mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact()

Commit Message

Vlastimil Babka March 14, 2019, 9:39 a.m. UTC

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 make 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 <[e-mail]>
---
 mm/page_alloc.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Patch

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0b9f577b1a2a..3127d47afaa7 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))
+		gfp_mask &= ~__GFP_COMP;
+
 	return make_alloc_exact(addr, order, size);
 }
 EXPORT_SYMBOL(alloc_pages_exact);