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To: "wuyifeng (C)" , akpm@linux-foundation.org Cc: linux-mm@kvack.org References: <17ad24e5-9ee0-4d94-be5f-3c28bd57460a@huawei.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: <17ad24e5-9ee0-4d94-be5f-3c28bd57460a@huawei.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: jtZ3LLPrmf_xkHG-tiKqb6u3oTxqm60_Wupae78FdAY_1758531625 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 10ED880006 X-Stat-Signature: eyqr48s6gnxkfa6nc44iu7f9rzbaknio X-Rspam-User: X-HE-Tag: 1758531629-874096 X-HE-Meta: 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 dgVW5mql 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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 22.09.25 08:19, wuyifeng (C) wrote: > Hi all, While reviewing the memory management code, I noticed a > potential inefficiency related to MAP_POPULATE used on writable > anonymous mappings.I verified the behavior on the mainline kernel > and wanted to share it for discussion. > > Test Environment: > Kernel version: 6.17.0-rc4-00083-gb9a10f876409 > Architecture: aarch64 > > Background: > For anonymous mappings with PROT_WRITE | PROT_READ, using MAP_POPULATE > is intended to pre-fault pages, so that subsequent accesses do not > trigger page faults. However,I observed that when MAP_POPULATE is used > on writable anonymous mappings, all pre-faulted pages are immediately > marked as dirty, even though the user program has not written to them. > > Minimal Reproduction: > > #define _GNU_SOURCE > #include > #include > #include > > int main() { > size_t len = 100*1024*1024; // 100MB > void *p = mmap(NULL, len, PROT_READ | PROT_WRITE, > MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE, -1, 0); > if (p == MAP_FAILED) { > perror("mmap"); > return 1; > } > pause(); > return 0; > } > > Observed Output (/proc//smaps): > ffff7a600000-ffff80a00000 rw-p 00000000 00:00 0 > Size: 102400 kB > KernelPageSize: 4 kB > MMUPageSize: 4 kB > Rss: 102400 kB > Pss: 102400 kB > Pss_Dirty: 102400 kB > Shared_Clean: 0 kB > Shared_Dirty: 0 kB > Private_Clean: 0 kB > Private_Dirty: 102400 kB > Referenced: 102400 kB > Anonymous: 102400 kB > KSM: 0 kB > LazyFree: 0 kB > AnonHugePages: 102400 kB > ShmemPmdMapped: 0 kB > FilePmdMapped: 0 kB > Shared_Hugetlb: 0 kB > Private_Hugetlb: 0 kB > Swap: 0 kB > SwapPss: 0 kB > Locked: 0 kB > THPeligible: 1 > VmFlags: rd wr mr mw me ac > > Code Path Analysis: > The behavior can be traced through the following kernel code path: > populate_vma_page_range() is invoked to pre-fault pages for the VMA. > Inside it: > > if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE) > gup_flags |= FOLL_WRITE; > > This sets FOLL_WRITE for writable anonymous VMAs. > > Later, in faultin_page(): > > if (*flags & FOLL_WRITE) > fault_flags |= FAULT_FLAG_WRITE; > > This effectively marks the page fault as a write. > Finally, in do_anonymous_page(): > > if (vma->vm_flags & VM_WRITE) > entry = pte_mkwrite(pte_mkdirty(entry), vma); > Yes, as MAP_POPULATE ends up triggering ordinary write faults through GUP, this is expected. For write faults it makes perfect sense to set the pte dirty as well: avoids the cost of setting the pte dirty immediately afterwards (either through another fault or through the hw). MADV_POPULATE_WRITE has the same behavior, but it's even documented to behave like that: "Populate (prefault) page tables writable, faulting in all pages in the range just as if manually writing to each each page;" > Here, the PTE is updated to writable and immediately marked dirty. > As a result, all pre-faulted pages are marked dirty, even though the > user program has not performed any writes. > For large anonymous mappings, this can trigger unnecessary swap-out > writebacks, generating avoidable I/O. Is this a theoretical issue? Applications are supposed to make use of that memory after all, and at that point, the folios will be dirty. > > Discussion: > Would it be possible to optimize this behavior: for example, by > populate pte as writable, but deferring the dirty bit until the user > actually writes to the page? The only way I would see us changing that is by passing from GUP that this is not an ordinary write fault but a populate_write fault. We certainly don't want to affect other fault+GUP behavior where we can avoid the cost of setting the dirty bit immediately afterwards. But then, it could be counter-productive for workloads that will just write to that memory (IOW, use it). -- Cheers David / dhildenb