Re: cgroups: warning for metadata allocation with GFP_NOFAIL (was Re: folio_alloc_buffers() doing allocations > order 1 with GFP_NOFAIL)

[Roman Gushchin <roman.gushchin@linux.dev>]

On Mon, Nov 06, 2023 at 06:57:05PM -0800, Christoph Lameter wrote:
> Right.. Well lets add the cgoup folks to this.

Hello!

I think it's the best thing we can do now. Thoughts?

From 5ed3e88f4f052b6ce8dbec0545dfc80eb7534a1a Mon Sep 17 00:00:00 2001
From: Roman Gushchin <roman.gushchin@linux.dev>
Date: Tue, 7 Nov 2023 09:18:02 -0800
Subject: [PATCH] mm: kmem: drop __GFP_NOFAIL when allocating objcg vectors

Objcg vectors attached to slab pages to store slab object ownership
information are allocated using gfp flags for the original slab
allocation. Depending on slab page order and the size of slab objects,
objcg vector can take several pages.

If the original allocation was done with the __GFP_NOFAIL flag, it
triggered a warning in the page allocation code. Indeed, order > 1
pages should not been allocated with the __GFP_NOFAIL flag.

Fix this by simple dropping the __GFP_NOFAIL flag when allocating
the objcg vector. It effectively allows to skip the accounting of a
single slab object under a heavy memory pressure.

An alternative would be to implement the mechanism to fallback to
order-0 allocations for accounting metadata, which is also not perfect
because it will increase performance penalty and memory footprint
of the kernel memory accounting under memory pressure.

Reported-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Matthew Wilcox <willy@infradead.org>
---
 mm/memcontrol.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 774bd6e21e27..1c1061df9cd1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2936,7 +2936,8 @@ void mem_cgroup_commit_charge(struct folio *folio, struct mem_cgroup *memcg)
  * Moreover, it should not come from DMA buffer and is not readily
  * reclaimable. So those GFP bits should be masked off.
  */
-#define OBJCGS_CLEAR_MASK	(__GFP_DMA | __GFP_RECLAIMABLE | __GFP_ACCOUNT)
+#define OBJCGS_CLEAR_MASK	(__GFP_DMA | __GFP_RECLAIMABLE | \
+				 __GFP_ACCOUNT | __GFP_NOFAIL)
 
 /*
  * mod_objcg_mlstate() may be called with irq enabled, so
-- 
2.42.0