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

["Paul E. McKenney" <paulmck@kernel.org>]

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