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[79.242.52.182]) by smtp.gmail.com with UTF8SMTPSA id ffacd0b85a97d-38f259f7998sm31659273f8f.82.2025.02.24.08.56.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Feb 2025 08:56:45 -0800 (PST) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-doc@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@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 , Muchun Song , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn Subject: [PATCH v2 19/20] fs/proc/task_mmu: remove per-page mapcount dependency for smaps/smaps_rollup (CONFIG_NO_PAGE_MAPCOUNT) Date: Mon, 24 Feb 2025 17:56:01 +0100 Message-ID: <20250224165603.1434404-20-david@redhat.com> X-Mailer: git-send-email 2.48.1 In-Reply-To: <20250224165603.1434404-1-david@redhat.com> References: <20250224165603.1434404-1-david@redhat.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Let's implement an alternative when per-page mapcounts in large folios are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT. When computing the output for smaps / smaps_rollups, in particular when calculating the USS (Unique Set Size) and the PSS (Proportional Set Size), we still rely on per-page mapcounts. To determine private vs. shared, we'll use folio_likely_mapped_shared(), similar to how we handle PM_MMAP_EXCLUSIVE. Similarly, we might now under-estimate the USS and count pages towards "shared" that are actually "private" ("exclusively mapped"). When calculating the PSS, we'll now also use the average per-page mapcount for large folios: this can result in both, an over-estimation and an under-estimation of the PSS. The difference is not expected to matter much in practice, but we'll have to learn as we go. We can now provide folio_precise_page_mapcount() only with CONFIG_PAGE_MAPCOUNT, and remove one of the last users of per-page mapcounts when CONFIG_NO_PAGE_MAPCOUNT is enabled. Document the new behavior. Signed-off-by: David Hildenbrand --- Documentation/filesystems/proc.rst | 13 +++++++++++++ fs/proc/internal.h | 8 ++++++++ fs/proc/task_mmu.c | 17 +++++++++++++++-- 3 files changed, 36 insertions(+), 2 deletions(-) diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst index 1aa190017f796..57d55274a1f42 100644 --- a/Documentation/filesystems/proc.rst +++ b/Documentation/filesystems/proc.rst @@ -506,6 +506,19 @@ Note that even a page which is part of a MAP_SHARED mapping, but has only a single pte mapped, i.e. is currently used by only one process, is accounted as private and not as shared. +Note that in some kernel configurations, all pages part of a larger allocation +(e.g., THP) might be considered "shared" if the large allocation is +considered "shared": if not all pages are exclusive to the same process. +Further, some kernel configurations might consider larger allocations "shared", +if they were at one point considered "shared", even if they would now be +considered "exclusive". + +Some kernel configurations do not track the precise number of times a page part +of a larger allocation is mapped. In this case, when calculating the PSS, the +average number of mappings per page in this larger allocation might be used +as an approximation for the number of mappings of a page. The PSS calculation +will be imprecise in this case. + "Referenced" indicates the amount of memory currently marked as referenced or accessed. diff --git a/fs/proc/internal.h b/fs/proc/internal.h index 16aa1fd260771..70205425a2daa 100644 --- a/fs/proc/internal.h +++ b/fs/proc/internal.h @@ -143,6 +143,7 @@ unsigned name_to_int(const struct qstr *qstr); /* Worst case buffer size needed for holding an integer. */ #define PROC_NUMBUF 13 +#ifdef CONFIG_PAGE_MAPCOUNT /** * folio_precise_page_mapcount() - Number of mappings of this folio page. * @folio: The folio. @@ -173,6 +174,13 @@ static inline int folio_precise_page_mapcount(struct folio *folio, return mapcount; } +#else /* !CONFIG_PAGE_MAPCOUNT */ +static inline int folio_precise_page_mapcount(struct folio *folio, + struct page *page) +{ + BUILD_BUG(); +} +#endif /* CONFIG_PAGE_MAPCOUNT */ /** * folio_average_page_mapcount() - Average number of mappings per page in this diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index d7ee842367f0f..7ca0bc3bf417d 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -707,6 +707,8 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page, struct folio *folio = page_folio(page); int i, nr = compound ? compound_nr(page) : 1; unsigned long size = nr * PAGE_SIZE; + bool exclusive; + int mapcount; /* * First accumulate quantities that depend only on |size| and the type @@ -747,18 +749,29 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page, dirty, locked, present); return; } + + if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) { + mapcount = folio_average_page_mapcount(folio); + exclusive = !folio_maybe_mapped_shared(folio); + } + /* * We obtain a snapshot of the mapcount. Without holding the folio lock * this snapshot can be slightly wrong as we cannot always read the * mapcount atomically. */ for (i = 0; i < nr; i++, page++) { - int mapcount = folio_precise_page_mapcount(folio, page); unsigned long pss = PAGE_SIZE << PSS_SHIFT; + + if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) { + mapcount = folio_precise_page_mapcount(folio, page); + exclusive = mapcount < 2; + } + if (mapcount >= 2) pss /= mapcount; smaps_page_accumulate(mss, folio, PAGE_SIZE, pss, - dirty, locked, mapcount < 2); + dirty, locked, exclusive); } }