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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 8C6EE20008 X-Stat-Signature: dhoxs6333inuumpscx714tdtxiep4pck X-Rspam-User: X-HE-Tag: 1740431424-113073 X-HE-Meta: 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 xH3SIBrD fDbphod7V628ROhtk0qjuZf//4huYGgmNIa1q5APg9vHYhnunufv57HnzukUXnrr5R8jc2h7LHSao2woyKm7epXqAnSUvxi6qsUeKnw/BFapmDViOId9BdS8XrqE24xbEhhyCS+j7FZarkAJ+SeSbuC6GVapiqFfpXsbzizhER15wZHKSpr4+OVk4t6Et0Pb/gGeAUkfu09h9Fv3V3Ela71kHi4wBNZqkCao8sw8vvcLYkZnOlZhVItkkRjCZ2jXzdUOxzYXgtoyk9JK5in60DGbKycXbvw5PypnNSCDYKWRi90/E/lkMUYKE2fRbaiWbmweV7p7WO9m1ySwCU9vKmTtmCZwCeFEE5P+uyUKU6783w+OdOCfbqTHZXRrhLVmi6GkBB8GFsjIAT45v7lqZu4kFhb9+H8WsuSQASOoEP8pfh8ivvnjRjU9yBxGoAFpnUPZsFKCOO1rjCBM5miqkRixm2g== 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 24.02.25 22:02, 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 are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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. > > Assume nr_pages = 512. > > With 255 we want to round down, with 256 we want to round up. > > 255 / 512 = 0 :) > 256 / 512 = 0 :( > > Compared to: > > (255 + (512 / 2)) / 512 = (255 + 256) / 512 = 0 :) > (256 + (512 / 2)) / 512 = (256 + 256) / 512 = 1 :) I think adding to the function doc: "The average is calculated by rounding to the nearest integer." might make it clearer. -- Cheers, David / dhildenb