Re: [PATCH 1/3] mm/slub: fix the race between validate_slab and slab_free

[Rongwei Wang]

On 6/17/22 5:40 PM, Vlastimil Babka wrote:
> On 6/8/22 14:23, Christoph Lameter wrote:
> > On Wed, 8 Jun 2022, Rongwei Wang wrote:
> >
> > > If available, I think document the issue and warn this incorrect behavior is
> > > OK. But it still prints a large amount of confusing messages, and disturbs us?
> >
> > Correct it would be great if you could fix this in a way that does not
> > impact performance.
> >
> > > > are current operations on the slab being validated.
> > > And I am trying to fix it in following way. In a short, these changes only
> > > works under the slub debug mode, and not affects the normal mode (I'm not
> > > sure). It looks not elegant enough. And if all approve of this way, I can
> > > submit the next version.
> >
> >
> > > Anyway, thanks for your time:).
> > > -wrw
> > >
> > > @@ -3304,7 +3300,7 @@ static void __slab_free(struct kmem_cache *s,
> > struct
> > > slab *slab,
> > >
> > >   {
> > >          void *prior;
> > > -       int was_frozen;
> > > +       int was_frozen, to_take_off = 0;
> > >          struct slab new;
> >
> > to_take_off has the role of !n ? Why is that needed?
> >
> > > -       do {
> > > -               if (unlikely(n)) {
> > > +               spin_lock_irqsave(&n->list_lock, flags);
> > > +               ret = free_debug_processing(s, slab, head, tail, cnt, addr);
> >
> > Ok so the idea is to take the lock only if kmem_cache_debug. That looks
> > ok. But it still adds a number of new branches etc to the free loop.
Hi, Vlastimil, sorry for missing your message long time.
> It also further complicates the already tricky code. I wonder if we should
> make more benefit from the fact that for kmem_cache_debug() caches we don't
> leave any slabs on percpu or percpu partial lists, and also in
> free_debug_processing() we aready take both list_lock and slab_lock. If we
> just did the freeing immediately there under those locks, we would be
> protected against other freeing cpus by that list_lock and don't need the
> double cmpxchg tricks.
enen, I'm not sure get your "don't need the double cmpxchg tricks" means 
completely. What you want to say is that replace cmpxchg_double_slab() 
here with following code when kmem_cache_debug(s)?

__slab_lock(slab);
if (slab->freelist == freelist_old && slab->counters == counters_old){
    slab->freelist = freelist_new;
    slab->counters = counters_new;
    __slab_unlock(slab);
    local_irq_restore(flags);
    return true;
}
__slab_unlock(slab);

If I make mistakes for your words, please let me know.
Thanks!
> What about against allocating cpus? More tricky as those will currently end
> up privatizing the freelist via get_partial(), only to deactivate it again,
> so our list_lock+slab_lock in freeing path would not protect in the
> meanwhile. But the allocation is currently very inefficient for debug
> caches, as in get_partial() it will take the list_lock to take the slab from
> partial list and then in most cases again in deactivate_slab() to return it.
It seems that I need speed some time to eat these words. Anyway, thanks.
> If instead the allocation path for kmem_cache_debug() cache would take a
> single object from the partial list (not whole freelist) under list_lock, it
> would be ultimately more efficient, and protect against freeing using
> list_lock. Sounds like an idea worth trying to me?

Hyeonggon had a similar advice that split freeing and allocating slab 
from debugging, likes below:


__slab_alloc() {
    if (kmem_cache_debug(s))
        slab_alloc_debug()
    else
        ___slab_alloc()
}

I guess that above code aims to solve your mentioned problem (idea)?

slab_free() {
    if (kmem_cache_debug(s))
        slab_free_debug()
    else
        __do_slab_free()
}

Currently, I only modify the code of freeing slab to fix the confusing 
messages of "slabinfo -v". If you agree, I can try to realize above 
mentioned slab_alloc_debug() code. Maybe it's also a challenge to me.

Thanks for your time.

> And of course we would stop creating the 'validate' sysfs files for
> non-debug caches.