. >> >> Passing the vmf to some callback might be the odd thing here, because the >> vmf->page contract is not really clear. > > I'm very sorry for introducing this regression. > Thank you for identifying and testing this issue. > > I will double-check this code path and apply your fix to run further > tests locally. > > Would it make sense to add a comment here like the below, to make this > subtle behavior clearer for future readers? > > diff --git a/mm/memory.c b/mm/memory.c > index 4134ac607ee0..2b6d6c863ecb 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4937,6 +4937,12 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > pte_unmap_unlock(vmf->pte, vmf->ptl); > pgmap = page_pgmap(vmf->page); > ret = pgmap->ops->migrate_to_ram(vmf); > + /* > + * migrate_to_ram() can split a large folio, updating > + * vmf->page to a different folio. Re-fetch folio for > + * correct unlock/put. > + */ "migrate_to_ram() might have split the folio." Should be sufficient I guess. -- Cheers, David