[PATCH v2] mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact()

[Vlastimil Babka <vbabka@suse.cz>]

alloc_pages_exact*() allocates a page of sufficient order and then splits it
to return only the number of pages requested. That makes it incompatible with
__GFP_COMP, because compound pages cannot be split.

As shown by [1] things may silently work until the requested size (possibly
depending on user) stops being power of two. Then for CONFIG_DEBUG_VM, BUG_ON()
triggers in split_page(). Without CONFIG_DEBUG_VM, consequences are unclear.

There are several options here, none of them great:

1) Don't do the spliting when __GFP_COMP is passed, and return the whole
compound page. However if caller then returns it via free_pages_exact(),
that will be unexpected and the freeing actions there will be wrong.

2) Warn and remove __GFP_COMP from the flags. But the caller wanted it, so
things may break later somewhere.

3) Warn and return NULL. However NULL may be unexpected, especially for
small sizes.

This patch picks option 3, as it's best defined.

[1] https://lore.kernel.org/lkml/20181126002805.GI18977@shao2-debian/T/#u

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
Sent v1 before amending commit, sorry.

 mm/page_alloc.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0b9f577b1a2a..dd3f89e8f88d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4752,7 +4752,7 @@ static void *make_alloc_exact(unsigned long addr, unsigned int order,
 /**
  * alloc_pages_exact - allocate an exact number physically-contiguous pages.
  * @size: the number of bytes to allocate
- * @gfp_mask: GFP flags for the allocation
+ * @gfp_mask: GFP flags for the allocation, must not contain __GFP_COMP
  *
  * This function is similar to alloc_pages(), except that it allocates the
  * minimum number of pages to satisfy the request.  alloc_pages() can only
@@ -4768,6 +4768,10 @@ void *alloc_pages_exact(size_t size, gfp_t gfp_mask)
 	unsigned long addr;
 
 	addr = __get_free_pages(gfp_mask, order);
+
+	if (WARN_ON_ONCE(gfp_mask & __GFP_COMP))
+		return NULL;
+
 	return make_alloc_exact(addr, order, size);
 }
 EXPORT_SYMBOL(alloc_pages_exact);
@@ -4777,7 +4781,7 @@ EXPORT_SYMBOL(alloc_pages_exact);
  *			   pages on a node.
  * @nid: the preferred node ID where memory should be allocated
  * @size: the number of bytes to allocate
- * @gfp_mask: GFP flags for the allocation
+ * @gfp_mask: GFP flags for the allocation, must not contain __GFP_COMP
  *
  * Like alloc_pages_exact(), but try to allocate on node nid first before falling
  * back.
@@ -4785,7 +4789,12 @@ EXPORT_SYMBOL(alloc_pages_exact);
 void * __meminit alloc_pages_exact_nid(int nid, size_t size, gfp_t gfp_mask)
 {
 	unsigned int order = get_order(size);
-	struct page *p = alloc_pages_node(nid, gfp_mask, order);
+	struct page *p;
+
+	if (WARN_ON_ONCE(gfp_mask & __GFP_COMP))
+		return NULL;
+
+	p = alloc_pages_node(nid, gfp_mask, order);
 	if (!p)
 		return NULL;
 	return make_alloc_exact((unsigned long)page_address(p), order, size);
-- 
2.20.1