Subject: Re: [PATCH 3/7] mm/page_alloc: Add a bulk page allocator

From: Mel Gorman <[email-masked]>
Date: Mon, 22 Mar 2021 08:30:39 +0000

On Fri, Mar 19, 2021 at 07:18:32PM +0100, Vlastimil Babka wrote:
> On 3/12/21 4:43 PM, Mel Gorman wrote:
> > This patch adds a new page allocator interface via alloc_pages_bulk,
> > and __alloc_pages_bulk_nodemask. A caller requests a number of pages
> > to be allocated and added to a list. They can be freed in bulk using
> > free_pages_bulk().
> > 
> > The API is not guaranteed to return the requested number of pages and
> > may fail if the preferred allocation zone has limited free memory, the
> > cpuset changes during the allocation or page debugging decides to fail
> > an allocation. It's up to the caller to request more pages in batch
> > if necessary.
> > 
> > Note that this implementation is not very efficient and could be improved
> > but it would require refactoring. The intent is to make it available early
> > to determine what semantics are required by different callers. Once the
> > full semantics are nailed down, it can be refactored.
> > 
> > Signed-off-by: Mel Gorman <[email-masked]>
> 
> Acked-by: Vlastimil Babka <[email-masked]>
> 
> Although maybe premature, if it changes significantly due to the users'
> performance feedback, let's see :)
> 

Indeed. The next version will have no users so that Jesper and Chuck
can check if an array-based or LRU based version is better. There were
also bugs such as broken accounting of stats that had to be fixed which
increases overhead.

> Some nits below:
> 
> ...
> 
> > @@ -4963,6 +4978,107 @@ static inline bool prepare_alloc_pages(gfp_t gfp, unsigned int order,
> >  	return true;
> >  }
> >  
> > +/*
> > + * This is a batched version of the page allocator that attempts to
> > + * allocate nr_pages quickly from the preferred zone and add them to list.
> > + *
> > + * Returns the number of pages allocated.
> > + */
> > +int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> > +			nodemask_t *nodemask, int nr_pages,
> > +			struct list_head *alloc_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;
> > +	int allocated = 0;
> > +
> > +	if (WARN_ON_ONCE(nr_pages <= 0))
> > +		return 0;
> > +
> > +	if (nr_pages == 1)
> > +		goto failed;
> > +
> > +	/* May set ALLOC_NOFRAGMENT, fragmentation will return 1 page. */
> > +	if (!prepare_alloc_pages(gfp, 0, preferred_nid, nodemask, &ac,
> > +	&alloc_gfp, &alloc_flags))
> 
> Unusual identation here.
> 

Fixed

> > +		return 0;
> > +	gfp = alloc_gfp;
> > +
> > +	/* Find an allowed local zone that meets the high watermark. */
> > +	for_each_zone_zonelist_nodemask(zone, z, ac.zonelist, ac.highest_zoneidx, ac.nodemask) {
> > +		unsigned long mark;
> > +
> > +		if (cpusets_enabled() && (alloc_flags & ALLOC_CPUSET) &&
> > +		    !__cpuset_zone_allowed(zone, gfp)) {
> > +			continue;
> > +		}
> > +
> > +		if (nr_online_nodes > 1 && zone != ac.preferred_zoneref->zone &&
> > +		    zone_to_nid(zone) != zone_to_nid(ac.preferred_zoneref->zone)) {
> > +			goto failed;
> > +		}
> > +
> > +		mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK) + nr_pages;
> > +		if (zone_watermark_fast(zone, 0,  mark,
> > +				zonelist_zone_idx(ac.preferred_zoneref),
> > +				alloc_flags, gfp)) {
> > +			break;
> > +		}
> > +	}
> > +	if (!zone)
> > +		return 0;
> 
> Why not also "goto failed;" here?

Good question. When first written, it was because the zone search for the
normal allocator was almost certainly going to fail to find a zone and
it was expected that callers would prefer to fail fast over blocking.
Now we know that sunrpc can sleep on a failing allocation and it would
be better to enter the single page allocator and reclaim pages instead of
"sleep and hope for the best".

-- 
Mel Gorman
SUSE Labs