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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-3a892e61f48sm10217418f8f.93.2025.06.30.04.01.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Jun 2025 04:01:46 -0700 (PDT) Message-ID: <42794499-64d0-44a1-ad70-a361e849b890@redhat.com> Date: Mon, 30 Jun 2025 13:01:44 +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> <8993dbc9-6c9a-4ac7-8c04-813851eba938@redhat.com> <2ca5cbb3-9795-49ef-ae53-11c3143edee1@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: <2ca5cbb3-9795-49ef-ae53-11c3143edee1@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 30.06.25 12:57, Ryan Roberts wrote: > On 30/06/2025 10:24, David Hildenbrand wrote: >> 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. > > OK, sorry I don't have the context of your actual series... I was just trying to > defend what my quote that Dev sent. I'll go take a look at your series. If we'd not do the split and always provide flags + non-inlined variant, then I'd agree. > >> >> 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 :) ) > > FWIW, I think we will need to honor the write bit for Dev's mprotect series (I > just sent review comments against his v4). So if you want most callers to just > call folio_pte_batch() and that will ignore the write bit, then I guess > folio_pte_batch_ext() will have to accept a FPB_HONOR_WRITE bit? Which I guess > aligns with what you are doing here to make all the flags HONOR. I have to take a look at that one. Maybe it's sufficient to know if any PTE was writable, just like we did with fork(). But yes, anybody who requires more advanced handling (flags) shall use the extended variant -- however that one will be called. -- Cheers, David / dhildenb