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[2003:d8:2f1a:7c00:4ac1:c2c4:4167:8a0f]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-395cb7eb8c2sm10072152f8f.85.2025.03.15.16.00.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 15 Mar 2025 16:00:10 -0700 (PDT) Message-ID: Date: Sun, 16 Mar 2025 00:00:09 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] block, fs: use FOLL_LONGTERM as gup_flags for direct IO To: John Hubbard , Christoph Hellwig , Matthew Wilcox , Jason Gunthorpe Cc: Sooyong Suk , viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, linux-mm@kvack.org, jaewon31.kim@gmail.com, spssyr@gmail.com, Zi Yan References: <20250306074056.246582-1-s.suk@samsung.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 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: i5WxXuWCmD1Nh78SYN3FQxnuAtk9fCVO6-fhR61yuA4_1742079612 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: C3E7D180004 X-Stat-Signature: ajwymcp8zt85bke5mnucjyxj7iqw64so X-HE-Tag: 1742079615-579894 X-HE-Meta: U2FsdGVkX19Xj8sfIzPsStFlYk3ir9bwA2zVRT7/Vk1VjfjznjgL0jEmDMZb0NGCEqa3y31Xd/lF9q4Yb7ukURxezh2g8SWuXLFqKo0ZOuLvT23YdT8Y62kEvwdip722zGWwYKs8sSA8JZGC7YJCEZmQlZBgqm9BJyUFxb8PhlfYZUUTRaJUrL+CZuD2ploEwrMeW9kZe6jLKx84c7HLLq5UGFtA/AftM3WTHWDBoCM3CQIFYz7PrnpAKyBLd7vOE3FW1VyrEfRa9iB7TJcgHOl8Va3KcU4Je5vKMUKT60PKB5qu9x0bjya0aNEcMiYCQrxpKOsfY5hynC8YkRRb8ANYTnzYWwT2JkbG7IGEqo9TlSqR+vNE/dzR6R9h+znnfWI+55h/tnF0F1lX3aRggeJLyFaziToJ8cQ+FWgA98Aygy9IH/mLhx+Fy0SjH449JZdybf1iJgxoCE2+pVwqXPrw/RC4xHHpjkC2MJ0GX+qrbsv1iwm45QiA2eKalRDj1UFQWmQZKXoKovBfe5v7TnH9AIlQLzUNd5lsdGqGG7+OLkau8GX3cPlBo6fKnUGHJTzpzSJudFc/w0qbc4tnHL2H9KdhTEF1bvERfptzERswgKJa75IGjrdhmMgSQrs5mmZHemiTeFqR/9O/TDaq+75q4njKigy4CA5GSxHz7ZH4hdFkDqQV+f7xsNpkqBWbFyQRFzj0teb5y/jU6r6d0+8rRiVDIl9Sak98aO6HAt4v/qSaHLLueMoqPYcnSj0V+YKmjSbFF8hgC0yDusvTVHSlBeNg5Z6AgYau9hFnUlUwcoG21w1wBtDxp7yVeJoUfUndCaPJvA5PlTMd/uvFe3klYGuIWxDLVEg0GPBGxwQyo/cFfh7mt34ZUeWXYASaWxP7XnPkb7DAMDb+xfG8WTNFS/CRbIahvlgeQyVqfGK5AiA7+B8xD42PkhFbwyts0X4eKfZrsvnIj7zbtmB 4loUw/WB SB86HeclyhWCOkcdbMQ4jdh+Mz+W7Ngv1st3iVCgLJgJh1obpP0ACIr7t3bpddEYCZlK6DGpQTjNSxkEBR4RZ4Y2XTnlD+8Ac8JLNGIFVzLqE7HsEq6hzVzuCxpWHZlGLUV8nmxvccmX84uMkzPStABS0EmqXCTYPGrkwsKJd80IlwcneRgEVNlHB9QzRzZWYl1+A 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 15.03.25 02:04, John Hubbard wrote: > On 3/13/25 3:49 PM, David Hildenbrand wrote: >> On 12.03.25 16:21, Christoph Hellwig wrote: >>> On Fri, Mar 07, 2025 at 08:23:08PM +0000, Matthew Wilcox wrote: >>>> Howver, the problem is real. >>> >>> What is the problem? >> >> I think the problem is the CMA allocation failure, not the latency. >> >> "if a large amount of direct IO is requested constantly, this can make >> pages in CMA pageblocks pinned and unable to migrate outside of the >> pageblock" >> >> We'd need a more reliable way to make CMA allocation -> page migration >> make progress. For example, after we isolated the pageblocks and >> migration starts doing its thing, we could disallow any further GUP >> pins. (e.g., make GUP spin or wait for migration to end) >> >> We could detect in GUP code that a folio is soon expected to be migrated >> by checking the pageblock (isolated) and/or whether the folio is locked. >> > > Jason Gunthorpe and Matthew both had some ideas about how to fix this [1], > which were very close (maybe the same) to what you're saying here: sleep > and spin in an killable loop. > > It turns out to be a little difficult to do this--I had trouble making > the folio's "has waiters" bit work for this, for example. And then...squirrel! > > However, I still believe, so far, this is the right approach. I'm just not > sure which thing to wait on, exactly. Zi Yan has a series to convert the "isolate" state of pageblocks to a separate pageblock bit; it could be considered a lock-bit. Currently, it's essentially the migratetype being MIGRATE_ISOLATE. As soon as a pageblock is isolated, one must be prepared for contained pages/folios to get migrated. The folio lock will only be grabbed once actually trying to migrate a folio IIRC, so it might not be the best choice: especially considering allocations that span many pageblocks. So maybe one would need a "has waiters" bit per pageblock, so relevant users (e.g., GUP) could wait on the isolate bit getting cleared. -- Cheers, David / dhildenb