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

Uladzislau Rezki
2020-10-01T19:32:23Z

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