Re: [PATCH v6 6/9] mm: multigenerational lru: aging

[Yu Zhao]

On Mon, Jan 10, 2022 at 04:01:13PM +0100, Michal Hocko wrote:
> On Thu 06-01-22 17:12:18, Michal Hocko wrote:
> > On Tue 04-01-22 13:22:25, Yu Zhao wrote:
> > > +static struct lru_gen_mm_walk *alloc_mm_walk(void)
> > > +{
> > > +	if (!current->reclaim_state || !current->reclaim_state->mm_walk)
> > > +		return kvzalloc(sizeof(struct lru_gen_mm_walk), GFP_KERNEL);
> 
> One thing I have overlooked completely.

I appreciate your attention to details but GFP_KERNEL is legit in the
reclaim path. It's been used many years in our production, e.g.,
  page reclaim
    swap_writepage()
      frontswap_store()
        zswap_frontswap_store()
          zswap_entry_cache_alloc(GFP_KERNEL)

(And I always test my changes with lockdep, kasan, DEBUG_VM, etc., no
 warnings ever seen from using GFP_KERNEL in the reclaim path.)

> You cannot really use GFP_KERNEL
> allocation here because the reclaim context can be constrained (e.g.
> GFP_NOFS). This allocation will not do any reclaim as it is PF_MEMALLOC
> but I suspect that the lockdep will complain anyway.
> 
> Also kvmalloc is not really great here. a) vmalloc path is never
> executed for small objects and b) we do not really want to make a
> dependency between vmalloc and the reclaim (by vmalloc -> reclaim ->
> vmalloc).
> 
> Even if we rule out vmalloc and look at kmalloc alone. Is this really
> safe? I do not see any recursion prevention in the SL.B code. Maybe this
> just happens to work but the dependency should be really documented so
> that future SL.B changes won't break the whole scheme. 

Affirmative, as Vlastimil has clarified.

Thanks!