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(p200300d82f4f0700c9db579f8b2b717c.dip0.t-ipconnect.de. [2003:d8:2f4f:700:c9db:579f:8b2b:717c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46e0d08996csm62883165e9.11.2025.09.23.07.09.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Sep 2025 07:09:10 -0700 (PDT) Message-ID: <6e4f6a37-2449-4089-8b3d-234ba86878e2@redhat.com> Date: Tue, 23 Sep 2025 16:09:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [DISCUSSION] Fixing bad pmd due to a race condition between change_prot_numa() and THP migration in pre-6.5 kernels. To: Harry Yoo Cc: Andrew Morton , Lorenzo Stoakes , Zi Yan , Baolin Wang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Kiryl Shutsemau , Hugh Dickins , Vlastimil Babka , Jann Horn , Pedro Falcato , Jane Chu , linux-mm@kvack.org, stable@vger.kernel.org References: <20250921232709.1608699-1-harry.yoo@oracle.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: F6ay5J1SfRMVe7pFODMhyr1L-uKcTxQ2k2TdULQjfbM_1758636553 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: D658D20011 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: whkh6tif9myhemo1w94oj7w5qh7rmpg5 X-HE-Tag: 1758636556-91250 X-HE-Meta: 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 jf7gEIrU 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 23.09.25 13:46, Harry Yoo wrote: > On Tue, Sep 23, 2025 at 11:00:57AM +0200, David Hildenbrand wrote: >> On 22.09.25 01:27, Harry Yoo wrote: >>> Hi. This is supposed to be a patch, but I think it's worth discussing >>> how it should be backported to -stable, so I've labeled it as [DISCUSSION]. >>> >>> The bug described below was unintentionally fixed in v6.5 and not >>> backported to -stable. So technically I would need to use "Option 3" [A], >> >> What is option 3? > > Citing Option 3 from [A]: Ah, I see. >> Send the patch, after verifying that it follows the above rules, to >> stable@vger.kernel.org and mention the kernel versions you wish it to be >> applied to. When doing so, you must note the upstream commit ID in the >> changelog of your submission with a separate line above the commit text, >> >> like this: >> commit upstream. >> >> Or alternatively: >> [ Upstream commit ] >> >> If the submitted patch deviates from the original upstream patch >> (for example because it had to be adjusted for the older API), >> this must be very clearly documented and justified in the patch description. > >> Just to clarify: it's fine to do a backport of a commit >> even though it was not tagged as a fix. > > Thanks for looking into it, David! > > Ok, I was worried that the original patch's description will confuse > people because 1) we don't allow pte_map_offset_lock() to fail in older > kernels, which the original patch relies on, and 2) the patch does not > mention the race condition (because it fixed the race 'accidentaly' :D). > > I'll backport the original patch but make it clear that: > > 1. while the original patch did not mention the race condition, > the patch fixes a it, and add link to this discussion. > > 2. we can't remove 1) pmd_trans_unstable() check in change_pte_range(), > and 2) "bad" pmd check in change_pmd_range() because we don't allow > pte_offset_map_lock() to fail(). > > 3. pmd_read_atomic() is used instead of pmdp_get_lockless() beucase it > does not exist in older kernels. Right, and backporting all the prerequisites might not be feasible/desirable. [...] >>> goto next; >> >> This is all because we are trying to be smart and walking page tables >> without the page table lock held. This is just absolutely nasty. > > commit 175ad4f1e7a2 ("mm: mprotect: use pmd_trans_unstable instead of > taking the pmd_lock") did this :( Right. I can understand why we would not want to grab the lock when there is a leaf page table. But everything else is just asking for trouble (as we saw :) ). > >> What about the following check: >> >> if (is_swap_pmd(*pmd) || pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) { >> >> Couldn't we have a similar race there when we are concurrently migrating? > > An excellent point! I agree that there could be a similar race, > but with something other than "bad pmd" error. Right, instead we'd go into change_pte_range() where we check pmd_trans_unstable(). After that, I'm not sure ... maybe we'll just retry or we'll accidentally try treating it as a PTE table. Looks like pmd_trans_unstable()->pud_none_or_trans_huge_or_dev_or_clear_bad() would return "0" in case we hit migration entry? :/ > > It'd be more robust to do something like: That's also what I had in mind. But all this lockless stuff makes me a bit nervous :) -- Cheers David / dhildenb