[RFC 4/8] mm: remove set_page_count() from page_frag_alloc_align

[Pasha Tatashin <pasha.tatashin@soleen.com>]

set_page_count() unconditionally resets the value of _ref_count and that
is dangerous, as it is not programmatically verified. Instead we rely on
comments like: "OK, page count is 0, we can safely set it".

Add a new refcount function: page_ref_add_return() to return the new
refcount value after adding to it. Use the return value to verify that
the _ref_count was indeed what we expected.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
 include/linux/page_ref.h | 13 +++++++++++++
 mm/page_alloc.c          |  6 ++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index b3ec2b231fc7..db7ccb461c3e 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -88,6 +88,19 @@ static inline void init_page_count(struct page *page)
 	set_page_count(page, 1);
 }
 
+static inline int page_ref_add_return(struct page *page, int nr)
+{
+	int ret;
+
+	VM_BUG_ON(nr <= 0);
+	ret = atomic_add_return(nr, &page->_refcount);
+	VM_BUG_ON_PAGE(ret <= 0, page);
+
+	if (page_ref_tracepoint_active(page_ref_mod_and_return))
+		__page_ref_mod_and_return(page, nr, ret);
+	return ret;
+}
+
 static inline void page_ref_add(struct page *page, int nr)
 {
 	int ret;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b37435c274cf..6af4596bddc2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5510,6 +5510,7 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
 	unsigned int size = PAGE_SIZE;
 	struct page *page;
 	int offset;
+	int refcnt;
 
 	if (unlikely(!nc->va)) {
 refill:
@@ -5548,8 +5549,9 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
 		/* if size can vary use size else just use PAGE_SIZE */
 		size = nc->size;
 #endif
-		/* OK, page count is 0, we can safely set it */
-		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
+		/* page count is 0, set it to PAGE_FRAG_CACHE_MAX_SIZE + 1 */
+		refcnt = page_ref_add_return(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
+		VM_BUG_ON_PAGE(refcnt != PAGE_FRAG_CACHE_MAX_SIZE + 1, page);
 
 		/* reset page count bias and offset to start of new frag */
 		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
-- 
2.33.0.1079.g6e70778dc9-goog