Re: SLAB_NO_CMPXCHG was:: [PATCH 6/6] slab: Introduce kmalloc_nolock() and kfree_nolock().

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

On Wed, Jun 25, 2025 at 4:38 AM Harry Yoo <harry.yoo@oracle.com> wrote:
>
> On Tue, Jun 24, 2025 at 10:13:49AM -0700, Alexei Starovoitov wrote:
> > On Thu, May 8, 2025 at 6:03 PM Harry Yoo <harry.yoo@oracle.com> wrote:
> > > > +     s = kmalloc_slab(size, NULL, alloc_gfp, _RET_IP_);
> > > > +
> > > > +     if (!(s->flags & __CMPXCHG_DOUBLE))
> > > > +             /*
> > > > +              * kmalloc_nolock() is not supported on architectures that
> > > > +              * don't implement cmpxchg16b.
> > > > +              */
> > > > +             return NULL;
> > >
> > > Hmm when someone uses slab debugging flags (e.g., passing boot
> > > parameter slab_debug=FPZ as a hardening option on production [1], or
> > > just for debugging), __CMPXCHG_DOUBLE is not set even when the arch
> > > supports it.
> > >
> > > Is it okay to fail all kmalloc_nolock() calls in such cases?
> >
> > I studied the code and the git history.
> > Looks like slub doesn't have to disable cmpxchg mode when slab_debug is on.
>
> A slight correction; Debug caches do not use cmpxchg mode at all by
> design. If a future change enables cmpxchg mode for them, it will cause
> the same consistency issue.
>
> > The commit 41bec7c33f37 ("mm/slub: remove slab_lock() usage for debug
> > operations")
> > removed slab_lock from debug validation checks.
>
> An excellent point!
>
> Yes, SLUB does not maintain cpu slab and percpu partial slabs on
> debug caches.

Ohh. That was a crucial detail I was missing.
Now I see that 90% of ___slab_alloc() logic is not executed
for debug slabs :)

> Alloc/free is done under n->list_lock, so no need for
> cmpxchg double and slab_lock() at all :)
>
> > So right now slab_lock() only serializes slab->freelist/counter update.
> > It's still necessary on arch-s that don't have cmpxchg, but that's it.
> > Only __update_freelist_slow() is using it.
>
> Yes.
>
> > The comment next to SLAB_NO_CMPXCHG is obsolete as well.
> > It's been there since days that slab_lock() was taken during
> > consistency checks.
>
> Yes.
>
> > I think the following diff is appropriate:
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 044e43ee3373..9d615cfd1b6f 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -286,14 +286,6 @@ static inline bool
> > kmem_cache_has_cpu_partial(struct kmem_cache *s)
> >  #define DEBUG_DEFAULT_FLAGS (SLAB_CONSISTENCY_CHECKS | SLAB_RED_ZONE | \
> >                                 SLAB_POISON | SLAB_STORE_USER)
> >
> > -/*
> > - * These debug flags cannot use CMPXCHG because there might be consistency
> > - * issues when checking or reading debug information
> > - */
> > -#define SLAB_NO_CMPXCHG (SLAB_CONSISTENCY_CHECKS | SLAB_STORE_USER | \
> > -                               SLAB_TRACE)
> > -
> > -
> >
> >  /*
> >   * Debugging flags that require metadata to be stored in the slab.  These get
> >   * disabled when slab_debug=O is used and a cache's min order increases with
> > @@ -6654,7 +6646,7 @@ int do_kmem_cache_create(struct kmem_cache *s,
> > const char *name,
> >         }
> >
> >  #ifdef system_has_freelist_aba
> > -       if (system_has_freelist_aba() && !(s->flags & SLAB_NO_CMPXCHG)) {
> > +       if (system_has_freelist_aba()) {
> >                 /* Enable fast mode */
> >                 s->flags |= __CMPXCHG_DOUBLE;
> >         }
> >
> > It survived my stress tests.
> > Thoughts?
>
> Perhaps it's better to change the condition in
> kmalloc_nolock_noprof() from;
>
>     if (!(s->flags & __CMPXCHG_DOUBLE))
>         return NULL;
>
> to;
>
>     if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s))
>         return NULL;
>
> Because debug caches do not use cmpxchg double (and that's why
> it survived your test), it is not accurate to set __CMPXCHG_DOUBLE.
>
> And it'll get into trouble anyway if debug caches use cmpxchg double.

All makes sense. Tested this suggestion. Indeed behaves as
you described :) It's quite a relief, since I was also worried
that __CMPXCHG_DOUBLE limitation of kmalloc_nolock() might hurt
debug_slab cases.

Only slub_tiny case left to do before I can post v2.