[044/163] mm/debug: handle page->mapping better in dump_page

Commit Message

Andrew Morton Aug. 7, 2020, 6:19 a.m. UTC

From: "Matthew Wilcox (Oracle)" <[e-mail]>
Subject: mm/debug: handle page->mapping better in dump_page

Patch series "Improvements for dump_page()", v2.

Here's a sample dump of a pagecache tail page with all of the patches
applied:

page:000000006d1c49ca refcount:6 mapcount:0 mapping:00000000136b8d90 index:0x109 pfn:0x6c645
head:000000008bd38076 order:2 compound_mapcount:0 compound_pincount:0
aops:xfs_address_space_operations ino:800042 dentry name:"fd"
flags: 0x4000000000012014(uptodate|lru|private|head)
raw: 4000000000000000 ffffd46ac1b19101 ffffffff00000202 dead000000000004
raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
head: 4000000000012014 ffffd46ac1b1bbc8 ffffd46ac1b1bc08 ffff91976f659560
head: 0000000000000108 ffff919773220680 00000006ffffffff 0000000000000000
page dumped because: testing


This patch (of 6):

If we can't call page_mapping() to get the page mapping, handle the
anon/ksm/movable bits correctly.

[[e-mail]: augmented code comment from John]
  Link: http://lkml.kernel.org/r/[e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Acked-by: Mike Rapoport <[e-mail]>
Reviewed-by: John Hubbard <[e-mail]>
Acked-by: Vlastimil Babka <[e-mail]>
Cc: William Kucharski <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/debug.c |   15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

Patch

--- a/mm/debug.c~mm-handle-page-mapping-better-in-dump_page
+++ a/mm/debug.c
@@ -69,8 +69,19 @@  void __dump_page(struct page *page, cons
 	}
 
 	if (page < head || (page >= head + MAX_ORDER_NR_PAGES)) {
-		/* Corrupt page, cannot call page_mapping */
-		mapping = page->mapping;
+		/*
+		 * Corrupt page, so we cannot call page_mapping. Instead, do a
+		 * safe subset of the steps that page_mapping() does. Caution:
+		 * this will be misleading for tail pages, PageSwapCache pages,
+		 * and potentially other situations. (See the page_mapping()
+		 * implementation for what's missing here.)
+		 */
+		unsigned long tmp = (unsigned long)page->mapping;
+
+		if (tmp & PAGE_MAPPING_ANON)
+			mapping = NULL;
+		else
+			mapping = (void *)(tmp & ~PAGE_MAPPING_FLAGS);
 		head = page;
 		compound = false;
 	} else {