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(p200300d82f40b30053f7d260aff47256.dip0.t-ipconnect.de. [2003:d8:2f40:b300:53f7:d260:aff4:7256]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a892e52b9esm9965303f8f.61.2025.06.30.02.24.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Jun 2025 02:24:09 -0700 (PDT) Message-ID: <8993dbc9-6c9a-4ac7-8c04-813851eba938@redhat.com> Date: Mon, 30 Jun 2025 11:24:08 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 1/4] mm: convert FPB_IGNORE_* into FPB_HONOR_* To: Ryan Roberts , Dev Jain , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Andrew Morton , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Pedro Falcato , Rik van Riel , Harry Yoo References: <20250627115510.3273675-1-david@redhat.com> <20250627115510.3273675-2-david@redhat.com> <5c3428c6-25be-4a94-811a-6bb6718f6c58@arm.com> <5375208d-2c11-4579-a303-e8416ab07159@arm.com> <79525362-2377-441b-8575-d2307bd77f26@arm.com> From: David Hildenbrand Content-Language: en-US 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: <79525362-2377-441b-8575-d2307bd77f26@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 30.06.25 11:18, Ryan Roberts wrote: > On 30/06/2025 10:08, David Hildenbrand wrote: >> On 30.06.25 11:04, Ryan Roberts wrote: >>> On 30/06/2025 04:34, Dev Jain wrote: >>>> >>>> On 29/06/25 2:30 am, David Hildenbrand wrote: >>>>> On 28.06.25 05:37, Dev Jain wrote: >>>>>> >>>>>> On 27/06/25 5:25 pm, David Hildenbrand wrote: >>>>>>> Honoring these PTE bits is the exception, so let's invert the meaning. >>>>>>> >>>>>>> With this change, most callers don't have to pass any flags. >>>>>>> >>>>>>> No functional change intended. >>>>>> >>>>>> FWIW I had proposed this kind of change earlier to Ryan (CCed) and >>>>>> he pointed out: "Doesn't that argument apply in reverse if you want >>>>>> to ignore something new in future? >>>>>> >>>>>> By default we are comparing all the bits in the pte when determining the >>>>>> batch. >>>>>> The flags request to ignore certain bits. >>>>> >>>>> That statement is not true: as default we ignore the write and young bit. And >>>>> we don't have flags for that ;) >>>>> >>>>> Now we also ignore the dirty and soft-dity bit as default, unless told not to >>>>> do that by one very specific caller. >>>>> >>>>>> If we want to ignore extra bits in >>>>>> future, we add new flags and the existing callers don't need to be updated. >>>>> >>>>> What stops you from using FPB_IGNORE_* for something else in the future? >>>>> >>>>> As a side note, there are not that many relevant PTE bits to worry about in >>>>> the near future ;) >>>>> >>>>> I mean, uffd-wp, sure, ... and before we add a FPB_HONOR_UFFD_WP to all users >>>>> to be safe (and changing the default to ignore), you could add a >>>>> FPB_IGNORE_UFFD_WP first, to then check who really can tolerate just ignoring >>>>> it (most of them, I assume). >>>> I agree. >>> >>> Meh. Personally I think if you start mixing HONOR and IGNORE flags, it becomes >>> very confusing to work out what is being checked for and what is not. I stand by >>> my original view. But yeah, writable and young confuse it a bit... How about >>> generalizing by explicitly requiring IGNORE flags for write and young, then also >>> create a flags macro for the common case? >>> >>> #define FPB_IGNORE_COMMON (FPB_IGNORE_WRITE | FPB_IGNORE_YOUNG |    \ >>>                FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY) >>> >>> It's not a hill I'm going to die on though... >> >> How about we make this function simpler, not more complicated? ;) > > I think here we both have different views of what is simpler... You are trying > to optimize for the callers writing less code. I'm trying to optimize for the > reader to be able to easily determine what the function will do for a given caller. See patch number #3: I want the default function -- folio_pte_batch() -- to not have any flags at all. And I don't want to achieve that by internally using flags when calling folio_pte_batch_ext(). If you don't specify flags (folio_pte_batch()), behave just as if calling folio_pte_batch_ext() without flags. Anything else would be more confusing IMHO. I agree that mixing HONOR and IGNORE is not a good idea. But then, it's really only uffd-wp that still could be batched, and likely we want it to be the default, and respect/honor/whatever instead in the cases where we really have to. (If we really want to go down that path and batch it :) ) -- Cheers, David / dhildenb