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

[John Hubbard <jhubbard@nvidia.com>]

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.)

In order to avoid this kind of crash, make dump_page() slightly more
robust, by providing a version of compound_page() that doesn't assert,
but just warns the first time that such a thing happens. And the first
time is usually enough.

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@lca.pw>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h | 17 +++++++++++++++--
 mm/debug.c         |  4 ++--
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dc7b87310c10..e3991fbb42c0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -779,6 +779,13 @@ 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 __compound_mapcount(struct page *page)
+{
+	page = compound_head(page);
+	return atomic_read(compound_mapcount_ptr(page)) + 1;
+}
+
 /*
  * Mapcount of compound page as a whole, does not include mapped sub-pages.
  *
@@ -787,8 +794,14 @@ extern void kvfree_sensitive(const void *addr, size_t len);
 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 __compound_mapcount(page);
+}
+
+static inline int dump_page_compound_mapcount(struct page *page)
+{
+	if (WARN_ON_ONCE(!PageCompound(page)))
+		return 0;
+	return __compound_mapcount(page);
 }
 
 /*
diff --git a/mm/debug.c b/mm/debug.c
index 4f376514744d..eab4244aabd8 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -91,7 +91,7 @@ void __dump_page(struct page *page, const char *reason)
 				"compound_mapcount:%d compound_pincount:%d\n",
 				page, page_ref_count(head), mapcount,
 				mapping, page_to_pgoff(page), head,
-				compound_order(head), compound_mapcount(page),
+				compound_order(head), dump_page_compound_mapcount(page),
 				compound_pincount(page));
 		} else {
 			pr_warn("page:%px refcount:%d mapcount:%d mapping:%p "
@@ -99,7 +99,7 @@ void __dump_page(struct page *page, const char *reason)
 				"compound_mapcount:%d\n",
 				page, page_ref_count(head), mapcount,
 				mapping, page_to_pgoff(page), head,
-				compound_order(head), compound_mapcount(page));
+				compound_order(head), dump_page_compound_mapcount(page));
 		}
 	else
 		pr_warn("page:%px refcount:%d mapcount:%d mapping:%p index:%#lx\n",
-- 
2.28.0