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(p200300d82f199c0005687df700e1293d.dip0.t-ipconnect.de. [2003:d8:2f19:9c00:568:7df7:e1:293d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-452730c747dsm18638905e9.31.2025.06.06.04.44.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 06 Jun 2025 04:44:13 -0700 (PDT) Message-ID: Date: Fri, 6 Jun 2025 13:44:12 +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] mm/gup: remove (VM_)BUG_ONs To: Lorenzo Stoakes Cc: Michal Hocko , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Jason Gunthorpe , John Hubbard , Peter Xu References: <20250604140544.688711-1-david@redhat.com> <1a65d0e6-6088-4a15-9c19-537203fe655c@redhat.com> <50ff9149-2824-4e57-8d74-d8d0c063c87e@lucifer.local> <1a7513cf-4a0a-4e58-b20d-31c1370b760f@lucifer.local> 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: <1a7513cf-4a0a-4e58-b20d-31c1370b760f@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 06.06.25 13:04, Lorenzo Stoakes wrote: > On Fri, Jun 06, 2025 at 12:28:28PM +0200, David Hildenbrand wrote: >> On 06.06.25 12:19, Lorenzo Stoakes wrote: >>> On Fri, Jun 06, 2025 at 12:13:27PM +0200, Michal Hocko wrote: >>>> On Fri 06-06-25 11:01:18, David Hildenbrand wrote: >>>>> On 06.06.25 10:31, Michal Hocko wrote: >>>> [...] >>>>>> Turning them into VM_WARN_ON >>>>>> should be reasonably safe as they are not enabled in production >>>>>> environment anyway so we cannot really rely on those. Having them in >>>>>> WARN form would be still useful for debugging and those that really need >>>>>> a crash dump while debugging can achieve the same result. >>>>> >>>>> One question is if we should be VM_WARN_ON vs. VM_WARN_ON_ONCE ... >>>> >>>> *WARN_ONCE ha a very limited use from code paths which are generally >>>> shared by many callers. You just see one and then nothing. Some time ago >>>> we have discussed an option to have _ONCE per call trace but I haven't >>>> see any follow up. >>>> >>>> Anyway starting without _ONCE seems like safer option because we are not >>>> losing potentially useful debugging information. Afterall this is >>>> debugging only thing. But no strong position on my side. >>>> >>>>> VM_BUG_ON is essentially a "once" thing so far, but then, we don't continue >>>>> ... so probably most should be _ONCE. >>>>> >>>>>> >>>>>> So while I agree that many of them could be dropped or made more clear >>>>>> those could be dealt with after a mass move. An advantage of this would >>>>>> be that we can drop VM_BUG_ON* and stop new instances from being added. >>>>> >>>>> As a first step we could probably just #define them to go to the >>>>> VM_WARN_ON_* variants and see what happens. >>>> >>>> You meand VM_BUG_ON expand to VM_WARN_ON by default? >>> >>> Sorry to interject in the conversation, but to boldly throw my two English pence >>> into the mix: >>> >>> As the "king of churn" (TM) you'll not be surprised to hear that I'm in favour >>> of us just doing a big patch and convert all VM_BUG_ON() -> VM_WARN_ON_ONCE() >>> and remove VM_BUG_ON*(). >>> >>> Pull the band-aid off... I think better than a #define if this indeed what you >>> meant David. >>> >>> But of course, you'd expect me to have this opinion ;) >> >> See my reply to Michal regarding keeping VM_BUG_ON() until we actually >> decided what the right cleanup is. > > Sure, but to me the concept of VM_BUG_ON() is surely fundamentally broken - if > BUG_ON() means 'stop everything we're going to corrupt' then it makes no sense > to add a '...but only if CONFIG_DEBUG_VM is set' in there. > > So to me the only assessment needed is 'do we want to warn on this or not?'. Well, when done carefully, it would be when reworking a VM_BUG_ON: (a) Should this really only be checked with DEBUG_VM or should this actually be a WARN_ON_ONCE() + recovery (b) Does this check even still make sense in current code, or were we just extra careful initially. (c) Do we even understand why it is checked or should we add a comment. (d) Should we use one of the _PAGE / _FOLIO / _VMA etc. variants instead or even add new ones. One could argue that the same is true for any other VM_WARN_ON ... but my point from the beginning was that if we're already touching them, why not spend some extra time and do it properly .. ... but yeah, 600 instances are a bit much. :) But yeah, VM_BUG_ON is easier to replace than BUG_ON, because ... not even Fedora is running with CONFIG_DEBUG_VM anymore, so these checks are mostly only there in actual debug kernels. So agreed, let's move forward with a simple conversion. > > And as you say, really WARN_ON_ONCE() seems appropriate, because nearly always > we will get flooded with useless information. > > I think this being debug code gives us a lot of leeway here. > > After the big change, we can always revisit individual cases and see if the > warning is valid at all. > >> >> I don't have a very strong opinion on any of this ... as long as VM_BUG_ON() >> goes away one way or the other. > > Am happy to come up with the churn-meister version of this patch and take the > heat :P I assume with "this patch" you mean "a patch that gets rid of VM_BUG_ON completely", because I want this patch here (that started the discussion) to go in first. Fascinating how you are always looking for work :P Thanks! -- Cheers, David / dhildenb