compound_head() vs uninitialized struct page poisoning

[Matthew Wilcox <willy@infradead.org>]

Hi Pavel,

This strikes me as wrong:

#define PF_HEAD(page, enforce)  PF_POISONED_CHECK(compound_head(page))

If we hit a page which is poisoned, PAGE_POISON_PATTERN is ~0, so PageTail
is set, and compound_head will return() 0xfff..ffe.  PagePoisoned()
will then try to derefence that pointer and we'll get an oops that isn't
obviously PagePoisoned.

I think this should have been:

#define PF_HEAD(page, enforce)  compound_head(PF_POISONED_CHECK(page))

One could make the argument for double-checking:

#define PF_HEAD(page, enforce)  PF_POISONED_CHECK(compound_head(PF_POISONED_CHECK(page)))

but I think this is overkill; if a tail page is initialised, then there's
no way that its head page should have been uninitialised.

Would a patch something along these lines make sense?  Compile-tested only.

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 9f8712a4b1a5..1d25d0899854 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -227,16 +227,18 @@ static inline void page_init_poison(struct page *page, size_t size)
 		VM_BUG_ON_PGFLAGS(PagePoisoned(page), page);		\
 		page; })
 #define PF_ANY(page, enforce)	PF_POISONED_CHECK(page)
-#define PF_HEAD(page, enforce)	PF_POISONED_CHECK(compound_head(page))
+#define PF_HEAD(page, enforce)	compound_head(PF_POISONED_CHECK(page))
 #define PF_ONLY_HEAD(page, enforce) ({					\
+		PF_POISONED_CHECK(page);				\
 		VM_BUG_ON_PGFLAGS(PageTail(page), page);		\
-		PF_POISONED_CHECK(page); })
+		page; })
 #define PF_NO_TAIL(page, enforce) ({					\
 		VM_BUG_ON_PGFLAGS(enforce && PageTail(page), page);	\
-		PF_POISONED_CHECK(compound_head(page)); })
+		compound_head(PF_POISONED_CHECK(page)); })
 #define PF_NO_COMPOUND(page, enforce) ({				\
+		PF_POISONED_CHECK(page); 				\
 		VM_BUG_ON_PGFLAGS(enforce && PageCompound(page), page);	\
-		PF_POISONED_CHECK(page); })
+		page; })
 
 /*
  * Macros to create function definitions for page flags