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[2003:cb:c735:1900:ac8b:7ae5:991f:54fc]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-439b02ce60asm119599345e9.7.2025.02.24.13.15.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Feb 2025 13:15:41 -0800 (PST) Message-ID: <567b02b0-3e39-4e3c-ba41-1bc59217a421@redhat.com> Date: Mon, 24 Feb 2025 22:15:40 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 16/20] fs/proc/page: remove per-page mapcount dependency for /proc/kpagecount (CONFIG_NO_PAGE_MAPCOUNT) To: Zi Yan Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, Andrew Morton , "Matthew Wilcox (Oracle)" , Tejun Heo , Zefan Li , Johannes Weiner , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Jonathan Corbet , Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Muchun Song , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn References: <20250224165603.1434404-1-david@redhat.com> <20250224165603.1434404-17-david@redhat.com> <8a5e94a2-8cd7-45f5-a2be-525242c0cd16@redhat.com> <9010E213-9FC5-4900-B971-D032CB879F2E@nvidia.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: <9010E213-9FC5-4900-B971-D032CB879F2E@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 24.02.25 22:10, Zi Yan wrote: > On 24 Feb 2025, at 16:02, David Hildenbrand wrote: > >> On 24.02.25 21:40, Zi Yan wrote: >>> On Mon Feb 24, 2025 at 11:55 AM EST, David Hildenbrand wrote: >>>> Let's implement an alternative when per-page mapcounts in large folios >>>> are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT. >>>> >>>> For large folios, we'll return the per-page average mapcount within the >>>> folio, except when the average is 0 but the folio is mapped: then we >>>> return 1. >>>> >>>> For hugetlb folios and for large folios that are fully mapped >>>> into all address spaces, there is no change. >>>> >>>> As an alternative, we could simply return 0 for non-hugetlb large folios, >>>> or disable this legacy interface with CONFIG_NO_PAGE_MAPCOUNT. >>>> >>>> But the information exposed by this interface can still be valuable, and >>>> frequently we deal with fully-mapped large folios where the average >>>> corresponds to the actual page mapcount. So we'll leave it like this for >>>> now and document the new behavior. >>>> >>>> Note: this interface is likely not very relevant for performance. If >>>> ever required, we could try doing a rather expensive rmap walk to collect >>>> precisely how often this folio page is mapped. >>>> >>>> Signed-off-by: David Hildenbrand >>>> --- >>>> Documentation/admin-guide/mm/pagemap.rst | 7 +++++- >>>> fs/proc/internal.h | 31 ++++++++++++++++++++++++ >>>> fs/proc/page.c | 19 ++++++++++++--- >>>> 3 files changed, 53 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst >>>> index caba0f52dd36c..49590306c61a0 100644 >>>> --- a/Documentation/admin-guide/mm/pagemap.rst >>>> +++ b/Documentation/admin-guide/mm/pagemap.rst >>>> @@ -42,7 +42,12 @@ There are four components to pagemap: >>>> skip over unmapped regions. >>>> * ``/proc/kpagecount``. This file contains a 64-bit count of the number of >>>> - times each page is mapped, indexed by PFN. >>>> + times each page is mapped, indexed by PFN. Some kernel configurations do >>>> + not track the precise number of times a page part of a larger allocation >>>> + (e.g., THP) is mapped. In these configurations, the average number of >>>> + mappings per page in this larger allocation is returned instead. However, >>>> + if any page of the large allocation is mapped, the returned value will >>>> + be at least 1. >>>> The page-types tool in the tools/mm directory can be used to query the >>>> number of times a page is mapped. >>>> diff --git a/fs/proc/internal.h b/fs/proc/internal.h >>>> index 1695509370b88..16aa1fd260771 100644 >>>> --- a/fs/proc/internal.h >>>> +++ b/fs/proc/internal.h >>>> @@ -174,6 +174,37 @@ static inline int folio_precise_page_mapcount(struct folio *folio, >>>> return mapcount; >>>> } >>>> +/** >>>> + * folio_average_page_mapcount() - Average number of mappings per page in this >>>> + * folio >>>> + * @folio: The folio. >>>> + * >>>> + * The average number of present user page table entries that reference each >>>> + * page in this folio as tracked via the RMAP: either referenced directly >>>> + * (PTE) or as part of a larger area that covers this page (e.g., PMD). >>>> + * >>>> + * Returns: The average number of mappings per page in this folio. 0 for >>>> + * folios that are not mapped to user space or are not tracked via the RMAP >>>> + * (e.g., shared zeropage). >>>> + */ >>>> +static inline int folio_average_page_mapcount(struct folio *folio) >>>> +{ >>>> + int mapcount, entire_mapcount; >>>> + unsigned int adjust; >>>> + >>>> + if (!folio_test_large(folio)) >>>> + return atomic_read(&folio->_mapcount) + 1; >>>> + >>>> + mapcount = folio_large_mapcount(folio); >>>> + entire_mapcount = folio_entire_mapcount(folio); >>>> + if (mapcount <= entire_mapcount) >>>> + return entire_mapcount; >>>> + mapcount -= entire_mapcount; >>>> + >>>> + adjust = folio_large_nr_pages(folio) / 2; >> >> Thanks for the review! >> >>> >>> Is there any reason for choosing this adjust number? A comment might be >>> helpful in case people want to change it later, either with some reasoning >>> or just saying it is chosen empirically. >> >> We're dividing by folio_large_nr_pages(folio) (shifting by folio_large_order(folio)), so this is not a magic number at all. >> >> So this should be "ordinary" rounding. > > I thought the rounding would be (mapcount + 511) / 512. Yes, that's "rounding up". > But > that means if one subpage is mapped, the average will be 1. > Your rounding means if at least half of the subpages is mapped, > the average will be 1. Others might think 1/3 is mapped, > the average will be 1. That is why I think adjust looks like > a magic number. I think all callers could tolerate (or benefit) from folio_average_page_mapcount() returning at least 1 in case any page is mapped. There was a reason why I decided to round to the nearest integer instead. Let me think about this once more, I went back and forth a couple of times on this. -- Cheers, David / dhildenb