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Howlett" , Vlastimil Babka , Jann Horn , Linux-MM , Kalesh Singh , SeongJae Park , Barry Song , Peter Xu References: 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: vHPt6tKkczpujYPB1oA03rd1K20UWJEsI_nJkFA3pz0_1757913464 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: rspam11 X-Rspamd-Queue-Id: EFEED100002 X-Stat-Signature: yxnm4fzarqwgjrczm81aqu4axb4b5qef X-Rspam-User: X-HE-Tag: 1757913467-530374 X-HE-Meta: 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 2noqD/rv gPSVxy2z0RqP1xcVOdeojWOKNKNzG5Jeulhzt4Las+l/sZ0ysLhQ85uZ3p1wyGQxUCMMqb17uw3Nf6g+AvzlhzCtENA== 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 15.09.25 04:50, Matthew Wilcox wrote: > On Mon, Sep 15, 2025 at 08:23:38AM +0800, Barry Song wrote: A couple of notes: >>> I wonder if we could fix this by adding a new syscall: >>> >>> mremap(addr, size, size, MREMAP_COW_NOW); >>> >>> That would create a new VMA that contains the COWed pages from the >>> old VMA, but crucially no longer attached to the anon_vma root of >>> the zygote. You wouldn't want to call this for every VMA, of course. >>> Just the ones which are likely to be fully COWed. MADV_POPULATE does that for writable vMAs (excluding the rmap opt, but that could likely be implemented). A student of mine implemented a MADV_UNSHARE that achieves the same by triggering unshare-faults even for non-writable VMAs (again, excluding the rmap opt). We used MADV_UNSHARE to break COW asynchronously to the already-running workload to keep fork() still short but avoid the overhead of COW faults later. [ insert usualy comment about no weird mremap flags ] >>> >>> Maybe this isn't practical, but I thought it worth suggesting. >> >> Lorenzo suggested possibly unlinking the child anon_vma from the root once all >> folios have been CoW-ed: >> >> "Right now, even if you entirely CoW everything in a VMA, we are still >> attached to parents with all the overhead. That's something I can look at. >> " >> >> My concern is that it’s difficult to determine whether a VMA has been completely >> CoW-ed, and a single shared folio would prevent the unlink. >> So I’m not sure this approach would work. > > I'm concerned that tracking how many folios remain shared may be > inefficient. Also that information needs to be gathered in both parent > and child. Yeah, not a fan. Tracking per MM might work, tracking per VMA is problematic due to the possibility for VMA splits. > >> You seem to be proposing a forced CoW as a way to safely unlink from the root. >> >> A side effect is the potential for sudden, heavy memory allocation, >> whereas CoW lets asynchronous tasks such as kswap work concurrently. > > Perhaps you could help us out with some stats on that -- how much > anonymous memory starts out shared between the zygote and a newly > spawned process? > >> Another issue is the extra memory use from folios that could have been >> shared but aren’t—likely minor on Android, since only a small portion >> of memory is actually shared, based on our observations. >> >> Calling mremap for each VMA might be difficult. Something applied to the >> whole process could be more practical—similar to exec, but only >> performing CoW and unlinking the anon_vma root. > > That seems like it would be worse for memory consumption than doing it > on the VMAs in question. MADV_UNSHARE we implemented simply took a range and one could apply it to the full process by supplying the full range. But yeah, the downside in any case is that you lose > > Another possibility would be for the zygote to set a flag on the VMA, > say EAGER_COW which forces a COW of all pages as soon as the first one > is COWed. But then we're paying at fault time rather than in a syscall > that we can predict. Right, or just avoid COW altogether (if fork time is irrelevant) and just copy during fork(). Either using a clone flag for the whole MM or using a new MADV option copy during fork. > > Another point in favour of COW_NOW or EAGER_COW is that we can choose to > allocate folios of the appropriate size at that time. Unless something's > changed, I think we always COW individual pages rather than multiple > pages at once. Yes. khugepaged will soon starting fixing that up later asynchronously I hope. But obviously, whenever we copy/unshare, we consume more memory. While this might possibly work for Android, I know that some workloads (was it websevers or webbrowsers for example?) spin up many instances through fork() to actually keep sharing pages and not break COW. So I would hope we can find a better optimization that doesn't rely on the workload to manually break COW and effectively consume more memory. -- Cheers David / dhildenb