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[2003:cb:c706:2000:e44c:bc46:d8d3:be5]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38f259d5e92sm20586344f8f.66.2025.02.20.02.59.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Feb 2025 02:59:13 -0800 (PST) Message-ID: <4cb93166-29fd-4aea-965b-5dfb62d4dc8c@redhat.com> Date: Thu, 20 Feb 2025 11:59:11 +0100 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 1/4] mm: page_ext: add an iteration API for page extensions To: Luiz Capitulino , linux-kernel@vger.kernel.org, linux-mm@kvack.org, yuzhao@google.com, pasha.tatashin@soleen.com Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, muchun.song@linux.dev References: <3f0e058aef3951b39cf6bb4259c247352d4fe736.1739931468.git.luizcap@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: <3f0e058aef3951b39cf6bb4259c247352d4fe736.1739931468.git.luizcap@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 19.02.25 03:17, Luiz Capitulino wrote: > The page extension implementation assumes that all page extensions of > a given page order are stored in the same memory section. The function > page_ext_next() relies on this assumption by adding an offset to the > current object to return the next adjacent page extension. > > This behavior works as expected for flatmem but fails for sparsemem when > using 1G pages. The commit cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for > gigantic folios") exposes this issue, making it possible for a crash when > using page_owner or page_table_check page extensions. > > The problem is that for 1G pages, the page extensions may span memory > section boundaries and be stored in different memory sections. This issue > was not visible before commit cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP > for gigantic folios") because alloc_contig_pages() never passed more than > MAX_PAGE_ORDER to post_alloc_hook(). However, the series introducing > mentioned commit changed this behavior allowing the full 1G page order > to be passed. > > Reproducer: > > 1. Build the kernel with CONFIG_SPARSEMEM=y and table extensions > support > 2. Pass 'default_hugepagesz=1 page_owner=on' in the kernel command-line > 3. Reserve one 1G page at run-time, this should crash (backtrace below) > > To address this issue, this commit introduces a new API for iterating > through page extensions. The main iteration loops are for_each_page_ext() > and for_each_page_ext_order(). Both must be called with the RCU read > lock taken. Here's an usage example: > > """ > struct page_ext_iter iter; > struct page_ext *page_ext; > > ... > > rcu_read_lock(); > for_each_page_ext_order(page, order, page_ext, iter) { > struct my_page_ext *obj = get_my_page_ext_obj(page_ext); > ... > } > rcu_read_unlock(); > """ > [...] > +struct page_ext *page_ext_iter_begin(struct page_ext_iter *iter, struct page *page); > +struct page_ext *page_ext_iter_next(struct page_ext_iter *iter); > + > +/** > + * page_ext_iter_get() - Get current page extension > + * @iter: page extension iterator. > + * > + * Return: NULL if no page_ext exists for this iterator. > + */ > +static inline struct page_ext *page_ext_iter_get(const struct page_ext_iter *iter) > +{ > + return iter->page_ext; > +} > + > +/** > + * for_each_page_ext(): iterate through page_ext objects. > + * @__page: the page we're interested in > + * @__pgcount: how many pages to iterate through > + * @__page_ext: struct page_ext pointer where the current page_ext > + * object is returned > + * @__iter: struct page_ext_iter object (defined in the stack) > + * > + * IMPORTANT: must be called with RCU read lock taken. > + */ > +#define for_each_page_ext(__page, __pgcount, __page_ext, __iter) \ > + __page_ext = page_ext_iter_begin(&__iter, __page); \ Doing stuff out of the loop is dangerous. Assume something does if (xxx) for_each_page_ext() Just move that inside the for(). for (__page_ext = page_ext_iter_begin(&__iter, __page), __iter.index = 0) > + for (__iter.index = 0; \ > + __page_ext && __iter.index < __pgcount; \ > + __page_ext = page_ext_iter_next(&__iter), \ > + __iter.index++) Hm, if we now have an index, why not turn iter.pfn -> iter.start_pfn, and only adjust the index in page_ext_iter_next? Then you can set the index to 0 in page_ext_iter_begin() and have here for (__page_ext = page_ext_iter_begin(&__iter, __page), __page_ext && __iter.index < __pgcount, __page_ext = page_ext_iter_next(&__iter);) A page_ext_iter_reset() could then simply reset the index=0 and lookup the page_ext(start_pfn + index) == page_ext(start_pfn) > + > +/** > + * for_each_page_ext_order(): iterate through page_ext objects > + * for a given page order > + * @__page: the page we're interested in > + * @__order: page order to iterate through > + * @__page_ext: struct page_ext pointer where the current page_ext > + * object is returned > + * @__iter: struct page_ext_iter object (defined in the stack) > + * > + * IMPORTANT: must be called with RCU read lock taken. > + */ > +#define for_each_page_ext_order(__page, __order, __page_ext, __iter) \ > + for_each_page_ext(__page, (1UL << __order), __page_ext, __iter) > + > #else /* !CONFIG_PAGE_EXTENSION */ > struct page_ext; > > diff --git a/mm/page_ext.c b/mm/page_ext.c > index 641d93f6af4c1..508deb04d5ead 100644 > --- a/mm/page_ext.c > +++ b/mm/page_ext.c > @@ -549,3 +549,44 @@ void page_ext_put(struct page_ext *page_ext) > > rcu_read_unlock(); > } > + > +/** > + * page_ext_iter_begin() - Prepare for iterating through page extensions. > + * @iter: page extension iterator. > + * @page: The page we're interested in. > + * > + * Must be called with RCU read lock taken. > + * > + * Return: NULL if no page_ext exists for this page. > + */ > +struct page_ext *page_ext_iter_begin(struct page_ext_iter *iter, struct page *page) > +{ > + iter->pfn = page_to_pfn(page); > + iter->page_ext = lookup_page_ext(page); > + > + return iter->page_ext; > +} > + > +/** > + * page_ext_iter_next() - Get next page extension > + * @iter: page extension iterator. > + * > + * Must be called with RCU read lock taken. > + * > + * Return: NULL if no next page_ext exists. > + */ > +struct page_ext *page_ext_iter_next(struct page_ext_iter *iter) > +{ > + if (WARN_ON_ONCE(!iter->page_ext)) > + return NULL; > + > + iter->pfn++; > +> + if (page_ext_iter_next_fast_possible(iter->pfn)) { > + iter->page_ext = page_ext_next(iter->page_ext); > + } else { > + iter->page_ext = lookup_page_ext(pfn_to_page(iter->pfn)); > + } > + > + return iter->page_ext; > +} We now always have a function call when calling into page_ext_iter_next(). Could we move that to the header and rather expose lookup_page_ext() ? -- Cheers, David / dhildenb