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

[hu.shengming]

Harry wrote:
> 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 confirming this.

> > 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.

Agreed. I will keep this patch focused on preventing the refill from
using pfmemalloc reserves and update it to use:

gfp | __GFP_NOMEMALLOC | __GFP_NOWARN

The handling of __GFP_NOFAIL in refill_sheaf() is a broader issue and
can be considered separately.

Thanks for the clarification!

--
With Best Regards,
Shengming