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[2003:cb:c731:3d00:918f:ce94:4280:80f0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-35dd04ca981sm9693509f8f.33.2024.06.03.14.01.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 Jun 2024 14:01:36 -0700 (PDT) Message-ID: Date: Mon, 3 Jun 2024 23:01:34 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [linus:master] [mm] efa7df3e3b: kernel_BUG_at_include/linux/page_ref.h To: Yang Shi Cc: Peter Xu , kernel test robot , Jason Gunthorpe , Vivek Kasireddy , Rik van Riel , oe-lkp@lists.linux.dev, lkp@intel.com, linux-kernel@vger.kernel.org, Andrew Morton , Matthew Wilcox , Christopher Lameter , linux-mm@kvack.org References: <202405311534.86cd4043-lkp@intel.com> <890e5a79-8574-4a24-90ab-b9888968d5e5@redhat.com> <0edfcfed-e8c4-4c46-bbce-528c07084792@redhat.com> <8da12503-839d-459f-a2fa-4abd6d21935d@redhat.com> <9d9a5730-161b-4a9d-a696-1cf6d0c5123c@redhat.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: 8bit X-Rspamd-Queue-Id: B8E41A0010 X-Stat-Signature: d3xinsjguze9cy5f49t88df3z4o8xqxb X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1717448502-57657 X-HE-Meta: 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 K4iyczT+ 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 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 03.06.24 22:44, Yang Shi wrote: > On Mon, Jun 3, 2024 at 1:38 PM David Hildenbrand wrote: >> >>>> try_get_folio() is all about grabbing a folio that might get freed >>>> concurrently. That's why it calls folio_ref_try_add_rcu() and does >>>> complicated stuff. >>> >>> IMHO we can define it.. e.g. try_get_page() wasn't defined as so. >>> >>> If we want to be crystal clear on that and if we think that's what we want, >>> again I would suggest we rename it differently from try_get_page() to avoid >>> future misuses, then add documents. We may want to also even assert the >> >> Yes, absolutely. >> >>> rcu/irq implications in try_get_folio() at entrance, then that'll be >>> detected even without TINY_RCU config. >>> >>>> >>>> On !CONFIG_TINY_RCU, it performs a folio_ref_add_unless(). That's >>>> essentially a atomic_add_unless(), which in the worst case ends up being a >>>> cmpxchg loop. >>>> >>>> >>>> Stating that we should be using try_get_folio() in paths where we are sure >>>> the folio refcount is not 0 is the same as using folio_try_get() where >>>> folio_get() would be sufficient. >>>> >>>> The VM_BUG_ON in folio_ref_try_add_rcu() really tells us here that we are >>>> using a function in the wrong context, although in our case, it is safe to >>>> use (there is now BUG). Which is true, because we know we have a folio >>>> reference and can simply use a simple folio_ref_add(). >>>> >>>> Again, just like we have folio_get() and folio_try_get(), we should >>>> distinguish in GUP whether we are adding more reference to a folio (and >>>> effectively do what folio_get() would), or whether we are actually grabbing >>>> a folio that could be freed concurrently (what folio_try_get() would do). >>> >>> Yes we can. Again, IMHO it's a matter of whether it will worth it. >>> >>> Note that even with SMP and even if we keep this code, the >>> atomic_add_unless only affects gup slow on THP only, and even with that >>> overhead it is much faster than before when that path was introduced.. and >>> per my previous experience we don't care too much there, really. >>> >>> So it's literally only three paths that are relevant here on the "unless" >>> overhead: >>> >>> - gup slow on THP (I just added it; used to be even slower..) >>> >>> - vivik's new path >>> >>> - hugepd (which may be gone for good in a few months..) >>> >>> IMHO none of them has perf concerns. The real perf concern paths is >>> gup-fast when pgtable entry existed, but that must use atomic_add_unless() >>> anyway. Even gup-slow !thp case won't regress as that uses try_get_page(). >> >> My point is primarily that we should be clear that the one thing is >> GUP-fast, and the other is for GUP-slow. >> >> Sooner or later we'll see more code that uses try_grab_page() to be >> converted to folios, and people might naturally use try_grab_folio(), >> just like we did with Vivik's code. >> >> And I don't think we'll want to make GUP-slow in general using >> try_grab_folio() in the future. >> >> So ... >> >>> >>> So again, IMHO the easist way to fix this WARN is we drop the TINY_RCU bit, >>> if nobody worries on UP perf. >>> >>> I don't have a strong opinion, if any of us really worry about above three >>> use cases on "unless" overhead, and think it worthwhile to add the code to >>> support it, I won't object. But to me it's adding pain with no real benefit >>> we could ever measure, and adding complexity to backport too since we'll >>> need a fix for as old as 6.6. >> >> ... for the sake of fixing this WARN, I don't primarily care. Adjusting >> the TINY_RCU feels wrong because I suspect somebody had good reasons to >> do it like that, and it actually reported something valuable (using the >> wrong function for the job). > > I think this is the major concern about what fix we should do. If that > tiny rcu optimization still makes sense and is useful, we'd better > keep it. But I can't tell. Leaving it as is may be safer. Willy moved that code in 020853b6f5e and I think it dates back to e286781d5f2e. That contained: + /* + * Preempt must be disabled here - we rely on rcu_read_lock doing + * this for us. + * + * Pagecache won't be truncated from interrupt context, so if we have + * found a page in the radix tree here, we have pinned its refcount by + * disabling preempt, and hence no need for the "speculative get" that + * SMP requires. + */ + VM_BUG_ON(page_count(page) == 0); + atomic_inc(&page->_count); + -- Cheers, David / dhildenb