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Shutemov" , Ryan Roberts Cc: Hugh Dickins , Mel Gorman , Linux-MM , Catalin Marinas , Matthew Wilcox References: <1cfae0c0-96a2-4308-9c62-f7a640520242@arm.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-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: E4E1218001F X-Stat-Signature: 5dr88ktqzaktd7txky1kykzhn7mxu54t X-HE-Tag: 1720102532-872534 X-HE-Meta: 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 9dutyI6R J9hMdBeL3cb6m/jvEY+FTir2VkJVg/tG6fgG6dP6jxOdg5flyEDkPl9X8iODgrN4M/mmjhGxGwykZSqIkTg0Frx2Jn1/sllrYjx1Vf4j35lywXvJyD83r44nC6lNbQzoyKz6gzlS39xow1aoJ4wMtN3g9H0ziy4QTXWdlRKr5zacXZMQl2TxuZqCfmEuiLsvidMQ4dIU3fzwBay+1rHPi7MQVFRF+qofUJGdAFIgnQel8hx1AIWeze1EgV/xYjD7v5zW5lnYMBajQQDmsl59JGu8aihcGnZkZK6nC+5ZnVyTtJ1O9hepk8kIYkdb+2eddk0be 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 04.07.24 16:07, Kirill A. Shutemov wrote: > On Thu, Jul 04, 2024 at 02:39:49PM +0100, Ryan Roberts wrote: >> On 04/07/2024 12:41, Kirill A. Shutemov wrote: >>> On Wed, Jul 03, 2024 at 06:37:48PM +0100, Ryan Roberts wrote: >>>> Hi Kirill, Hugh, Mel, >>>> >>>> We recently had a problem reported at [1] that due to aarch64 arch requiring >>>> that atomic RMW instructions raise a read fault, followed by a write fault, this >>>> causes a huge zero page to be faulted in during the read fault, then the write >>>> fault shatters the huge zero page, installing small zero pages for every PTE in >>>> the PMD region, except the faulting address which gets a writable private page. >>>> >>>> A number of ways were discussed to solve that problem. But it got me wondering >>>> why we have this behaviour in general for huge zero page? This seems like odd >>>> behaviour to me. Surely it would be less effort and more aligned with the app's >>>> expectations to notice the huge zero page in the PMD, remove it, and install a >>>> THP, as would have been done if pmd_none() was true? Or if there is a reason to >>>> shatter on write, why not do away with the huge zero page and save some memory, >>>> and just install a PMD's worth of small zero pages on fault? >>>> >>>> Perhaps replacing the huge zero page with a huge THP on write fault would have >>>> been a better behavior at the time, but perhaps changing that behaviour now >>>> risks a memory bloat regression in some workloads? >>> >>> Yeah, I agree that WP fault on zero page page should give THP. I think >>> treating zero page as none PMD on write page fault should be safe and >>> reasonable. >> >> So you're not concerned about potential for memory bloat regressions in apps >> that are deployed today? I'm a bit nervous to make the change without a bunch of >> testing... > > No, I am not concern. It is silly to expect different result depending on > what comes first read or write fault to the page. It should be consistent. > > On related note, I think we need to drop __split_huge_zero_page_pmd(). We > can just unmap it on split and let caller populate it as needed as we do > for file mappings. We'll likely have to take care of UFFD-WP being set on the PMD. -- Cheers, David / dhildenb