mm/util: 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 implementation of snapshot_page() comes from Matthew
Wilcox with suggestions for improvements from David Hildenbrand.  All bugs
and misconceptions are mine.

[luizcap@redhat.com: fix set_ps_flags() commentary]
  Link: https://lkml.kernel.org/r/d5c75701-b353-4536-a306-187fab0655b3@redhat.com
Link: https://lkml.kernel.org/r/637a03a05cb2e3df88f84ff9e9f9642374ef813a.1752499009.git.luizcap@redhat.com
Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Tested-by: Harry Yoo <harry.yoo@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 38 deletions

diff --git a/mm/debug.c b/mm/debug.c
index e2973e1b3812f..b4388f4dcd4d2 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)