From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx: augmented code comment from John] Link: http://lkml.kernel.org/r/15cff11a-6762-8a6a-3f0e-dd227280cd6f@xxxxxxxxxx Link: http://lkml.kernel.org/r/20200709202117.7216-1-willy@xxxxxxxxxxxxx Link: http://lkml.kernel.org/r/20200709202117.7216-2-willy@xxxxxxxxxxxxx Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Cc: William Kucharski <william.kucharski@xxxxxxxxxx> Cc: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- 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 { _
