Re: [PATCH v4 2/5] mm/cma: populate ZONE_CMA

[Vlastimil Babka]

On 08/09/2016 08:39 AM, js1304@xxxxxxxxx wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>
> Until now, reserved pages for CMA are managed in the ordinary zones
> where page's pfn are belong to. This approach has numorous problems
> and fixing them isn't easy. (It is mentioned on previous patch.)
> To fix this situation, ZONE_CMA is introduced in previous patch, but,
> not yet populated. This patch implement population of ZONE_CMA
> by stealing reserved pages from the ordinary zones.
>
> Unlike previous implementation that kernel allocation request with
> __GFP_MOVABLE could be serviced from CMA region, allocation request only
> with GFP_HIGHUSER_MOVABLE can be serviced from CMA region in the new
> approach. This is an inevitable design decision to use the zone
> implementation because ZONE_CMA could contain highmem. Due to this
> decision, ZONE_CMA will work like as ZONE_HIGHMEM or ZONE_MOVABLE.
>
> I don't think it would be a problem because most of file cache pages
> and anonymous pages are requested with GFP_HIGHUSER_MOVABLE. It could
> be proved by the fact that there are many systems with ZONE_HIGHMEM and
> they work fine. Notable disadvantage is that we cannot use these pages
> for blockdev file cache page, because it usually has __GFP_MOVABLE but
> not __GFP_HIGHMEM and __GFP_USER. But, in this case, there is pros and
> cons. In my experience, blockdev file cache pages are one of the top
> reason that causes cma_alloc() to fail temporarily. So, we can get more
> guarantee of cma_alloc() success by discarding that case.
>
> Implementation itself is very easy to understand. Steal when cma area is
> initialized and recalculate various per zone stat/threshold.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

[...]

> @@ -145,6 +145,28 @@ err:
>  static int __init cma_init_reserved_areas(void)
>  {
>  	int i;
> +	struct zone *zone;
> +	unsigned long start_pfn = UINT_MAX, end_pfn = 0;
> +
> +	if (!cma_area_count)
> +		return 0;
> +
> +	for (i = 0; i < cma_area_count; i++) {
> +		if (start_pfn > cma_areas[i].base_pfn)
> +			start_pfn = cma_areas[i].base_pfn;
> +		if (end_pfn < cma_areas[i].base_pfn + cma_areas[i].count)
> +			end_pfn = cma_areas[i].base_pfn + cma_areas[i].count;
> +	}
> +
> +	for_each_populated_zone(zone) {
> +		if (!is_zone_cma(zone))
> +			continue;
> +
> +		/* ZONE_CMA doesn't need to exceed CMA region */
> +		zone->zone_start_pfn = max(zone->zone_start_pfn, start_pfn);
> +		zone->spanned_pages = min(zone_end_pfn(zone), end_pfn) -
> +					zone->zone_start_pfn;

Hmm is this a dead code? for_each_populated_zone() will skip zones where 
zone->present_pages is 0, which is AFAICS the result for ZONE_CMA
after it's initialized by calculate_node_totalpages() (after Patch 1/5).
The present_pages seem to be only increased later in this function by 
cma_activate_area() -> init_cma_reserved_pageblock().

> +	}
>
>  	for (i = 0; i < cma_area_count; i++) {
>  		int ret = cma_activate_area(&cma_areas[i]);
> @@ -153,6 +175,24 @@ static int __init cma_init_reserved_areas(void)
>  			return ret;
>  	}

[...]

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f6c4358..352096e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1600,16 +1600,38 @@ void __init page_alloc_init_late(void)
>  }
>
>  #ifdef CONFIG_CMA
> +static void __init adjust_present_page_count(struct page *page, long count)
> +{
> +	struct zone *zone = page_zone(page);
> +
> +	/* We don't need to hold a lock since it is boot-up process */
> +	zone->present_pages += count;
> +}
> +
>  /* Free whole pageblock and set its migration type to MIGRATE_CMA. */
>  void __init init_cma_reserved_pageblock(struct page *page)
>  {
>  	unsigned i = pageblock_nr_pages;
> +	unsigned long pfn = page_to_pfn(page);
>  	struct page *p = page;
> +	int nid = page_to_nid(page);
> +
> +	/*
> +	 * ZONE_CMA will steal present pages from other zones by changing
> +	 * page links so page_zone() is changed. Before that,
> +	 * we need to adjust previous zone's page count first.
> +	 */
> +	adjust_present_page_count(page, -pageblock_nr_pages);

So in previous version I said this (and you replied):

> > > Ideally, zone's start_pfn and spanned_pages should be also adjusted
> > > if we stole from the beginning/end (which I suppose should be quite
> > > common?).
>
> It would be possible. Maybe, there is a reason I didn't do that but I
> don't remember it. I will think more.

What's the outcome? :) Is stealing from beginning/end of zone common for 
CMA? Are we losing zone->contiguous and add iterations to compaction 
scanner needlessly?

Thanks.