Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

Paul E. McKenney
2020-08-14T23:27:47Z

On Fri, Aug 14, 2020 at 11:52:06PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 14, 2020 at 01:41:40PM -0700, Paul E. McKenney wrote:
> > > And that enforces the GFP_NOLOCK allocation mode or some other solution
> > > unless you make a new rule that calling call_rcu() is forbidden while
> > > holding zone lock or any other lock which might be nested inside the
> > > GFP_NOWAIT zone::lock held region.
> >
> > Again, you are correct. Maybe the forecasted weekend heat will cause
> > my brain to hallucinate a better solution, but in the meantime, the
> > GFP_NOLOCK approach looks good from this end.
>
> So I hate __GFP_NO_LOCKS for a whole number of reasons:
>
> - it should be called __GFP_LOCKLESS if anything
> - it sprinkles a bunch of ugly branches around the allocator fast path
> - it only works for order==0
>
> Combined I really odn't think this should be a GFP flag. How about a
> special purpose allocation function, something like so..

This looks entirely reasonable to me!

Thanx, Paul

> ---
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 901a21f61d68..cdec9c99fba7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4875,6 +4875,47 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
> }
> EXPORT_SYMBOL(__alloc_pages_nodemask);
>
> +struct page *__rmqueue_lockless(struct zone *zone, struct per_cpu_pages *pcp)
> +{
> + struct list_head *list;
> + struct page *page;
> + int migratetype;
> +
> + for (migratetype = 0; migratetype < MIGRATE_PCPTYPES; migratetype++) {
> + list = &pcp->list[migratetype];
> + page = list_first_entry_or_null(list, struct page, lru);
> + if (page && check_new_pcp(page)) {
> + list_del(&page->lru);
> + pcp->count--;
> + return page;
> + }
> + }
> +
> + return NULL;
> +}
> +
> +struct page *__alloc_page_lockless(void)
> +{
> + struct zonelist *zonelist = node_zonelist(numa_node_id(), GFP_KERNEL);
> + struct per_cpu_pages *pcp;
> + struct page *page = NULL;
> + unsigned long flags;
> + struct zoneref *z;
> + struct zone *zone;
> +
> + for_each_zone_zonelist(zone, z, zonelist, ZONE_NORMAL) {
> + local_irq_save(flags);
> + pcp = &this_cpu_ptr(zone->pageset)->pcp;
> + page = __rmqueue_lockless(zone, pcp);
> + local_irq_restore(flags);
> +
> + if (page)
> + break;
> + }
> +
> + return page;
> +}
> +
> /*
> * Common helper functions. Never use with __GFP_HIGHMEM because the returned
> * address cannot represent highmem pages. Use alloc_pages and then kmap if