Re: [PATCH 4/8] mm: Add __dump_folio()

[Matthew Wilcox <willy@infradead.org>]

Further testing revealed some more problems.  We were getting confused
between various different pointers leading to spurious messages about
the page not matching the folio and passing the wrong pointer to
__dump_folio().  Here's a fix-3 patch which I tested like so:

+static int __init page_dump(void)
+{
+       struct page *page;
+
+       printk("testing page dump\n");
+
+       page = alloc_page(GFP_KERNEL);
+       dump_page(page, "single");
+       put_page(page);
+       page = alloc_pages(GFP_KERNEL | __GFP_COMP, 2);
+       dump_page(page, "head");
+       dump_page(page + 1, "tail 1");
+       dump_page(page + 2, "tail 2");
+       dump_page(page + 3, "tail 3");
+       put_page(page);
+
+       return 0;
+}
+
+module_init(page_dump);

(needed some extra debug to check the values of the pointers being
passed to __dump_folio() which we wouldn't want to include)

---
 mm/debug.c | 36 ++++++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 14 deletions(-)

diff --git a/mm/debug.c b/mm/debug.c
index e7aa8a9d5d86..4dae73bc4530 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -121,23 +121,31 @@ static void __dump_page(const struct page *page)
 again:
 	memcpy(&precise, page, sizeof(*page));
 	foliop = page_folio(&precise);
-	idx = folio_page_idx(foliop, page);
-	if (idx != 0) {
-		if (idx < MAX_FOLIO_NR_PAGES) {
-			memcpy(&folio, foliop, 2 * sizeof(struct page));
-			nr_pages = folio_nr_pages(&folio);
-		}
+	if (foliop == (struct folio *)&precise) {
+		idx = 0;
+		if (!folio_test_large(foliop))
+			goto dump;
+		foliop = (struct folio *)page;
+	} else {
+		idx = folio_page_idx(foliop, page);
+	}
 
-		if (idx > nr_pages) {
-			if (loops-- > 0)
-				goto again;
-			printk("page does not match folio\n");
-			precise.compound_head &= ~1UL;
-			foliop = (struct folio *)&precise;
-			idx = 0;
-		}
+	if (idx < MAX_FOLIO_NR_PAGES) {
+		memcpy(&folio, foliop, 2 * sizeof(struct page));
+		nr_pages = folio_nr_pages(&folio);
+		foliop = &folio;
+	}
+
+	if (idx > nr_pages) {
+		if (loops-- > 0)
+			goto again;
+		printk("page does not match folio\n");
+		precise.compound_head &= ~1UL;
+		foliop = (struct folio *)&precise;
+		idx = 0;
 	}
 
+dump:
 	__dump_folio(foliop, &precise, pfn, idx);
 }
 
-- 
2.43.0