[PATCH v2] slub: Clear __GFP_COMP flag when allocating 0 order page

[Haifeng Xu]

We encounter warning messages when failing to create a new slab like
this:

page allocation failure: order:0, mode:0x1004000(GFP_NOWAIT|__GFP_COMP),
nodemask=(null)

It's a bit confusing for users because __GFP_COMP flag is used to create
compound page which implies the order should not be 0. This is because
minimum order will be tried if higher-order allocation fails and the
minimum order is 0. It's pointless to allocate 0 order page with __GFP_COMP
flag. Therefore, clear the __GFP_COMP flag when falling back to 0 order
allocation which makes the order and gfp flags matched.

Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
Suggested-by: Christoph Lameter (Ampere) <cl@xxxxxxxxx>
---
Changes since v1:
- Place the check in allocate_slab() where the minimum order allocation
  is performed.
---
 mm/slub.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index a307d319e82c..e7bf1a1a31a8 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2027,6 +2027,15 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 	if (unlikely(!slab)) {
 		oo = s->min;
 		alloc_gfp = flags;
+		unsigned int order = oo_order(oo);
+
+		/*
+		 * If fallback to the minimum order allocation and
+		 * the order is 0, clear the __GFP_COMP flag.
+		 */
+		if (order == 0)
+			alloc_gfp = alloc_gfp & ~__GFP_COMP;
+
 		/*
 		 * Allocation may have failed due to fragmentation.
 		 * Try a lower order alloc if possible
-- 
2.25.1