Subject: Re: [PATCH 2/9] mm/page_alloc: Add a bulk page allocator

From: Mel Gorman <[email-masked]>
Date: Mon, 12 Apr 2021 11:59:38 +0100

On Mon, Apr 12, 2021 at 12:21:42PM +0200, Vlastimil Babka wrote:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 8a3e13277e22..eb547470a7e4 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4965,6 +4965,124 @@ static inline bool prepare_alloc_pages(gfp_t gfp_mask, unsigned int order,
> >  	return true;
> >  }
> >  
> > +/*
> > + * __alloc_pages_bulk - Allocate a number of order-0 pages to a list
> > + * @gfp: GFP flags for the allocation
> > + * @preferred_nid: The preferred NUMA node ID to allocate from
> > + * @nodemask: Set of nodes to allocate from, may be NULL
> > + * @nr_pages: The number of pages desired on the list
> > + * @page_list: List to store the allocated pages
> > + *
> > + * This is a batched version of the page allocator that attempts to
> > + * allocate nr_pages quickly and add them to a list.
> > + *
> > + * Returns the number of pages on the list.
> > + */
> > +int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> > +			nodemask_t *nodemask, int nr_pages,
> > +			struct list_head *page_list)
> > +{
> > +	struct page *page;
> > +	unsigned long flags;
> > +	struct zone *zone;
> > +	struct zoneref *z;
> > +	struct per_cpu_pages *pcp;
> > +	struct list_head *pcp_list;
> > +	struct alloc_context ac;
> > +	gfp_t alloc_gfp;
> > +	unsigned int alloc_flags;
> 
> Was going to complain that this is not set to ALLOC_WMARK_LOW. Must be faster
> next time...
> 

Good that you caught it anyway!

> > +	int allocated = 0;
> > +
> > +	if (WARN_ON_ONCE(nr_pages <= 0))
> > +		return 0;
> > +
> > +	/* Use the single page allocator for one page. */
> > +	if (nr_pages == 1)
> > +		goto failed;
> > +
> > +	/* May set ALLOC_NOFRAGMENT, fragmentation will return 1 page. */
> 
> I don't understand this comment. Only alloc_flags_nofragment() sets this flag
> and we don't use it here?
> 

It's there as a reminder that there are non-obvious consequences
to ALLOC_NOFRAGMENT that may affect the bulk allocation success
rate. __rmqueue_fallback will only select pageblock_order pages and if that
fails, we fall into the slow path that allocates a single page. I didn't
deal with it because it was not obvious that it's even relevant but I bet
in 6 months time, I'll forget that ALLOC_NOFRAGMENT may affect success
rates without the comment. I'm waiting for a bug that can trivially trigger
a case with a meaningful workload where the success rate is poor enough to
affect latency before adding complexity. Ideally by then, the allocation
paths would be unified a bit better.

> > +	gfp &= gfp_allowed_mask;
> > +	alloc_gfp = gfp;
> > +	if (!prepare_alloc_pages(gfp, 0, preferred_nid, nodemask, &ac, &alloc_gfp, &alloc_flags))
> > +		return 0;
> > +	gfp = alloc_gfp;
> > +
> > +	/* Find an allowed local zone that meets the high watermark. */
> 
> Should it say "low watermark"?
> 

Yeah, that's leftover from an earlier prototype :(

-- 
Mel Gorman
SUSE Labs