mm, debug: print raw struct page data in __dump_page()

__dump_page() is used when a page metadata inconsistency is detected,
either by standard runtime checks, or extra checks in CONFIG_DEBUG_VM
builds.  It prints some of the relevant metadata, but not the whole
struct page, which is based on unions and interpretation is dependent on
the context.

This means that sometimes e.g.  a VM_BUG_ON_PAGE() checks certain field,
which is however not printed by __dump_page() and the resulting bug
report may then lack clues that could help in determining the root
cause.  This patch solves the problem by simply printing the whole
struct page word by word, so no part is missing, but the interpretation
of the data is left to developers.  This is similar to e.g.  x86_64 raw
stack dumps.

Example output:

 page:ffffea00000475c0 count:1 mapcount:0 mapping:          (null) index:0x0
 flags: 0x100000000000400(reserved)
 raw: 0100000000000400 0000000000000000 0000000000000000 00000001ffffffff
 raw: ffffea00000475e0 ffffea00000475e0 0000000000000000 0000000000000000
 page dumped because: VM_BUG_ON_PAGE(1)

[[email protected]: suggested print_hex_dump()]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Acked-by: Andrey Ryabinin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/debug.c

@@ -59,6 +59,10 @@ void __dump_page(struct page *page, const char *reason)
 
 	pr_emerg("flags: %#lx(%pGp)\n", page->flags, &page->flags);
 
+	print_hex_dump(KERN_ALERT, "raw: ", DUMP_PREFIX_NONE, 32,
+			sizeof(unsigned long), page,
+			sizeof(struct page), false);
+
 	if (reason)
 		pr_alert("page dumped because: %s\n", reason);