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[2003:cb:c713:a600:7ca0:23b3:d48a:97c7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42802e2f525sm57618195e9.0.2024.07.26.14.48.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Jul 2024 14:48:02 -0700 (PDT) Message-ID: <9e671388-a5c6-4de1-8c85-b7af8aee7f44@redhat.com> Date: Fri, 26 Jul 2024 23:48:01 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 1/2] mm: let pte_lockptr() consume a pte_t pointer To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Muchun Song , Oscar Salvador , Qi Zheng References: <20240725183955.2268884-1-david@redhat.com> <20240725183955.2268884-2-david@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: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: A0A81100010 X-Stat-Signature: mxb75hf9giq78ghbzwadptt4ipfqxnuy X-Rspam-User: X-HE-Tag: 1722030486-849542 X-HE-Meta: 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 hNse1Zyn 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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 26.07.24 23:28, Peter Xu wrote: > On Fri, Jul 26, 2024 at 06:02:17PM +0200, David Hildenbrand wrote: >> On 26.07.24 17:36, Peter Xu wrote: >>> On Thu, Jul 25, 2024 at 08:39:54PM +0200, David Hildenbrand wrote: >>>> pte_lockptr() is the only *_lockptr() function that doesn't consume >>>> what would be expected: it consumes a pmd_t pointer instead of a pte_t >>>> pointer. >>>> >>>> Let's change that. The two callers in pgtable-generic.c are easily >>>> adjusted. Adjust khugepaged.c:retract_page_tables() to simply do a >>>> pte_offset_map_nolock() to obtain the lock, even though we won't actually >>>> be traversing the page table. >>>> >>>> This makes the code more similar to the other variants and avoids other >>>> hacks to make the new pte_lockptr() version happy. pte_lockptr() users >>>> reside now only in pgtable-generic.c. >>>> >>>> Maybe, using pte_offset_map_nolock() is the right thing to do because >>>> the PTE table could have been removed in the meantime? At least it sounds >>>> more future proof if we ever have other means of page table reclaim. >>> >>> I think it can't change, because anyone who wants to race against this >>> should try to take the pmd lock first (which was held already)? >> >> That doesn't explain why it is safe for us to assume that after we took the >> PMD lock that the PMD actually still points at a completely empty page >> table. Likely it currently works by accident, because we only have a single >> such user that makes this assumption. It might certainly be a different once >> we asynchronously reclaim page tables. > > I think it's safe because find_pmd_or_thp_or_none() returned SUCCEED, and > we're holding i_mmap lock for read. I don't see any way that this pmd can > become a non-pgtable-page. > > I meant, AFAIU tearing down pgtable in whatever sane way will need to at > least take both mmap write lock and i_mmap write lock (in this case, a file > mapping), no? Skimming over [1] where I still owe a review I think we can now do it now purely under the read locks, with the PMD lock held. I think this is also what collapse_pte_mapped_thp() ends up doing: replace a PTE table that maps a folio by a PMD (present or none, depends) that maps a folio only while holding the mmap lock in read mode. Of course, here the table is not empty but we need similar ways of making PT walkers aware of concurrent page table retraction. IIRC, that was the magic added to __pte_offset_map(), such that pte_offset_map_nolock/pte_offset_map_lock can fail on races. But if we hold the PMD lock, nothing should actually change (so far my understanding) -- we cannot suddenly rip out a page table. [1] https://lkml.kernel.org/r/cover.1719570849.git.zhengqi.arch@bytedance.com > >> >> But yes, the PMD cannot get modified while we hold the PMD lock, otherwise >> we'd be in trouble >> >>> >>> I wonder an open coded "ptlock_ptr(page_ptdesc(pmd_page(*pmd)))" would be >>> nicer here, but only if my understanding is correct. >> >> I really don't like open-coding that. Fortunately we were able to limit the >> use of ptlock_ptr to a single user outside of arch/x86/xen/mmu_pv.c so far. > > I'm fine if you prefer like that; I don't see it a huge deal to me. Let's keep it like that, unless we can come up with something neater. At least it makes the code also more consistent with similar code in that file and the overhead should be minimal. I was briefly thinking about actually testing if the PT is full of pte_none(), either as a debugging check or to also handle what is currently handled via: if (likely(!vma->anon_vma && !userfaultfd_wp(vma))) { Seems wasteful just because some part of a VMA might have a private page mapped / uffd-wp active to let all other parts suffer. Will think about if that is really worth it. ... also because I still want to understand why the PTL of the PMD table is required at all. What if we lock it first and somebody else wants to lock it after us while we already ripped it out? Sure there must be some reason for the lock, I just don't understand it yet :/. -- Cheers, David / dhildenb