mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact()

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 splitting 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 may have
really 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 2, because as Michal Hocko put it: "callers wanted
it" is much less probable than "caller is simply confused and more gfp
flags is surely better than fewer".

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

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Vlastimil Babka <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Acked-by: Mel Gorman <[e-mail]>
Cc: Takashi Iwai <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/page_alloc.c

···
 /**
  * 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
···
 	unsigned int order = get_order(size);
 	unsigned long addr;
 
+	if (WARN_ON_ONCE(gfp_mask & __GFP_COMP))
+		gfp_mask &= ~__GFP_COMP;
+
 	addr = __get_free_pages(gfp_mask, order);
 	return make_alloc_exact(addr, order, size);
 }
···
  *			   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.
···
 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))
+		gfp_mask &= ~__GFP_COMP;
+
+	p = alloc_pages_node(nid, gfp_mask, order);
 	if (!p)
 		return NULL;
 	return make_alloc_exact((unsigned long)page_address(p), order, size);