Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Sep 30, 2020 at 12:35:57PM +0200, Michal Hocko wrote:
> On Wed 30-09-20 00:07:42, Uladzislau Rezki wrote:
> [...]
> > <snip>
> > bool is_pcp_cache_empty(gfp_t gfp)
> > {
> >     struct per_cpu_pages *pcp;
> >     struct zoneref *ref;
> >     unsigned long flags;
> >     bool empty;
> > 
> >     ref = first_zones_zonelist(node_zonelist(
> >             numa_node_id(), gfp), gfp_zone(gfp), NULL);
> >     if (!ref->zone)
> >             return true;
> > 
> >     local_irq_save(flags);
> >     pcp = &this_cpu_ptr(ref->zone->pageset)->pcp;
> >     empty = list_empty(&pcp->lists[gfp_migratetype(gfp)]);
> >     local_irq_restore(flags);
> > 
> >     return empty;
> > }
> > 
> > disable_irq();
> > if (!is_pcp_cache_empty(GFP_NOWAIT))
> >     __get_free_page(GFP_NOWAIT);
> > enable_irq();
> > <snip>
> > 
> > Do you mean to have something like above? I mean some extra API
> > function that returns true or false if fast-fast allocation can
> > either occur or not. Above code works just fine and never touches
> > main zone->lock.
> 
> The above code works with the _current_ implementation and it restricts
> its implementation to some degree. Future changes might get harder to
> implement with a pattern like this. I do not think we want users to be
> aware of internal implementation details like pcp caches, migrate types
> or others. While pcp caches are here for years and unlikely to change in
> a foreseeable future many details are changing on regular basis.
>
I see your view. That was en example for better understanding.

Thanks.

--
Vlad Rezki