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Tue, 17 Jun 2025 10:27:34 -0400 X-MC-Unique: DWISZpDLPRuc0IZtUfGB1g-1 X-Mimecast-MFC-AGG-ID: DWISZpDLPRuc0IZtUfGB1g_1750170453 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 42BA71955F37; Tue, 17 Jun 2025 14:27:33 +0000 (UTC) Received: from fedora.redhat.com (unknown [10.22.80.174]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id B2D2019560B2; Tue, 17 Jun 2025 14:27:31 +0000 (UTC) From: Luiz Capitulino To: david@redhat.com, willy@infradead.org Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lcapitulino@gmail.com, shivankg@amd.com Subject: [RFC 1/3] mm: introduce snapshot_page() Date: Tue, 17 Jun 2025 10:27:08 -0400 Message-ID: <48d49e3371c35a2da6f4c42d5d9f14b6476ff734.1750170418.git.luizcap@redhat.com> In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-Rspamd-Queue-Id: C55F6C000C X-Stat-Signature: 1yhg9idwfyp11ceq13bor1nh8n1nij7i X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1750170460-758928 X-HE-Meta: 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 CI82uVAD Kxdh1kOx4W+bf4LkgIQUMT5g1Rms/B1v86C4jvFINimC8BbJlIDnWP3jd8zpbHJAHmawSla2TdKJ1/oAiVwvcaamOdqG5ocKEy2eHUR+inAqet9vFGy/vgRZvrBhYcHG5AWDGAU1lZCHqll9tmL+GHHIFdJOvyRJgiBienN4vteKLGNFJQAGr8fle8L0oL1T6c9V15bO55u1tyRO1enw7idEgR5SjkXc7VYJ0tkJ5jGLNL6o0orLltAJ1vuhBcYRkxKIRfLaIkAGuwWCbIa1QyGHTpO5hd1/TnNYLL4JNQssfi9OO+x3TSx38qALYkNmYYoECmxyqURsuK3oXPxTNG/ZCarUb9TZKUYtPO/kLDCfiJUex7Ckkbn0soyfPFHGcfSkC 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 implemenetation 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 | 20 +++++++++++++ mm/debug.c | 42 +++------------------------ mm/util.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 95 insertions(+), 38 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 0ef2ba0c667a..4d83674e32a7 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -4184,4 +4184,24 @@ static inline bool page_pool_page_is_pp(struct page *page) } #endif +#define PAGE_SNAPSHOT_FAITHFUL (1 << 0) +#define PAGE_SNAPSHOT_PG_HUGE_ZERO (1 << 1) +#define PAGE_SNAPSHOT_PG_FREE (1 << 2) +#define PAGE_SNAPSHOT_PG_IDLE (1 << 3) + +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 & 0x1; +} + +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..8c56e10aca5b 100644 --- a/mm/util.c +++ b/mm/util.c @@ -1171,3 +1171,74 @@ 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) +{ + if (is_huge_zero_folio(folio)) + ps->flags |= PAGE_SNAPSHOT_PG_HUGE_ZERO; + if (folio_ref_count(folio) == 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)); +} -- 2.49.0