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Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn Subject: [PATCH v3 19/20] fs/proc/task_mmu: remove per-page mapcount dependency for smaps/smaps_rollup (CONFIG_NO_PAGE_MAPCOUNT) Date: Mon, 3 Mar 2025 17:30:12 +0100 Message-ID: <20250303163014.1128035-20-david@redhat.com> X-Mailer: git-send-email 2.48.1 In-Reply-To: <20250303163014.1128035-1-david@redhat.com> References: <20250303163014.1128035-1-david@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: MzQ7fmQSk0H--G9sRAEOV1C8Xcmo6hf8FigIwD_J2ds_1741019460 X-Mimecast-Originator: redhat.com content-type: text/plain; charset="US-ASCII"; x-default=true X-Rspam-User: X-Rspamd-Queue-Id: 3AEB1120024 X-Rspamd-Server: rspam09 X-Stat-Signature: sns1c9uexa3y978cjkjhggixd6iwwnnm X-HE-Tag: 1741019463-946058 X-HE-Meta: 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 tPBVQDcF 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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List-ID: List-Subscribe: List-Unsubscribe: Let's implement an alternative when per-page mapcounts in large folios are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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 | 22 +++++++++++++++++++--- fs/proc/internal.h | 8 ++++++++ fs/proc/task_mmu.c | 17 +++++++++++++++-- 3 files changed, 42 insertions(+), 5 deletions(-) diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst index 1aa190017f796..c9e62e8e0685e 100644 --- a/Documentation/filesystems/proc.rst +++ b/Documentation/filesystems/proc.rst @@ -502,9 +502,25 @@ process, its PSS will be 1500. "Pss_Dirty" is the portion of PSS which consists of dirty pages. ("Pss_Clean" is not included, but it can be calculated by subtracting "Pss_Dirty" from "Pss".) -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. +Traditionally, a page is accounted as "private" if it is mapped exactly once, +and a page is accounted as "shared" when mapped multiple times, even when +mapped in the same process multiple times. Note that this accounting is +independent of MAP_SHARED. + +In some kernel configurations, the semantics of pages part of a larger +allocation (e.g., THP) can differ: a page is accounted as "private" if all +pages part of the corresponding large allocation are *certainly* mapped in the +same process, even if the page is mapped multiple times in that process. A +page is accounted as "shared" if any page page of the larger allocation +is *maybe* mapped in a different process. In some cases, a large allocation +might be treated as "maybe mapped by multiple processes" even though this +is no longer the case. + +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 96ea58e843114..8c921bc8652d9 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 5043376ebd476..061f16b767118 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); } }