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Howlett" , Vlastimil Babka , Jann Horn , Linux-MM , Kalesh Singh , SeongJae Park , Barry Song , Peter Xu References: <18361483-5089-4414-b974-4c481189b9fa@redhat.com> <585b0ca3-cc56-4f74-9950-800d6faf8012@lucifer.local> 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: <585b0ca3-cc56-4f74-9950-800d6faf8012@lucifer.local> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: jCfJl9sg1wuvFiI06U4VyvbvCMS-Tci17s0CTVYbEOs_1757932193 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: EF14E1C0006 X-Rspamd-Server: rspam05 X-Stat-Signature: duenphq3t1eb3geeq94fse9q6gmwnm6k X-Rspam-User: X-HE-Tag: 1757932195-68587 X-HE-Meta: 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 tcQ1A3nX 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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 15.09.25 11:42, Lorenzo Stoakes wrote: > On Mon, Sep 15, 2025 at 07:17:41AM +0200, David Hildenbrand wrote: >> 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). > > I think the rmap bit is non-trivial as per my other replies. > >> >> 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 ] > > Yup > >> >>>>> >>>>> 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. > > Well, I think it's possible (maybe)... with a rework :>) > > "Lorenzo's grand rework" etc. etc. > >> >>> >>>> 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 > > You just lose? :P I assume you forgot to finish this thought :>) "you lose the memory savings of COW" -- was still tired there. > > I wonder from rmap point of view whether you could actually simply check to > see if you're fully CoW'd. > > E.g.: > > madvise(..., MADV_ISOLATE_COWED) > > And have it take the anon_vma write lock from root, have it walk the rmap, > go and check to see if every folio in the VMA is now CoW'd, and if so, > detatch the CoW'd anon_vma from its parent/root? TBH, this all feels like things we should be optimizing internally somehow. And don't get me started on MADV_ISOLATE_COWED eww NO_COWS eww COW NOW eww > > This would be a sort of after-the-fact thing, but maybe could be done > periodically. > > Of course then if you had one folio that was not yet CoW'd, that'd prevent > this from completing. [...] >>> >>> 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 only at mTHP granularity a. once the relevant series lands and b. if > mTHP is enabled (I mean for sub-PMD sized/aligned ranges) :>) Well, we need khugepaged in one form or the other form mTHP in any case :P And the glorious future will have all sizes enabled as default. -- Cheers David / dhildenb