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Thu, 29 Aug 2024 16:58:44 +0000 (UTC) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, cgroups@vger.kernel.org, x86@kernel.org, linux-fsdevel@vger.kernel.org, David Hildenbrand , 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 Subject: [PATCH v1 13/17] fs/proc/page: remove per-page mapcount dependency for /proc/kpagecount (CONFIG_NO_PAGE_MAPCOUNT) Date: Thu, 29 Aug 2024 18:56:16 +0200 Message-ID: <20240829165627.2256514-14-david@redhat.com> In-Reply-To: <20240829165627.2256514-1-david@redhat.com> References: <20240829165627.2256514-1-david@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: B5A41140011 X-Stat-Signature: xfp3hrabg1mkj4kbownsqribxes891r3 X-HE-Tag: 1724950740-809104 X-HE-Meta: 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 8yqw1OTR rWof7zXrd+1UZ0OHmCRqs7kNmhx3VZAJsVjgx61oPkQld33JcVIwSYRoBg9YvYlEXa55k5+u+8qHX2qiT9khmsh2liRYD1eNUV9ybP3UbLf3lznxMG6KTs1PrAEte17Pq7WtbyJZ9MOyg/4j6XJDQFdSNfsZAlZCdkL4sJhSjx2763BjaM+i1V03uNsWY0stLKTVtQEEIXGP/FEoPkF6JTIB/acHWkVi+sXwQ0b5MdxKMdzdM6f+Wq1DuCKS0Ow6mqVfOQFWKtmI0c6wNnr8jGx0f+2KktJRMJprjTKUDFqaRyGtSbw08+lmwzufLEL6nJEpkJZTXhFKAuz1GYwaIVt3sEKGMVnGgq/R/Vk6H7klTSPUPkXHY7MxoVQ5F7D2Zg/Uf 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List-ID: List-Subscribe: List-Unsubscribe: Let's implement an alternative when per-page mapcounts in large folios are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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. Signed-off-by: David Hildenbrand --- Documentation/admin-guide/mm/pagemap.rst | 7 +++++- fs/proc/internal.h | 31 ++++++++++++++++++++++++ fs/proc/page.c | 18 +++++++++++--- 3 files changed, 52 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 cc520168f8b69..3c687f97e18c4 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; + return ((mapcount + adjust) >> folio_large_order(folio)) + + entire_mapcount; +} /* * array.c */ diff --git a/fs/proc/page.c b/fs/proc/page.c index a55f5acefa974..c7838de949287 100644 --- a/fs/proc/page.c +++ b/fs/proc/page.c @@ -67,9 +67,21 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf, * memmaps that were actually initialized. */ page = pfn_to_online_page(pfn); - if (page) - mapcount = folio_precise_page_mapcount(page_folio(page), - page); + if (page) { + struct folio *folio = page_folio(page); + +#ifdef CONFIG_PAGE_MAPCOUNT + mapcount = folio_precise_page_mapcount(folio, page); +#else /* !CONFIG_PAGE_MAPCOUNT */ + /* + * Indicate the per-page average, but at least "1" for + * mapped folios. + */ + mapcount = folio_average_page_mapcount(folio); + if (!mapcount && folio_test_large(folio) && folio_mapped(folio)) + mapcount = 1; +#endif /* !CONFIG_PAGE_MAPCOUNT */ + } if (put_user(mapcount, out)) { ret = -EFAULT;