Re: [PATCH 1/2] mm: slub: allocate slab object extensions non-contiguously

[Suren Baghdasaryan <surenb@google.com>]

On Tue, May 20, 2025 at 7:13 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 20/05/2025 14:46, Usama Arif wrote:
> >
> >
> > On 20/05/2025 14:44, Kent Overstreet wrote:
> >> On Tue, May 20, 2025 at 01:25:46PM +0100, Usama Arif wrote:
> >>> When memory allocation profiling is running on memory bound services,
> >>> allocations greater than order 0 for slab object extensions can fail,
> >>> for e.g. zs_handle zswap slab which will be 512 objsperslab x 16 bytes
> >>> per slabobj_ext (order 1 allocation). Use kvcalloc to improve chances
> >>> of the allocation being successful.
> >>>
> >>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> >>> Reported-by: Vlad Poenaru <vlad.wing@gmail.com>
> >>> Closes: https://lore.kernel.org/all/17fab2d6-5a74-4573-bcc3-b75951508f0a@gmail.com/
> >>> ---
> >>>  mm/slub.c | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/mm/slub.c b/mm/slub.c
> >>> index dc9e729e1d26..bf43c403ead2 100644
> >>> --- a/mm/slub.c
> >>> +++ b/mm/slub.c
> >>> @@ -1989,7 +1989,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> >>>     gfp &= ~OBJCGS_CLEAR_MASK;
> >>>     /* Prevent recursive extension vector allocation */
> >>>     gfp |= __GFP_NO_OBJ_EXT;
> >>> -   vec = kcalloc_node(objects, sizeof(struct slabobj_ext), gfp,
> >>> +   vec = kvcalloc_node(objects, sizeof(struct slabobj_ext), gfp,
> >>>                        slab_nid(slab));
> >>
> >> And what's the latency going to be on a vmalloc() allocation when we're
> >> low on memory?
> >
> > Would it not be better to get the allocation slighly slower than to not get
> > it at all?
>
> Also a majority of them are less than 1 page. kvmalloc of less than 1 page
> falls back to kmalloc. So vmalloc will only be on those greater than 1 page
> size, which are in the minority (for e.g. zs_handle, request_sock_subflow_v6,
> request_sock_subflow_v4...).

Not just the majority. For all of these kvmalloc allocations kmalloc
will be tried first and vmalloc will be used only if the former
failed: https://elixir.bootlin.com/linux/v6.14.7/source/mm/util.c#L665
That's why I think this should not regress normal case when slab has
enough space to satisfy the allocation.