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[2003:d8:2f14:2400:afc:9797:137c:a25b]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-403ec628ff7sm7538809f8f.4.2025.09.24.03.06.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 24 Sep 2025 03:06:20 -0700 (PDT) Message-ID: <61880ecc-7618-4688-af84-02f31e058aea@redhat.com> Date: Wed, 24 Sep 2025 12:06:19 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm: always call rmap_walk() on locked folios To: Lokesh Gidra , akpm@linux-foundation.org Cc: linux-mm@kvack.org, kaleshsingh@google.com, ngeoffray@google.com, jannh@google.com, Lorenzo Stoakes , Harry Yoo , Peter Xu , Suren Baghdasaryan , Barry Song , SeongJae Park References: <20250923071019.775806-1-lokeshgidra@google.com> <20250923071019.775806-2-lokeshgidra@google.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: <20250923071019.775806-2-lokeshgidra@google.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: GRyX-GgpSR0iBT5bPy7GItD2dKP5JIRXCf2WUsiA0k8_1758708381 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: 7F456A0002 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: jokht5jnxm5rpfia1gngprnty83hr4xo X-HE-Tag: 1758708384-501059 X-HE-Meta: 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 9XdrmuGo y3kCIpirjFXBXbwHw+Ir1Z00SBJ182S8xw/lOQozSMhqfA26B35X03L2opGBZUGZVg/0BSXfEBu00kFz6GLlZNBjgZuZhiV5HmGGq1SSD4stLgqWDILYzzzR6Z/xMUsKuZJ7PYKKkr1w63SHiIzC1n5pC9tEiDFA3SEQ3nj55CxliTAoemLwz78qOjzUi9UQuAMTouYaDaA1nH2ZP6xJeZ11EaKwe1+n384apXmFHEWHBWkjTZ9nU+AUOiPp4SIqenX4gjgpj+1+Uomo9UwCHa6y46refjnRCQ+2Fu2XZ+XwQX84cERek6LfjDVBiJsS8rbkYsWo/dHBAhyb3ttqT7Dp7eCOaPXQoqwA41TME1RfjC9V3zsfZHw+rfdiVDP15IczcnKjlkHjNuZ30XG0g30BEOHkFR+e/vgTooUNnPT+uKfhfJLYrwo7fl/NEGi9W8yqcn6qMTuI1jLdGdZyI5gOaVHemBFVYJba4 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 23.09.25 09:10, Lokesh Gidra wrote: > Guarantee that rmap_walk() is called on locked folios so that threads > changing folio->mapping and folio->index for non-KSM anon folios can > serialize on fine-grained folio lock rather than anon_vma lock. Other > folio types are already always locked before rmap_walk(). With this, we > are going from 'not necessarily' locking the non-KSM anon folio to > 'definitely' locking it during rmap walks. > > This patch is in preparation for removing anon_vma write-lock from > UFFDIO_MOVE. > > With this patch, three functions are now expected to be called with a > locked folio. To be careful of not missing any case, here is the > exhaustive list of all their callers. > > 1) rmap_walk() is called from: > > a) folio_referenced() > b) damon_folio_mkold() > c) damon_folio_young() > d) page_idle_clear_pte_refs() > e) try_to_unmap() > f) try_to_migrate() > g) folio_mkclean() > h) remove_migration_ptes() > > In the above list, first 4 are changed in this patch to try-lock non-KSM > anon folios, similar to other types of folios. The remaining functions > in the list already hold folio lock when calling rmap_walk(). > > 2) folio_lock_anon_vma_read() is called from following functions: > > a) collect_procs_anon() > b) page_idle_clear_pte_refs() > c) damon_folio_mkold() > d) damon_folio_young() > e) folio_referenced() > f) try_to_unmap() > g) try_to_migrate() > > All the functions in above list, except collect_procs_anon(), are > covered by the rmap_walk() list above. For collect_procs_anon(), with > kill_procs_now() changed to take folio lock in this patch ensures that > all callers of folio_lock_anon_vma_read() now hold the lock. > > 3) folio_get_anon_vma() is called from following functions, all of which > already hold the folio lock: > > a) move_pages_huge_pmd() > b) __folio_split() > c) move_pages_ptes() > d) migrate_folio_unmap() > e) unmap_and_move_huge_page() > > Functionally, this patch doesn't break the logic because rmap walkers > generally do some other check to see if what is expected to mapped did > happen so it's fine, or otherwise treat things as best-effort. > > Among the 4 functions changed in this patch, folio_referenced() is the > only core-mm function, and is also frequently accessed. To assess the > impact of locking non-KSM anon folios in > shrink_active_list()->folio_referenced() path, we performed an app cycle > test on an arm64 android device. During the whole duration of the test > there were over 140k invocations of shrink_active_list(), out of which > over 29k had at least one non-KSM anon folio on which folio_referenced() > was called. In none of these invocations folio_trylock() failed. Yeah, that's the expectation: it should be a rare event. > > Of course, we now take a lock where we wouldn't previously have. In the > past it would have had a major impact in causing a CoW write fault to > copy a page in do_wp_page(), as commit 09854ba94c6a ("mm: do_wp_page() > simplification") caused a failure to obtain folio lock to result in a > page copy even if one wasn't necessary. > > However, since commit 6c287605fd56 ("mm: remember exclusively mapped > anonymous pages with PG_anon_exclusive"), and the introduction of the > folio anon exclusive flag, this issue is significantly mitigated. > > The only case remaining that we might worry about from this perspective > is that of read-only folios immediately after fork where the anon > exclusive bit will not have been set yet. > > We note however in the case of read-only just-forked folios that > wp_can_reuse_anon_folio() will notice the raised reference count > established by shrink_active_list() via isolate_lru_folios() and refuse > to reuse in any case, so this will in fact have no impact - the folio > lock is ultimately immaterial here. > > All-in-all it appears that there is little opportunity for meaningful > negative impact from this change. Yes, that's a good summary now, thanks a bunch! [...] > > static ssize_t page_idle_bitmap_read(struct file *file, struct kobject *kobj, > diff --git a/mm/rmap.c b/mm/rmap.c > index 0bc7cf8b7359..fd9f18670440 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -489,17 +489,15 @@ void __init anon_vma_init(void) > * if there is a mapcount, we can dereference the anon_vma after observing > * those. > * > - * NOTE: the caller should normally hold folio lock when calling this. If > - * not, the caller needs to double check the anon_vma didn't change after > - * taking the anon_vma lock for either read or write (UFFDIO_MOVE can modify it > - * concurrently without folio lock protection). See folio_lock_anon_vma_read() > - * which has already covered that, and comment above remap_pages(). > + * NOTE: the caller should hold folio lock when calling this. > */ > struct anon_vma *folio_get_anon_vma(const struct folio *folio) > { > struct anon_vma *anon_vma = NULL; > unsigned long anon_mapping; > > + VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio); > + As raised in v1, I am not sure about device-private dax folios that are anonymous where we could end up here through memory_failure_dev_pagemap() and not having the folio locked. If so we might have to throw in an actual folio lock somewhere on that path. -- Cheers David / dhildenb