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

Andrew Morton
2020-08-07T06:19:36Z

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
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.

[akpm@linux-foundation.org: augmented code comment from John]
Link: http://lkml.kernel.org/r/15cff11a-6762-8a6a-3f0e-dd227280cd6f@nvidia.com
Link: http://lkml.kernel.org/r/20200709202117.7216-1-willy@infradead.org
Link: http://lkml.kernel.org/r/20200709202117.7216-2-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- 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 {
_