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
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
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: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/debug.c b/mm/debug.c
index 4f37651..567b80b 100644 (file)
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -69,8 +69,19 @@ void __dump_page(struct page *page, const char *reason)
        }
 
        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 {