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(p200300d82f1bb8006fdb1af24fbd1fdf.dip0.t-ipconnect.de. [2003:d8:2f1b:b800:6fdb:1af2:4fbd:1fdf]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-450d8006ff8sm200519545e9.34.2025.06.04.07.58.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Jun 2025 07:58:27 -0700 (PDT) Message-ID: <5575b0cf-de59-4b4e-b339-c310f079bda7@redhat.com> Date: Wed, 4 Jun 2025 16:58:25 +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: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jason Gunthorpe , John Hubbard , Peter Xu , Linus Torvalds References: <20250604140544.688711-1-david@redhat.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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 04.06.25 16:48, Lorenzo Stoakes wrote: > +Linus in case he has an opinion about BUG_ON() in general... > > On Wed, Jun 04, 2025 at 04:05:44PM +0200, David Hildenbrand wrote: >> Especially once we hit one of the assertions in >> sanity_check_pinned_pages(), observing follow-up assertions failing >> in other code can give good clues about what went wrong, so use >> VM_WARN_ON_ONCE instead. > > I guess the situation where you'd actually want a BUG_ON() is one where > carrying on might cause further corruption so you just want things to stop. Yes. Like, serious data corruption would be avoidable. > > But usually we're already pretty screwed if the thing happened right? So > it's rare if ever that this would be legit? > > Linus's point of view is that we shouldn't use them _at all_ right? So > maybe even this situation isn't one where we'd want to use one? I think the grey zone is actual data corruption. But one has to have a pretty good reason to use a BUG_ON and not a WARN_ON_ONCE() + recovery. > >> >> While at it, let's just convert all VM_BUG_ON to VM_WARN_ON_ONCE as >> well. Add one comment for the pfn_valid() check. > > Yeah VM_BUG_ON() is just _weird_. Maybe we should get rid of all of them > full stop? That's my thinking a well. > >> >> We have to introduce VM_WARN_ON_ONCE_VMA() to make that fly. > > I checked the implementation vs. the other VM_WARN_ON_ONCE_*()'s and it > looks good. > > I wonder if we can find a way to not duplicate this code... but one for a > follow up I think :>) > >> >> Drop the BUG_ON after mmap_read_lock_killable(), if that ever returns >> something > 0 we're in bigger trouble. Convert the other BUG_ON's into >> VM_WARN_ON_ONCE as well, they are in a similar domain "should never >> happen", but more reasonable to check for during early testing. >> >> Cc: Andrew Morton >> Cc: Lorenzo Stoakes >> Cc: "Liam R. Howlett" >> Cc: Vlastimil Babka >> Cc: Mike Rapoport >> Cc: Suren Baghdasaryan >> Cc: Michal Hocko >> Cc: Jason Gunthorpe >> Cc: John Hubbard >> Cc: Peter Xu >> Signed-off-by: David Hildenbrand > > LGTM so, > > Reviewed-by: Lorenzo Stoakes > > Thanks! > One nit below. > >> --- >> >> Wanted to do this for a long time, but my todo list keeps growing ... > > Sounds familiar :) Merge window a chance to do some of these things... > >> >> Based on mm/mm-unstable >> >> --- >> include/linux/mmdebug.h | 12 ++++++++++++ >> mm/gup.c | 41 +++++++++++++++++++---------------------- >> 2 files changed, 31 insertions(+), 22 deletions(-) >> >> diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h >> index a0a3894900ed4..14a45979cccc9 100644 >> --- a/include/linux/mmdebug.h >> +++ b/include/linux/mmdebug.h >> @@ -89,6 +89,17 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi); >> } \ >> unlikely(__ret_warn_once); \ >> }) >> +#define VM_WARN_ON_ONCE_VMA(cond, vma) ({ \ >> + static bool __section(".data..once") __warned; \ >> + int __ret_warn_once = !!(cond); \ >> + \ >> + if (unlikely(__ret_warn_once && !__warned)) { \ >> + dump_vma(vma); \ >> + __warned = true; \ >> + WARN_ON(1); \ >> + } \ >> + unlikely(__ret_warn_once); \ >> +}) > > An aside, I wonder if we could somehow make this generic for various > WARN_ON_ONCE()'s? Yeah, probably. Maybe it will get .... ugly :) > >> #define VM_WARN_ON_VMG(cond, vmg) ({ \ >> int __ret_warn = !!(cond); \ >> \ >> @@ -115,6 +126,7 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi); >> #define VM_WARN_ON_FOLIO(cond, folio) BUILD_BUG_ON_INVALID(cond) >> #define VM_WARN_ON_ONCE_FOLIO(cond, folio) BUILD_BUG_ON_INVALID(cond) >> #define VM_WARN_ON_ONCE_MM(cond, mm) BUILD_BUG_ON_INVALID(cond) >> +#define VM_WARN_ON_ONCE_VMA(cond, vma) BUILD_BUG_ON_INVALID(cond) >> #define VM_WARN_ON_VMG(cond, vmg) BUILD_BUG_ON_INVALID(cond) >> #define VM_WARN_ONCE(cond, format...) BUILD_BUG_ON_INVALID(cond) >> #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond) >> diff --git a/mm/gup.c b/mm/gup.c >> index e065a49842a87..3c3931fcdd820 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -64,11 +64,11 @@ static inline void sanity_check_pinned_pages(struct page **pages, >> !folio_test_anon(folio)) >> continue; >> if (!folio_test_large(folio) || folio_test_hugetlb(folio)) >> - VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page); >> + VM_WARN_ON_ONCE_PAGE(!PageAnonExclusive(&folio->page), page); >> else >> /* Either a PTE-mapped or a PMD-mapped THP. */ >> - VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page) && >> - !PageAnonExclusive(page), page); >> + VM_WARN_ON_ONCE_PAGE(!PageAnonExclusive(&folio->page) && >> + !PageAnonExclusive(page), page); > > Nit but wouldn't VM_WARN_ON_ONCE_FOLIO() work better here? No, we want the actual problematic page here, as that can give us clues what is going wrong. For the small-folio case above we could use it, though. -- Cheers, David / dhildenb