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[2003:d8:2f3c:3a00:5662:26b3:3e5d:438e]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-455f6794a00sm2066425e9.37.2025.07.11.04.56.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 11 Jul 2025 04:56:35 -0700 (PDT) Message-ID: <40d3d56a-e434-4d3d-ba0e-3f1204edbda5@redhat.com> Date: Fri, 11 Jul 2025 13:56:34 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/4] mm/util: introduce snapshot_page() To: Luiz Capitulino , willy@infradead.org, akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, shivankg@amd.com, sj@kernel.org References: <88d956248f7528b7998ef00ca8742061839d1036.1751914235.git.luizcap@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: <88d956248f7528b7998ef00ca8742061839d1036.1751914235.git.luizcap@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: Y89A4qsfyJUNKR84MiIOb5SfmpQAsWcXzqAJAn4k8VA_1752234996 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 936C64000C X-Stat-Signature: jkna51oqjp4z7f5duwain6i5sdhbae7a X-HE-Tag: 1752235001-722155 X-HE-Meta: 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 lxgPoE+C 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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 07.07.25 20:50, Luiz Capitulino wrote: > This commit refactors __dump_page() into snapshot_page(). > > snapshot_page() tries to take a faithful snapshot of a page and its > folio representation. The snapshot is returned in the struct > page_snapshot parameter along with additional flags that are best > retrieved at snapshot creation time to reduce race windows. > > This function is intended to be used by callers that need a stable > representation of a struct page and struct folio so that pointers > or page information doesn't change while working on a page. > > The idea and original implementation of snapshot_page() comes from > Matthew Wilcox with suggestions for improvements from David Hildenbrand. > All bugs and misconceptions are mine. > > Signed-off-by: Luiz Capitulino > --- > include/linux/mm.h | 19 ++++++++++++ > mm/debug.c | 42 +++---------------------- > mm/util.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++ > 3 files changed, 100 insertions(+), 38 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 0ef2ba0c667a..090968c6eebb 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -4184,4 +4184,23 @@ static inline bool page_pool_page_is_pp(struct page *page) > } > #endif > > +#define PAGE_SNAPSHOT_FAITHFUL (1 << 0) > +#define PAGE_SNAPSHOT_PG_FREE (1 << 1) Can we call this "BUDDY" instead of FREE There are other types of free pages -- in particular free hugetlb folios -- which won't be covered by this check. We really only care about "buddy" pages, which correspond to "free" pages. That's what we check and alter expose :) > +#define PAGE_SNAPSHOT_PG_IDLE (1 << 2) > +> +struct page_snapshot { > + struct folio folio_snapshot; > + struct page page_snapshot; > + unsigned long pfn; > + unsigned long idx; > + unsigned long flags; > +}; > + > +static inline bool snapshot_page_is_faithful(const struct page_snapshot *ps) > +{ > + return ps->flags & PAGE_SNAPSHOT_FAITHFUL; > +} > + > +void snapshot_page(struct page_snapshot *ps, const struct page *page); > + > #endif /* _LINUX_MM_H */ > diff --git a/mm/debug.c b/mm/debug.c > index 907382257062..7349330ea506 100644 > --- a/mm/debug.c > +++ b/mm/debug.c > @@ -129,47 +129,13 @@ static void __dump_folio(struct folio *folio, struct page *page, > > static void __dump_page(const struct page *page) > { > - struct folio *foliop, folio; > - struct page precise; > - unsigned long head; > - unsigned long pfn = page_to_pfn(page); > - unsigned long idx, nr_pages = 1; > - int loops = 5; > - > -again: > - memcpy(&precise, page, sizeof(*page)); > - head = precise.compound_head; > - if ((head & 1) == 0) { > - foliop = (struct folio *)&precise; > - idx = 0; > - if (!folio_test_large(foliop)) > - goto dump; > - foliop = (struct folio *)page; > - } else { > - foliop = (struct folio *)(head - 1); > - idx = folio_page_idx(foliop, page); > - } > + struct page_snapshot ps; > > - if (idx < MAX_FOLIO_NR_PAGES) { > - memcpy(&folio, foliop, 2 * sizeof(struct page)); > - nr_pages = folio_nr_pages(&folio); > - if (nr_pages > 1) > - memcpy(&folio.__page_2, &foliop->__page_2, > - sizeof(struct page)); > - foliop = &folio; > - } > - > - if (idx > nr_pages) { > - if (loops-- > 0) > - goto again; > + snapshot_page(&ps, page); > + if (!snapshot_page_is_faithful(&ps)) > pr_warn("page does not match folio\n"); > - precise.compound_head &= ~1UL; > - foliop = (struct folio *)&precise; > - idx = 0; > - } > > -dump: > - __dump_folio(foliop, &precise, pfn, idx); > + __dump_folio(&ps.folio_snapshot, &ps.page_snapshot, ps.pfn, ps.idx); > } > > void dump_page(const struct page *page, const char *reason) > diff --git a/mm/util.c b/mm/util.c > index 0b270c43d7d1..c38d213be83f 100644 > --- a/mm/util.c > +++ b/mm/util.c > @@ -25,6 +25,7 @@ > #include > #include > #include > +#include > > #include > > @@ -1171,3 +1172,79 @@ int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma) > return 0; > } > EXPORT_SYMBOL(compat_vma_mmap_prepare); > + > +static void set_flags(struct page_snapshot *ps, const struct folio *folio, > + const struct page *page) > +{ > + /* > + * Caveats on high order pages: PG_buddy and PG_slab will only be set > + * on the head page. > + */ Talking about slab here is a bit misleading, IIRC they are proper compound pages today. For PG_buddy (which was renamed to PGTY_buddy), we are dealing with non-compound higher allocations. So talking about "head" is also a bit misleading. /* * Only the first page of a high-order buddy page has PageBuddy() set. * So we have to check manually whether this page is part of a high- * order buddy page. */ > + if (PageBuddy(page)) > + ps->flags |= PAGE_SNAPSHOT_PG_FREE; > + else if (page_count(page) == 0 && is_free_buddy_page(page)) > + ps->flags |= PAGE_SNAPSHOT_PG_FREE; > + > + if (folio_test_idle(folio)) > + ps->flags |= PAGE_SNAPSHOT_PG_IDLE; > +} > + > +/* I would suggest writing proper kernel doc (you mostly have that already) Something like /** * snapshot_page() - Create a snapshot of a "struct page" * @ps: struct page_snapshot to store the page snapshot * @page: the page we want to snapshot * * Create a snapshot of a page and store its struct page and struct * folio ... > + * Create a snapshot of a page and store its struct page and struct folio > + * representations in a struct page_snapshot. > + * > + * @ps: struct page_snapshot to store the page snapshot > + * @page: the page we want to snapshot > + * > + * Note that creating a faithful snapshot of a page may fail if the page > + * compound keeps changing (eg. due to folio split). In this case we set > + * ps->faithful to false and the snapshot will assume that @page refers > + * to a single page. > + */ -- Cheers, David / dhildenb