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[80.187.83.189]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fa9d7f91esm72241855e9.20.2025.10.09.01.19.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 Oct 2025 01:19:34 -0700 (PDT) Message-ID: Date: Thu, 9 Oct 2025 10:19:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Bug: Performance regression in 1013af4f585f: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race From: David Hildenbrand To: Jann Horn , "Uschakow, Stanislav" Cc: "linux-mm@kvack.org" , "trix@redhat.com" , "ndesaulniers@google.com" , "nathan@kernel.org" , "akpm@linux-foundation.org" , "muchun.song@linux.dev" , "mike.kravetz@oracle.com" , "lorenzo.stoakes@oracle.com" , "liam.howlett@oracle.com" , "osalvador@suse.de" , "vbabka@suse.cz" , "stable@vger.kernel.org" References: <4d3878531c76479d9f8ca9789dc6485d@amazon.de> 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: GMCzfJYiReO6_hgojYIUPLrVPg3MIgI9twDdL9TyVg8_1759997976 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: A86E3A0004 X-Stat-Signature: ta9d437h1ftwas85szen9148rta1uz16 X-Rspam-User: X-HE-Tag: 1759997978-203841 X-HE-Meta: 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 5mpCDX0w 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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 09.10.25 09:40, David Hildenbrand wrote: > On 01.09.25 12:58, Jann Horn wrote: >> Hi! >> >> On Fri, Aug 29, 2025 at 4:30 PM Uschakow, Stanislav wrote: >>> We have observed a huge latency increase using `fork()` after ingesting the CVE-2025-38085 fix which leads to the commit `1013af4f585f: mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race`. On large machines with 1.5TB of memory with 196 cores, we identified mmapping of 1.2TB of shared memory and forking itself dozens or hundreds of times we see a increase of execution times of a factor of 4. The reproducer is at the end of the email. >> >> Yeah, every 1G virtual address range you unshare on unmap will do an >> extra synchronous IPI broadcast to all CPU cores, so it's not very >> surprising that doing this would be a bit slow on a machine with 196 >> cores. >> >>> My observation/assumption is: >>> >>> each child touches 100 random pages and despawns >>> on each despawn `huge_pmd_unshare()` is called >>> each call to `huge_pmd_unshare()` syncrhonizes all threads using `tlb_remove_table_sync_one()` leading to the regression >> >> Yeah, makes sense that that'd be slow. >> >> There are probably several ways this could be optimized - like maybe >> changing tlb_remove_table_sync_one() to rely on the MM's cpumask >> (though that would require thinking about whether this interacts with >> remote MM access somehow), or batching the refcount drops for hugetlb >> shared page tables through something like struct mmu_gather, or doing >> something special for the unmap path, or changing the semantics of >> hugetlb page tables such that they can never turn into normal page >> tables again. However, I'm not planning to work on optimizing this. > > I'm currently looking at the fix and what sticks out is "Fix it with an > explicit broadcast IPI through tlb_remove_table_sync_one()". > > (I don't understand how the page table can be used for "normal, > non-hugetlb". I could only see how it is used for the remaining user for > hugetlb stuff, but that's different question) > > How does the fix work when an architecture does not issue IPIs for TLB > shootdown? To handle gup-fast on these architectures, we use RCU. > > So I'm wondering whether we use RCU somehow. > > But note that in gup_fast_pte_range(), we are validating whether the PMD > changed: > > if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) || > unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) { > gup_put_folio(folio, 1, flags); > goto pte_unmap; > } > > > So in case the page table got reused in the meantime, we should just > back off and be fine, right? Wrong page table level. We'd have to check when processing a PMD leave whether the PUD changed as well. -- Cheers David / dhildenb