Re: [PATCH] mm/slub: prevent pfmemalloc objects from entering the barn

[Harry Yoo]

On 7/20/26 5:41 PM, hu.shengming@xxxxxxxxxx wrote:
> Hao wrote:
>> On Sun, Jul 19, 2026 at 11:37:01AM +0800, hu.shengming@xxxxxxxxxx wrote:
>>> From: Shengming Hu <hu.shengming@xxxxxxxxxx>
>>>
>>> kmem_cache_return_sheaf() may refill a partially consumed sheaf before
>>> placing it in the barn. Without an explicit restriction, this refill may
>>> draw objects from pfmemalloc slabs and consume emergency reserves.
>>>
>>> Add __GFP_NOMEMALLOC so returned sheaves are refilled only from normal
>>> memory. If that fails, flush and free the sheaf instead.
>>>
>>> Signed-off-by: Shengming Hu <hu.shengming@xxxxxxxxxx>
>>> ---
>>>  mm/slub.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index 53b4976d3831..34f17ecbde87 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -5123,7 +5123,7 @@ void kmem_cache_return_sheaf(struct kmem_cache *s, gfp_t gfp,
>>>       * simply flush and free it.
>>>       */
>>>      if (!barn || data_race(barn->nr_full) >= MAX_FULL_SHEAVES ||
>>> -        refill_sheaf(s, sheaf, gfp)) {
>>> +        refill_sheaf(s, sheaf, gfp | __GFP_NOMEMALLOC)) {
>>
>> maybe we can add __GFP_NOWARN as this is just an refilling attempt?
>> refilling failure could be acceptable.

Adding __GFP_NOMEMALLOC | __GFP_NOWARN makes sense to me.
Analogous to __pcs_replace_empty_main().

We don't really put pfmemalloc sheaves back to the barn but as you
pointed out kmem_cache_return_sheaf() could fill the non-pfmemalloc
sheaf with objects from pfmemalloc slabs.

> Thanks for the review!
> 
> Good suggestion. Since refilling the sheaf is only a best-effort attempt
> and failure is acceptable here, adding __GFP_NOWARN makes sense.
>
> After taking another look, maybe we should also clear __GFP_NOFAIL if
> it is present in the caller-provided GFP flags? Otherwise, the allocation
> could retry indefinitely, which may not be appropriate for this optional
> refill path.

Hmm, you might argue that it's not worth trying to refill the whole
sheaf w/ __GFP_NOFAIL in refill_sheaf() if e.g.) that end up reclaiming
more than one page under high memory pressure. (Just like how we clear
__GFP_NOFAIL when allocating high-order sheaves).

But that should not be part of this patch at least.

-- 
Cheers,
Harry / Hyeonggon