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To: Pedro Falcato Cc: "wuyifeng (C)" , akpm@linux-foundation.org, 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: iUmqd1EiBNKli4JD1UI5onODJsa6bEqGAh8BqeZWiDY_1758545206 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: 42F1E120007 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: qgmwdyj3754rowge1ccz9mp3qgn1cjfe X-HE-Tag: 1758545211-936992 X-HE-Meta: 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 69SI5U3E 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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 11:37, Pedro Falcato wrote: > On Mon, Sep 22, 2025 at 11:07:43AM +0200, David Hildenbrand wrote: >> On 22.09.25 10:45, Pedro Falcato wrote: >>> On Mon, Sep 22, 2025 at 02:19:51PM +0800, 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); >>>> >>>> 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. >>>> >>>> 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? >>> >>> How would we know if the user wrote to the page, since we marked it writeable? >> >> On access, either HW sets the dirty bit if it supports it, or we get another >> fault and set the dirty bit manually. >> >> What happens on architectures where the HW doesn't support setting the dirty >> bit is that performing a pte_mkwrite() checks whether the pte is dirty. If >> it's not dirty the HW write bit will not be set and instead the next >> pte_mkdirty() will set the actual HW write bit. >> >> See pte_mkwrite() handling in arch/sparc/include/asm/pgtable_64.h or >> arch/s390/include/asm/pgtable.h >> >> Of course, setting the dirty bit either way on later access comes with a >> price. > > Ah, yes, the details were a little fuzzy in my head, thanks. > I'm trying to swap in (ha!) the details again. We still proactively mark anon > folios dirty anyway for $reasons, so optimizing it might be difficult? Not sure > if it is _worth_ optimizing for anyway. I remembered the same thing (proactively mark anon folios dirty) but I didn't easily spot it in the code. Did you spot it? I only found the folio_mark_dirty() calls when unmapping anon pages and we stumble over a dirty pte. -- Cheers David / dhildenb