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[2003:cb:c70d:3a00:d73c:6a8:ca9f:1df7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-436e2e92f6dsm20457255e9.41.2025.01.08.05.19.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Jan 2025 05:19:40 -0800 (PST) Message-ID: Date: Wed, 8 Jan 2025 14:19:38 +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 mm-unstable v2] mm/hugetlb_vmemmap: fix memory loads ordering To: Yu Zhao , Andrew Morton Cc: Mateusz Guzik , "Matthew Wilcox (Oracle)" , Muchun Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Will Deacon References: <20250108074822.722696-1-yuzhao@google.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: <20250108074822.722696-1-yuzhao@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 08.01.25 08:48, Yu Zhao wrote: > Using x86_64 as an example, for a 32KB struct page[] area describing a > 2MB hugeTLB, HVO reduces the area to 4KB by the following steps: > 1. Split the (r/w vmemmap) PMD mapping the area into 512 (r/w) PTEs; > 2. For the 8 PTEs mapping the area, remap PTE 1-7 to the page mapped > by PTE 0, and at the same time change the permission from r/w to > r/o; > 3. Free the pages PTE 1-7 used to map, hence the reduction from 32KB > to 4KB. > > However, the following race can happen due to improperly memory loads > ordering: > CPU 1 (HVO) CPU 2 (speculative PFN walker) > > page_ref_freeze() > synchronize_rcu() > rcu_read_lock() > page_is_fake_head() is false > vmemmap_remap_pte() > XXX: struct page[] becomes r/o > > page_ref_unfreeze() > page_ref_count() is not zero > > atomic_add_unless(&page->_refcount) > XXX: try to modify r/o struct page[] > > Specifically, page_is_fake_head() must be ordered after > page_ref_count() on CPU 2 so that it can only return true for this > case, to avoid the later attempt to modify r/o struct page[]. > > This patch adds the missing memory barrier and makes the tests on > page_is_fake_head() and page_ref_count() done in the proper order. > > Fixes: bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers") > Reported-by: Will Deacon > Closes: https://lore.kernel.org/20241128142028.GA3506@willie-the-truck/ > Signed-off-by: Yu Zhao > --- > include/linux/page-flags.h | 37 +++++++++++++++++++++++++++++++++++++ > include/linux/page_ref.h | 2 +- > 2 files changed, 38 insertions(+), 1 deletion(-) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index 691506bdf2c5..16fa8f0cea02 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -225,11 +225,48 @@ static __always_inline const struct page *page_fixed_fake_head(const struct page > } > return page; > } > + > +static __always_inline bool page_count_writable(const struct page *page, int u) > +{ > + if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key)) > + return true; > + > + /* > + * The refcount check is ordered before the fake-head check to prevent > + * the following race: > + * CPU 1 (HVO) CPU 2 (speculative PFN walker) > + * > + * page_ref_freeze() > + * synchronize_rcu() > + * rcu_read_lock() > + * page_is_fake_head() is false > + * vmemmap_remap_pte() > + * XXX: struct page[] becomes r/o > + * > + * page_ref_unfreeze() > + * page_ref_count() is not zero > + * > + * atomic_add_unless(&page->_refcount) > + * XXX: try to modify r/o struct page[] > + * > + * The refcount check also prevents modification attempts to other (r/o) > + * tail pages that are not fake heads. > + */ > + if (atomic_read_acquire(&page->_refcount) == u) > + return false; > + > + return page_fixed_fake_head(page) == page; > +} > #else > static inline const struct page *page_fixed_fake_head(const struct page *page) > { > return page; > } > + > +static inline bool page_count_writable(const struct page *page, int u) > +{ > + return true; > +} > #endif > > static __always_inline int page_is_fake_head(const struct page *page) > diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h > index 8c236c651d1d..544150d1d5fd 100644 > --- a/include/linux/page_ref.h > +++ b/include/linux/page_ref.h > @@ -234,7 +234,7 @@ static inline bool page_ref_add_unless(struct page *page, int nr, int u) > > rcu_read_lock(); > /* avoid writing to the vmemmap area being remapped */ > - if (!page_is_fake_head(page) && page_ref_count(page) != u) > + if (page_count_writable(page, u)) > ret = atomic_add_unless(&page->_refcount, nr, u); > rcu_read_unlock(); > LGTM, thanks! Reviewed-by: David Hildenbrand -- Cheers, David / dhildenb