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[2003:d8:2f23:9c00:d1f6:f7fe:8f14:7e34]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45b9a6ecfafsm339135615e9.21.2025.09.09.04.50.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Sep 2025 04:50:52 -0700 (PDT) Message-ID: <655d2d29-8fe6-4684-aba4-4803bed0d4d0@redhat.com> Date: Tue, 9 Sep 2025 13:50:51 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/gup: Drain batched mlock folio processing before attempting migration To: Will Deacon , Hugh Dickins Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Keir Fraser , Jason Gunthorpe , John Hubbard , Frederick Mayle , Andrew Morton , Peter Xu , Rik van Riel , Vlastimil Babka , Ge Yang References: <20250815101858.24352-1-will@kernel.org> <9e7d31b9-1eaf-4599-ce42-b80c0c4bb25d@google.com> <8376d8a3-cc36-ae70-0fa8-427e9ca17b9b@google.com> <7ce169c2-09b7-39e3-d00b-ba1db6dd258c@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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: QPqgfnGMp6F2S46JRE0aT-c41SgspEBLlx4r0TCAgZk_1757418654 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: rspam11 X-Rspamd-Queue-Id: D4D0440002 X-Stat-Signature: 8c1deeitfxudu7x4uj1mcahj5yy7pfxb X-Rspam-User: X-HE-Tag: 1757418657-551999 X-HE-Meta: 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 R63qAFSG rrCqpS87fp4exY4pETA3GqNYAb0kfDchmg05hEHyjIL4i/3RL9DYyvFGF8pgJzDWJEt6mK8r6xeYGf8j4C+uUJPchdmR3QFuq79ypoLH5ZPJFkKIXbYb425cbpPy6n/kCBedxf8MmYqGJbvTHY5ahG8o2RMYTHJvYgawtzHufkxMLD37Wjk8pVyqBIXpkxaPj+KZGsx5MVVNBw2xB4LSCBk2URNeVSqITqLzlispGvp4bZi2teY0H1mVtWVtVk3rjjS2SM7zebNqZSz7sPSifGA9vjGZQ+oVxIWppw1dpDlsEPwfOXA2teGAXxJnzPFYQmU/OFNiZD91AxhI944bi+J38Os7bHxkW8+sp+tYXCe9TDpAI+KrCiYP+Rk+1mRDRmuNx3wzHmotHonAlB+vCjSGZwtjt25B+QDXoafR6QFRdNvdms9NZ99j23VYKSHV3sbvh 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 09.09.25 13:39, Will Deacon wrote: > On Fri, Aug 29, 2025 at 08:46:52AM -0700, Hugh Dickins wrote: >> On Fri, 29 Aug 2025, Will Deacon wrote: >>> On Thu, Aug 28, 2025 at 01:47:14AM -0700, Hugh Dickins wrote: >>>> diff --git a/mm/gup.c b/mm/gup.c >>>> index adffe663594d..9f7c87f504a9 100644 >>>> --- a/mm/gup.c >>>> +++ b/mm/gup.c >>>> @@ -2291,6 +2291,8 @@ static unsigned long collect_longterm_unpinnable_folios( >>>> struct folio *folio; >>>> long i = 0; >>>> >>>> + lru_add_drain(); >>>> + >>>> for (folio = pofs_get_folio(pofs, i); folio; >>>> folio = pofs_next_folio(folio, pofs, &i)) { >>>> >>>> @@ -2307,7 +2309,8 @@ static unsigned long collect_longterm_unpinnable_folios( >>>> continue; >>>> } >>>> >>>> - if (!folio_test_lru(folio) && drain_allow) { >>>> + if (drain_allow && folio_ref_count(folio) != >>>> + folio_expected_ref_count(folio) + 1) { >>>> lru_add_drain_all(); >>> >>> How does this synchronise with the folio being added to the mlock batch >>> on another CPU? >>> >>> need_mlock_drain(), which is what I think lru_add_drain_all() ends up >>> using to figure out which CPU batches to process, just looks at the >>> 'nr' field in the batch and I can't see anything in mlock_folio() to >>> ensure any ordering between adding the folio to the batch and >>> incrementing its refcount. >>> >>> Then again, my hack to use folio_test_mlocked() would have a similar >>> issue because the flag is set (albeit with barrier semantics) before >>> adding the folio to the batch, meaning the drain could miss the folio. >>> >>> I guess there's some higher-level synchronisation making this all work, >>> but it would be good to understand that as I can't see that >>> collect_longterm_unpinnable_folios() can rely on much other than the pin. >> >> No such strict synchronization: you've been misled if people have told >> you that this pinning migration stuff is deterministically successful: >> it's best effort - or will others on the Cc disagree? >> >> Just as there's no synchronization between the calculation inside >> folio_expected_ref_count() and the reading of folio's refcount. >> >> It wouldn't make sense for this unpinnable collection to anguish over >> such synchronization, when a moment later the migration is liable to >> fail (on occasion) for other transient reasons. All ending up reported >> as -ENOMEM apparently? that looks unhelpful. > > I see this was tangentially discussed with David on the patches you sent > and I agree that it's a distinct issue from what we're solving here, > however, -ENOMEM is a particularly problematic way to report transient > errors with migration due to a race. For KVM, the -ENOMEM will bubble > back up to userspace and the VMM is likely to destroy the VM altogether > whereas -EAGAIN would return back to the guest and retry the faulting > instruction. Migration code itself will retry multiple times, which usually takes care of most races. Not all of course. Now, I recall John was working on that at some point (I recall an RFC patch, but I might be daydreaming), and I recall discussions at LSF/MM around improving handling when we are mixing a flood of short-therm gup with a single long-term gup that wants to migrate these (short-term pinned) pages. Essentially, we would have to temporarily prevent new short-term GUP pins in order to make the long-term GUP-pin succeed in migrating the folio. -- Cheers David / dhildenb