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Mon, 07 Jul 2025 14:51:07 -0400 X-MC-Unique: 0Y-PJ3nsNjKuMpUol191nw-1 X-Mimecast-MFC-AGG-ID: 0Y-PJ3nsNjKuMpUol191nw_1751914266 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 5D27419560AE; Mon, 7 Jul 2025 18:51:06 +0000 (UTC) Received: from fedora.redhat.com (unknown [10.22.80.96]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id E3B3219560B2; Mon, 7 Jul 2025 18:51:04 +0000 (UTC) From: Luiz Capitulino To: david@redhat.com, willy@infradead.org, akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, shivankg@amd.com, sj@kernel.org Subject: [PATCH v2 2/4] mm/util: introduce snapshot_page() Date: Mon, 7 Jul 2025 14:50:44 -0400 Message-ID: <88d956248f7528b7998ef00ca8742061839d1036.1751914235.git.luizcap@redhat.com> In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 X-Rspamd-Queue-Id: 6384A20015 X-Stat-Signature: keqkkxsfy8rgrcdpehkutmb3kpuqids4 X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1751914270-556822 X-HE-Meta: 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 wB8bPYaM 7sNUpWgO0DaC7rnZJ1HG9KXA7OiLlITt2AfY2TAzM2Agr/sf966nmSwlT39RDIvCyClvjt+E76vEM8qCiLSaiVX6uViG/C0YcKcpRX0BBqRs69+VwVWZz9Ziev27fXzhOvh+QQhFpc/ypJQEFwFeX3Q0LLtzfgciIuG/nXg5WYUfWWmGC5UyL9ALx7BaJczKuVLN16gUrati7CAXqo6UiJJkNi/AGf86VeF6QdgMTwSrN0ix3I2wUnrp1H+H4a9IJKzv9jBLxVHAXsGFS3NkXlwyCwAzwlkflD+N4ybcs728NPmwm1/lLE5obVeAkeDgbeB64BWUNTidULpVGuJiuf6lC2LOrwrrL2IYD2DFi7HtmhwrvI7lXGIyMVjRDCsk5Yagg 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: This commit refactors __dump_page() into snapshot_page(). snapshot_page() tries to take a faithful snapshot of a page and its folio representation. The snapshot is returned in the struct page_snapshot parameter along with additional flags that are best retrieved at snapshot creation time to reduce race windows. This function is intended to be used by callers that need a stable representation of a struct page and struct folio so that pointers or page information doesn't change while working on a page. The idea and original implementation of snapshot_page() comes from Matthew Wilcox with suggestions for improvements from David Hildenbrand. All bugs and misconceptions are mine. Signed-off-by: Luiz Capitulino --- include/linux/mm.h | 19 ++++++++++++ mm/debug.c | 42 +++---------------------- mm/util.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 100 insertions(+), 38 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 0ef2ba0c667a..090968c6eebb 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -4184,4 +4184,23 @@ static inline bool page_pool_page_is_pp(struct page *page) } #endif +#define PAGE_SNAPSHOT_FAITHFUL (1 << 0) +#define PAGE_SNAPSHOT_PG_FREE (1 << 1) +#define PAGE_SNAPSHOT_PG_IDLE (1 << 2) + +struct page_snapshot { + struct folio folio_snapshot; + struct page page_snapshot; + unsigned long pfn; + unsigned long idx; + unsigned long flags; +}; + +static inline bool snapshot_page_is_faithful(const struct page_snapshot *ps) +{ + return ps->flags & PAGE_SNAPSHOT_FAITHFUL; +} + +void snapshot_page(struct page_snapshot *ps, const struct page *page); + #endif /* _LINUX_MM_H */ diff --git a/mm/debug.c b/mm/debug.c index 907382257062..7349330ea506 100644 --- a/mm/debug.c +++ b/mm/debug.c @@ -129,47 +129,13 @@ static void __dump_folio(struct folio *folio, struct page *page, static void __dump_page(const struct page *page) { - struct folio *foliop, folio; - struct page precise; - unsigned long head; - unsigned long pfn = page_to_pfn(page); - unsigned long idx, nr_pages = 1; - int loops = 5; - -again: - memcpy(&precise, page, sizeof(*page)); - head = precise.compound_head; - if ((head & 1) == 0) { - foliop = (struct folio *)&precise; - idx = 0; - if (!folio_test_large(foliop)) - goto dump; - foliop = (struct folio *)page; - } else { - foliop = (struct folio *)(head - 1); - idx = folio_page_idx(foliop, page); - } + struct page_snapshot ps; - if (idx < MAX_FOLIO_NR_PAGES) { - memcpy(&folio, foliop, 2 * sizeof(struct page)); - nr_pages = folio_nr_pages(&folio); - if (nr_pages > 1) - memcpy(&folio.__page_2, &foliop->__page_2, - sizeof(struct page)); - foliop = &folio; - } - - if (idx > nr_pages) { - if (loops-- > 0) - goto again; + snapshot_page(&ps, page); + if (!snapshot_page_is_faithful(&ps)) pr_warn("page does not match folio\n"); - precise.compound_head &= ~1UL; - foliop = (struct folio *)&precise; - idx = 0; - } -dump: - __dump_folio(foliop, &precise, pfn, idx); + __dump_folio(&ps.folio_snapshot, &ps.page_snapshot, ps.pfn, ps.idx); } void dump_page(const struct page *page, const char *reason) diff --git a/mm/util.c b/mm/util.c index 0b270c43d7d1..c38d213be83f 100644 --- a/mm/util.c +++ b/mm/util.c @@ -25,6 +25,7 @@ #include #include #include +#include #include @@ -1171,3 +1172,79 @@ int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma) return 0; } EXPORT_SYMBOL(compat_vma_mmap_prepare); + +static void set_flags(struct page_snapshot *ps, const struct folio *folio, + const struct page *page) +{ + /* + * Caveats on high order pages: PG_buddy and PG_slab will only be set + * on the head page. + */ + if (PageBuddy(page)) + ps->flags |= PAGE_SNAPSHOT_PG_FREE; + else if (page_count(page) == 0 && is_free_buddy_page(page)) + ps->flags |= PAGE_SNAPSHOT_PG_FREE; + + if (folio_test_idle(folio)) + ps->flags |= PAGE_SNAPSHOT_PG_IDLE; +} + +/* + * Create a snapshot of a page and store its struct page and struct folio + * representations in a struct page_snapshot. + * + * @ps: struct page_snapshot to store the page snapshot + * @page: the page we want to snapshot + * + * Note that creating a faithful snapshot of a page may fail if the page + * compound keeps changing (eg. due to folio split). In this case we set + * ps->faithful to false and the snapshot will assume that @page refers + * to a single page. + */ +void snapshot_page(struct page_snapshot *ps, const struct page *page) +{ + unsigned long head, nr_pages = 1; + struct folio *foliop, folio; + int loops = 5; + + ps->pfn = page_to_pfn(page); + ps->flags = PAGE_SNAPSHOT_FAITHFUL; + +again: + memcpy(&ps->page_snapshot, page, sizeof(*page)); + head = ps->page_snapshot.compound_head; + if ((head & 1) == 0) { + foliop = (struct folio *)&ps->page_snapshot; + ps->idx = 0; + if (!folio_test_large(foliop)) { + set_flags(ps, page_folio(page), page); + goto out; + } + foliop = (struct folio *)page; + } else { + foliop = (struct folio *)(page->compound_head - 1); + ps->idx = folio_page_idx(foliop, page); + } + + if (ps->idx < MAX_FOLIO_NR_PAGES) { + memcpy(&folio, foliop, 2 * sizeof(struct page)); + nr_pages = folio_nr_pages(&folio); + if (nr_pages > 1) + memcpy(&folio.__page_2, &foliop->__page_2, + sizeof(struct page)); + set_flags(ps, foliop, page); + foliop = &folio; + } + + if (ps->idx > nr_pages) { + if (loops-- > 0) + goto again; + ps->page_snapshot.compound_head &= ~1UL; + foliop = (struct folio *)&ps->page_snapshot; + ps->flags = 0; + ps->idx = 0; + } + +out: + memcpy(&ps->folio_snapshot, foliop, sizeof(struct folio)); +} -- 2.50.0