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[80.187.83.110]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46e723614c9sm145456425e9.14.2025.10.06.01.18.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 06 Oct 2025 01:18:38 -0700 (PDT) Message-ID: <9f3e4031-9e13-4f8b-a7fb-8db4166c47f0@redhat.com> Date: Mon, 6 Oct 2025 10:18:30 +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> <6e4f6a37-2449-4089-8b3d-234ba86878e2@redhat.com> <9b05b974-7478-4c99-9c4f-6593e0fd4f93@redhat.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: 8qPC2PmL9pcyFXr8Nycn3YqPCpexqUCkvU4-Yw_g31o_1759738719 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: 8BBE31C0004 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: ad9baype468zo7zyzc4ou9je49woqpcu X-HE-Tag: 1759738722-226143 X-HE-Meta: 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 gkjTrBDy 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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 02.10.25 16:07, Harry Yoo wrote: > On Wed, Sep 24, 2025 at 05:52:14PM +0200, David Hildenbrand wrote: >> On 24.09.25 13:54, Harry Yoo wrote: >>> On Tue, Sep 23, 2025 at 04:09:06PM +0200, David Hildenbrand wrote: >>>> 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: >>> In case is_swap_pmd() or pmd_trans_huge() returned true, but another >>> kernel thread splits THP after we checked it, __split_huge_pmd() or >>> change_huge_pmd() will just return without actually splitting or changing >>> pmd entry, if it turns out that evaluating >>> (is_swap_pmd() || pmd_trans_huge() || pmd_devmap()) as true >>> was false positive due to race condition, because they both double check >>> after acquiring pmd lock: >>> >>> 1) __split_huge_pmd() checks if it's either pmd_trans_huge(), pmd_devmap() >>> or is_pmd_migration_entry() under pmd lock. >>> >>> 2) change_huge_pmd() checks if it's either is_swap_pmd(), >>> pmd_trans_huge(), or pmd_devmap() under pmd lock. >>> >>> And if either function simply returns because it was not a THP, >>> pmd migration entry, or pmd devmap, khugepaged cannot colleapse >>> huge page because we're holding mmap_lock in read mode. >>> >>> And then we call change_pte_range() and that's safe. >>> >>>> After that, I'm not sure ... maybe we'll just retry >>> >>> Or as you mentioned, if we are misled into thinking it is not a THP, >>> PMD devmap, or swap PMD due to race condition, we'd end up going into >>> change_pte_range(). >>> >>>> or we'll accidentally try treating it as a PTE table. >>> >>> But then pmd_trans_unstable() check should prevent us from treating >>> it as PTE table (and we're still holding mmap_lock here). >>> In such case we don't retry but skip it instead. >>> >>>> Looks like >>>> pmd_trans_unstable()->pud_none_or_trans_huge_or_dev_or_clear_bad() would >>> >>> I think you mean >>> pmd_trans_unstable()->pmd_none_or_trans_huge_or_clear_bad()? >> >> Yes! >> >>> >>>> return "0" >>>> in case we hit migration entry? :/ >>> >>> pmd_none_or_trans_huge_or_clear_bad() open-coded is_swap_pmd(), as it >>> eventually checks !pmd_none() && !pmd_present() case. > > > Apologies for the late reply. > >> Ah, right, I missed the pmd_present() while skimming over this extremely >> horrible function. >> >> So pmd_trans_unstable()->pmd_none_or_trans_huge_or_clear_bad() would return >> "1" and make us retry. > > We don't retry in pre-6.5 kernels because retrying is a new behavior > after commit 670ddd8cdcbd1. :/ >>>>> 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 :) >>> >>> Yeah the code is not very straightforward... :/ >>> >>> But technically the diff that I pasted here should be enough to fix >>> this... or do you have any alternative approach in mind? >> >> Hopefully, I'm not convinced this code is not buggy, but at least regarding >> concurrent migration it should be fine with that. > > I've been thinking about this... > > Actually, it'll make more sense to open-code what pte_map_offset_lock() > does in the mainline: > > 1. do not remove the "bad pte" checks, because pte_offset_map() in pre-6.5 > kernels doesn't do the check for us unlike the mainline. > 2. check is_swap_pmd(), pmd_trans_huge(), pmd_devmap() without ptl, but > atomically. > 3. after acquiring ptl in change_pte_range(), check if pmd has changed > since step 1 and 2. if yes, retry (like mainline). if no, we're all good. > > What do you think? Only for -stable, right? Does not sound too wrong for me, but I would have to take a closer look at the end result! -- Cheers David / dhildenb