[PATCH v2] mm, dump_page: do not crash with bad compound_mapcount()

[John Hubbard]

If a compound page is being split while dump_page() is being run on that
page, we can end up calling compound_mapcount() on a page that is no
longer compound. This leads to a crash (already seen at least once in
the field), due to the VM_BUG_ON_PAGE() assertion inside
compound_mapcount().

(The above is from Matthew Wilcox's analysis of Qian Cai's bug report.)

A similar problem is possible, via compound_pincount() instead of
compound_mapcount().

In order to avoid this kind of crash, make dump_page() slightly more
robust, by providing a pair of simpler routines that don't contain
assertions: head_mapcount() and head_pincount().

For debug tools, we don't want to go *too* far in this direction, but
this is a simple small fix, and the crash has already been seen, so it's
a good trade-off.

Reported-by: Qian Cai <cai@xxxxxx>
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
Hi,

I'm assuming that a fix is not required for -stable, but let me know if
others feel differently. The dump_page() code has changed a lot in that
area.

Changes since v1 [1]:

1) Rebased onto mmotm

2) Used a simpler head_*count() approach.

3) Added Matthew's Suggested-by: tag

4) Support pincount as well as mapcount.

[1] https://lore.kernel.org/linux-mm/20200804183943.1244828-1-jhubbard@xxxxxxxxxx/

thanks,
John Hubbard

 include/linux/mm.h | 14 ++++++++++++--
 mm/debug.c         |  6 +++---
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4f12b2465e80..8ab941cf73f4 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -776,6 +776,11 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags)
 extern void kvfree(const void *addr);
 extern void kvfree_sensitive(const void *addr, size_t len);
 
+static inline int head_mapcount(struct page *head)
+{
+	return atomic_read(compound_mapcount_ptr(head)) + 1;
+}
+
 /*
  * Mapcount of compound page as a whole, does not include mapped sub-pages.
  *
@@ -785,7 +790,7 @@ static inline int compound_mapcount(struct page *page)
 {
 	VM_BUG_ON_PAGE(!PageCompound(page), page);
 	page = compound_head(page);
-	return atomic_read(compound_mapcount_ptr(page)) + 1;
+	return head_mapcount(page);
 }
 
 /*
@@ -898,11 +903,16 @@ static inline bool hpage_pincount_available(struct page *page)
 	return PageCompound(page) && compound_order(page) > 1;
 }
 
+static inline int head_pincount(struct page *head)
+{
+	return atomic_read(compound_pincount_ptr(head));
+}
+
 static inline int compound_pincount(struct page *page)
 {
 	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
 	page = compound_head(page);
-	return atomic_read(compound_pincount_ptr(page));
+	return head_pincount(page);
 }
 
 static inline void set_compound_order(struct page *page, unsigned int order)
diff --git a/mm/debug.c b/mm/debug.c
index c27fff1e3ca8..69b60637112b 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -102,12 +102,12 @@ void __dump_page(struct page *page, const char *reason)
 		if (hpage_pincount_available(page)) {
 			pr_warn("head:%p order:%u compound_mapcount:%d compound_pincount:%d\n",
 					head, compound_order(head),
-					compound_mapcount(head),
-					compound_pincount(head));
+					head_mapcount(head),
+					head_pincount(head));
 		} else {
 			pr_warn("head:%p order:%u compound_mapcount:%d\n",
 					head, compound_order(head),
-					compound_mapcount(head));
+					head_mapcount(head));
 		}
 	}
 	if (PageKsm(page))
-- 
2.28.0