Re: [PATCH RFC 10/19] slab: remove cpu (partial) slabs usage from allocation paths

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, Oct 30, 2025 at 6:09 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 10/30/25 05:32, Harry Yoo wrote:
> > On Thu, Oct 23, 2025 at 03:52:32PM +0200, Vlastimil Babka wrote:
> >> diff --git a/mm/slub.c b/mm/slub.c
> >> index e2b052657d11..bd67336e7c1f 100644
> >> --- a/mm/slub.c
> >> +++ b/mm/slub.c
> >> @@ -4790,66 +4509,15 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> >>
> >>      stat(s, ALLOC_SLAB);
> >>
> >> -    if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
> >> -            freelist = alloc_single_from_new_slab(s, slab, orig_size, gfpflags);
> >> -
> >> -            if (unlikely(!freelist))
> >> -                    goto new_objects;
> >> -
> >> -            if (s->flags & SLAB_STORE_USER)
> >> -                    set_track(s, freelist, TRACK_ALLOC, addr,
> >> -                              gfpflags & ~(__GFP_DIRECT_RECLAIM));
> >> -
> >> -            return freelist;
> >> -    }
> >> -
> >> -    /*
> >> -     * No other reference to the slab yet so we can
> >> -     * muck around with it freely without cmpxchg
> >> -     */
> >> -    freelist = slab->freelist;
> >> -    slab->freelist = NULL;
> >> -    slab->inuse = slab->objects;
> >> -    slab->frozen = 1;
> >> -
> >> -    inc_slabs_node(s, slab_nid(slab), slab->objects);
> >> +    freelist = alloc_single_from_new_slab(s, slab, orig_size, gfpflags);
> >>
> >> -    if (unlikely(!pfmemalloc_match(slab, gfpflags) && allow_spin)) {
> >> -            /*
> >> -             * For !pfmemalloc_match() case we don't load freelist so that
> >> -             * we don't make further mismatched allocations easier.
> >> -             */
> >> -            deactivate_slab(s, slab, get_freepointer(s, freelist));
> >> -            return freelist;
> >> -    }
> >> +    if (unlikely(!freelist))
> >> +            goto new_objects;
> >
> > We may end up in an endless loop in !allow_spin case?
> > (e.g., kmalloc_nolock() is called in NMI context and n->list_lock is
> > held in the process context on the same CPU)
> >
> > Allocate a new slab, but somebody is holding n->list_lock, so trylock fails,
> > free the slab, goto new_objects, and repeat.
>
> Ugh, yeah. However, AFAICS this possibility already exists prior to this
> patch, only it's limited to SLUB_TINY/kmem_cache_debug(s). But we should fix
> it in 6.18 then.
> How? Grab the single object and defer deactivation of the slab minus one
> object? Would work except for kmem_cache_debug(s) we open again a race for
> inconsistency check failure, and we have to undo the simple slab freeing fix
>  and handle the accounting issue differently again.
> Fail the allocation for the debug case to avoid the consistency check
> issues? Would it be acceptable for kmalloc_nolock() users?

You mean something like:
diff --git a/mm/slub.c b/mm/slub.c
index a8fcc7e6f25a..e9a8b75f31d7 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4658,8 +4658,11 @@ static void *___slab_alloc(struct kmem_cache
*s, gfp_t gfpflags, int node,
        if (kmem_cache_debug(s)) {
                freelist = alloc_single_from_new_slab(s, slab,
orig_size, gfpflags);

-               if (unlikely(!freelist))
+               if (unlikely(!freelist)) {
+                       if (!allow_spin)
+                               return NULL;
                        goto new_objects;
+               }

or I misunderstood the issue?