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Message-ID: <48d49e3371c35a2da6f4c42d5d9f14b6476ff734.1750170418.git.luizcap@redhat.com>
Date: Tue, 17 Jun 2025 10:27:08 -0400
From: Luiz Capitulino <luizcap@...hat.com>
To: david@...hat.com,
willy@...radead.org
Cc: akpm@...ux-foundation.org,
linux-kernel@...r.kernel.org,
linux-mm@...ck.org,
lcapitulino@...il.com,
shivankg@....com
Subject: [RFC 1/3] mm: introduce snapshot_page()
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 <luizcap@...hat.com>
---
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
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