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

[Matthew Wilcox <willy@infradead.org>]

On Fri, Mar 01, 2024 at 10:21:10AM +0000, Ryan Roberts wrote:
> > +	page_cma = is_migrate_cma_page(page);
> 
> Problem is here: is_migrate_cma_page() is a macro that resolves to this:

Ah, yeah, maybe somebody should be testing with CONFIG_CMA enabled.
Ahem.

> page_cma = get_pfnblock_flags_mask(page, page_to_pfn(page), MIGRATETYPE_MASK) == MIGRATE_CMA;
> 
> And since page is on the stack, page_to_pfn() gives a very wrong answer.
> 
> I confirmed that the problem goes away for both cases above, when changing the line to:
> 
> page_cma = get_pfnblock_flags_mask(page, pfn, MIGRATETYPE_MASK) == MIGRATE_CMA;

Thanks!  I think what we end up wanting is ...

From f005189ad418d05d168c0ff00daecc2a444733ef Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Date: Fri, 1 Mar 2024 16:11:20 -0500
Subject: [PATCH] mm: Fix __dump_folio

Ryan Roberts reports that (if you have CONFIG_CMA enabled), calling
__dump_folio() will panic as we call is_migrate_cma_page() with a
stack copy of struct page, which gets passed to page_to_pfn().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/mmzone.h | 3 +++
 mm/debug.c             | 4 +---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 633812a1d220..c11b7cde81ef 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -76,9 +76,12 @@ extern const char * const migratetype_names[MIGRATE_TYPES];
 #ifdef CONFIG_CMA
 #  define is_migrate_cma(migratetype) unlikely((migratetype) == MIGRATE_CMA)
 #  define is_migrate_cma_page(_page) (get_pageblock_migratetype(_page) == MIGRATE_CMA)
+#  define is_migrate_cma_folio(folio, pfn)	(MIGRATE_CMA ==		\
+	get_pfnblock_flags_mask(&folio->page, pfn, MIGRATETYPE_MASK))
 #else
 #  define is_migrate_cma(migratetype) false
 #  define is_migrate_cma_page(_page) false
+#  define is_migrate_cma_folio(folio, pfn) false
 #endif
 
 static inline bool is_migrate_movable(int mt)
diff --git a/mm/debug.c b/mm/debug.c
index 32ac7d79fd04..e7aa8a9d5d86 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -55,7 +55,6 @@ static void __dump_folio(struct folio *folio, struct page *page,
 		unsigned long pfn, unsigned long idx)
 {
 	struct address_space *mapping = folio_mapping(folio);
-	bool page_cma;
 	int mapcount = 0;
 	char *type = "";
 
@@ -98,9 +97,8 @@ static void __dump_folio(struct folio *folio, struct page *page,
 	 * state for debugging, it should be fine to accept a bit of
 	 * inaccuracy here due to racing.
 	 */
-	page_cma = is_migrate_cma_page(page);
 	pr_warn("%sflags: %pGp%s\n", type, &folio->flags,
-		page_cma ? " CMA" : "");
+		is_migrate_cma_folio(folio, pfn) ? " CMA" : "");
 	pr_warn("page_type: %pGt\n", &folio->page.page_type);
 
 	print_hex_dump(KERN_WARNING, "raw: ", DUMP_PREFIX_NONE, 32,
-- 
2.43.0