linux-kernel - Re: [PATCH v5 3/6] mm/cma: populate ZONE

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Date:   Thu, 22 Sep 2016 14:45:46 +0900
From:   Joonsoo Kim <iamjoonsoo.kim@....com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        mgorman@...hsingularity.net, Laura Abbott <lauraa@...eaurora.org>,
        Minchan Kim <minchan@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Michal Nazarewicz <mina86@...a86.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/6] mm/cma: populate ZONE_CMA

On Wed, Sep 21, 2016 at 11:20:11AM +0200, Vlastimil Babka wrote:
> On 08/29/2016 07:07 AM, js1304@...il.com wrote:
> >From: Joonsoo Kim <iamjoonsoo.kim@....com>
> >
> >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@....com>
> 
> ...
> 
> >@@ -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_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, so what happens on a system with multiple nodes? Each will have
> its own ZONE_CMA, and all will have the same start pfn and spanned
> pages?

Each of zone_start_pfn and spanned_pages are initialized in
calculate_node_totalpages() which considers node boundary. So, they will
have not the same start pfn and spanned pages. However, each would
contain unnecessary holes.

> 
> > /* 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);
> >
> > 	do {
> > 		__ClearPageReserved(p);
> > 		set_page_count(p, 0);
> >-	} while (++p, --i);
> >+
> >+		/* Steal pages from other zones */
> >+		set_page_links(p, ZONE_CMA, nid, pfn);
> >+	} while (++p, ++pfn, --i);
> >+
> >+	adjust_present_page_count(page, pageblock_nr_pages);
> 
> This seems to assign pages to ZONE_CMA on the proper node, which is
> good. But then ZONE_CMA on multiple nodes will have unnecessary
> holes in the spanned pages, as each will contain only a subset.

True, I will fix it and respin the series.

Thanks.