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

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Message-ID: <CAJuCfpHqGQKgU=rnJbZnbyTs3vKL-gEjLp1Yw1idWUzdkjZsLA@mail.gmail.com>
Date: Tue, 20 May 2025 10:20:23 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Usama Arif <usamaarif642@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	hannes@...xchg.org, shakeel.butt@...ux.dev, vlad.wing@...il.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH 1/2] mm: slub: allocate slab object extensions non-contiguously

On Tue, May 20, 2025 at 9:41 AM Kent Overstreet
<kent.overstreet@...ux.dev> wrote:
>
> On Tue, May 20, 2025 at 08:20:38AM -0700, Suren Baghdasaryan wrote:
> > On Tue, May 20, 2025 at 7:13 AM Usama Arif <usamaarif642@...il.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@...il.com>
> > > >>> Reported-by: Vlad Poenaru <vlad.wing@...il.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.
>
> And you really should consider just letting the extension vector
> allocation fail if we're under that much memory pressure.

I see your point. One case we would want to use vmalloc is if the
allocation is sizable (multiple pages), so failing it does not mean
critical memory pressure level yet. I don't think today's extension
vectors would be large enough to span multiple pages. That would
require a rather large obj_per_slab and in most cases I think this
change would not affect current behavior, the allocations will be
smaller than PAGE_SIZE and kvmalloc will fail anyway.
I guess the question is whether we want to fail if allocation size is
higher than PAGE_SIZE but less than PAGE_ALLOC_COSTLY_ORDER. Failing
that I think is reasonable and I don't think any extension vector will
be large enough to reach PAGE_ALLOC_COSTLY_ORDER. So, I'm ok with
dropping this part of the patchset.

>
> Failing allocations is an important mechanism for load shedding,
> otherwise stuff just piles up - it's a big cause of our terrible
> behaviour when we're thrashing.
>
> It's equivalent to bufferbloat in the networking world.