Re: [PATCH v2 1/2] mm, slab: move memcg charging to post-alloc hook

[Vlastimil Babka <vbabka@suse.cz>]

On 4/3/24 1:39 PM, Aishwarya TCV wrote:
> 
> 
> On 25/03/2024 08:20, Vlastimil Babka wrote:
>> The MEMCG_KMEM integration with slab currently relies on two hooks
>> during allocation. memcg_slab_pre_alloc_hook() determines the objcg and
>> charges it, and memcg_slab_post_alloc_hook() assigns the objcg pointer
>> to the allocated object(s).
>> 
>> As Linus pointed out, this is unnecessarily complex. Failing to charge
>> due to memcg limits should be rare, so we can optimistically allocate
>> the object(s) and do the charging together with assigning the objcg
>> pointer in a single post_alloc hook. In the rare case the charging
>> fails, we can free the object(s) back.
>> 
>> This simplifies the code (no need to pass around the objcg pointer) and
>> potentially allows to separate charging from allocation in cases where
>> it's common that the allocation would be immediately freed, and the
>> memcg handling overhead could be saved.
>> 
>> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
>> Link: https://lore.kernel.org/all/CAHk-=whYOOdM7jWy5jdrAm8LxcgCMFyk2bt8fYYvZzM4U-zAQA@mail.gmail.com/
>> Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
>> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>>  mm/slub.c | 180 +++++++++++++++++++++++++++-----------------------------------
>>  1 file changed, 77 insertions(+), 103 deletions(-)
> 
> Hi Vlastimil,
> 
> When running the LTP test "memcg_limit_in_bytes" against next-master
> (next-20240402) kernel with Arm64 on JUNO, oops is observed in our CI. I
> can send the full logs if required. It is observed to work fine on
> softiron-overdrive-3000.
> 
> A bisect identified 11bb2d9d91627935c63ea3e6a031fd238c846e1 as the first
> bad commit. Bisected it on the tag "next-20240402" at repo
> "https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git".
> 
> This works fine on  Linux version v6.9-rc2

Oops, sorry, can you verify that this fixes it?
Thanks.

----8<----
From b0597c220624fef4f10e26079a3ff1c86f02a12b Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Wed, 3 Apr 2024 17:45:15 +0200
Subject: [PATCH] fixup! mm, slab: move memcg charging to post-alloc hook

The call to memcg_alloc_abort_single() is wrong, it expects a pointer to
single object, not an array.

Reported-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index f5b151a58b7d..b32e79629ae7 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2100,7 +2100,7 @@ bool memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
 		return true;
 
 	if (likely(size == 1)) {
-		memcg_alloc_abort_single(s, p);
+		memcg_alloc_abort_single(s, *p);
 		*p = NULL;
 	} else {
 		kmem_cache_free_bulk(s, size, p);
-- 
2.44.0