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[2003:cb:c737:1900:16b0:8dc:77e:31af]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3730815b7e3sm10986513f8f.53.2024.08.26.08.21.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 Aug 2024 08:21:34 -0700 (PDT) Message-ID: Date: Mon, 26 Aug 2024 17:21:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 01/14] mm: pgtable: introduce pte_offset_map_{ro|rw}_nolock() To: Qi Zheng , hughd@google.com, willy@infradead.org, muchun.song@linux.dev, vbabka@kernel.org, akpm@linux-foundation.org, rppt@kernel.org, vishal.moola@gmail.com, peterx@redhat.com, ryan.roberts@arm.com, christophe.leroy2@cs-soprasteria.com Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org References: 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-Stat-Signature: kqquua5orxsk8xe7g9a86q4w9j3mqm6w X-Rspam-User: X-Rspamd-Queue-Id: 9F56612000C X-Rspamd-Server: rspam02 X-HE-Tag: 1724685699-592164 X-HE-Meta: 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 d4rDAMlo 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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 22.08.24 09:13, Qi Zheng wrote: > Currently, the usage of pte_offset_map_nolock() can be divided into the > following two cases: > > 1) After acquiring PTL, only read-only operations are performed on the PTE > page. In this case, the RCU lock in pte_offset_map_nolock() will ensure > that the PTE page will not be freed, and there is no need to worry > about whether the pmd entry is modified. There is also the usage where we don't grab the PTL at all, and only do a racy (read-only) lookup. > > 2) After acquiring PTL, the pte or pmd entries may be modified. At this > time, we need to ensure that the pmd entry has not been modified > concurrently. > > To more clearing distinguish between these two cases, this commit > introduces two new helper functions to replace pte_offset_map_nolock(). > For 1), just rename it to pte_offset_map_ro_nolock(). For 2), in addition > to changing the name to pte_offset_map_rw_nolock(), it also outputs the > pmdval when successful. This can help the caller recheck *pmd once the PTL > is taken. In some cases, that is, either the mmap_lock for write, or > pte_same() check on contents, is also enough to ensure that the pmd entry > is stable. But in order to prevent the interface from being abused, we > choose to pass in a dummy local variable instead of NULL. > > Subsequent commits will convert pte_offset_map_nolock() into the above > two functions one by one, and finally completely delete it. > > Signed-off-by: Qi Zheng > --- > Documentation/mm/split_page_table_lock.rst | 7 ++++ > include/linux/mm.h | 5 +++ > mm/pgtable-generic.c | 43 ++++++++++++++++++++++ > 3 files changed, 55 insertions(+) > > diff --git a/Documentation/mm/split_page_table_lock.rst b/Documentation/mm/split_page_table_lock.rst > index e4f6972eb6c04..08d0e706a32db 100644 > --- a/Documentation/mm/split_page_table_lock.rst > +++ b/Documentation/mm/split_page_table_lock.rst > @@ -19,6 +19,13 @@ There are helpers to lock/unlock a table and other accessor functions: > - pte_offset_map_nolock() > maps PTE, returns pointer to PTE with pointer to its PTE table > lock (not taken), or returns NULL if no PTE table; What will happen to pte_offset_map_nolock() after this series? Does it still exist or will it become an internal helper? > + - pte_offset_map_ro_nolock() > + maps PTE, returns pointer to PTE with pointer to its PTE table > + lock (not taken), or returns NULL if no PTE table; > + - pte_offset_map_rw_nolock() > + maps PTE, returns pointer to PTE with pointer to its PTE table > + lock (not taken) and the value of its pmd entry, or returns NULL > + if no PTE table; [...] > +pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd, > + unsigned long addr, pmd_t *pmdvalp, > + spinlock_t **ptlp) > +{ > + pmd_t pmdval; > + pte_t *pte; > + > + BUG_ON(!pmdvalp); As raised, no BUG_ON please. VM_WARN_ON_ONCE() is helpful during early testing and should catch these kind of things. If someone thinks not requiring a non-NULL pointer is better, please speak up, I'm not married to that idea :) > + pte = __pte_offset_map(pmd, addr, &pmdval); > + if (likely(pte)) > + *ptlp = pte_lockptr(mm, &pmdval); > + *pmdvalp = pmdval; > + return pte; > +} > + > /* > * pte_offset_map_lock(mm, pmd, addr, ptlp), and its internal implementation > * __pte_offset_map_lock() below, is usually called with the pmd pointer for > @@ -356,6 +383,22 @@ pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd, > * recheck *pmd once the lock is taken; in practice, no callsite needs that - > * either the mmap_lock for write, or pte_same() check on contents, is enough. > * > + * pte_offset_map_ro_nolock(mm, pmd, addr, ptlp), above, is like > + * pte_offset_map(); but when successful, it also outputs a pointer to the > + * spinlock in ptlp - as pte_offset_map_lock() does, but in this case without > + * locking it. This helps the caller to avoid a later pte_lockptr(mm, *pmd), > + * which might by that time act on a changed *pmd: pte_offset_map_ro_nolock() > + * provides the correct spinlock pointer for the page table that it returns. > + * For readonly case, the caller does not need to recheck *pmd after the lock is > + * taken, because the RCU lock will ensure that the PTE page will not be freed. > + * > + * pte_offset_map_rw_nolock(mm, pmd, addr, pmdvalp, ptlp), above, is like > + * pte_offset_map_ro_nolock(); but when successful, it also outputs the > + * pdmval. For cases where pte or pmd entries may be modified, that is, maywrite > + * case, this can help the caller recheck *pmd once the lock is taken. In some > + * cases, that is, either the mmap_lock for write, or pte_same() check on > + * contents, is also enough to ensure that the pmd entry is stable. > + * > * Note that free_pgtables(), used after unmapping detached vmas, or when > * exiting the whole mm, does not take page table lock before freeing a page > * table, and may not use RCU at all: "outsiders" like khugepaged should avoid In general to me a step into the right direction. Likely the documentation could be further clarified in some aspects: Like that the use of pte_offset_map_ro_nolock() does not allow to easily identify if the page table was replaced in the meantime. Even after grabbing the PTL, we might be looking either at a page table that is still mapped or one that was unmapped and is about to get freed. But for R/O access this is usually sufficient AFAIUK. Or that "RO" / "RW" expresses the intended semantics, not that the *kmap* will be RO/RW protected. -- Cheers, David / dhildenb