linux-kernel - Re: [PATCH v6 3/6] mm/cma: populate ZONE

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Date:   Tue, 18 Oct 2016 17:27:30 +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 v6 3/6] mm/cma: populate ZONE_CMA

On Tue, Oct 18, 2016 at 09:42:57AM +0200, Vlastimil Babka wrote:
> On 10/14/2016 05:03 AM, js1304@...il.com wrote:
> >@@ -145,6 +145,35 @@ static int __init cma_activate_area(struct cma *cma)
> > static int __init cma_init_reserved_areas(void)
> > {
> > 	int i;
> >+	struct zone *zone;
> >+	pg_data_t *pgdat;
> >+
> >+	if (!cma_area_count)
> >+		return 0;
> >+
> >+	for_each_online_pgdat(pgdat) {
> >+		unsigned long start_pfn = UINT_MAX, end_pfn = 0;
> >+
> >+		for (i = 0; i < cma_area_count; i++) {
> >+			if (pfn_to_nid(cma_areas[i].base_pfn) !=
> >+				pgdat->node_id)
> >+				continue;
> >+
> >+			start_pfn = min(start_pfn, cma_areas[i].base_pfn);
> >+			end_pfn = max(end_pfn, cma_areas[i].base_pfn +
> >+						cma_areas[i].count);
> >+		}
> >+
> >+		if (!end_pfn)
> >+			continue;
> >+
> >+		zone = &pgdat->node_zones[ZONE_CMA];
> >+
> >+		/* 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, do the max/min here work as intended? IIUC the initial

Yeap.

> zone_start_pfn is UINT_MAX and zone->spanned_pages is 1? So at least
> the max/min should be swapped?

No. CMA zone's start/end pfn are updated as node's start/end pfn.

> Also the zone_end_pfn(zone) on the second line already sees the
> changes to zone->zone_start_pfn in the first line, so it's kind of a
> mess. You should probably cache zone_end_pfn() to a temporary
> variable before changing zone_start_pfn.

You're right although it doesn't cause any problem. I look at the code
again and find that max/min isn't needed. Calculated start/end pfn
should be inbetween node's start/end pfn so max(zone->zone_start_pfn,
start_pfn) will return start_pfn and messed up min(zone_end_pfn(zone),
end_pfn) will return end_pfn in all the cases.

Anyway, I will fix it as following.

zone->zone_start_pfn = start_pfn
zone->spanned_pages = end_pfn - start_pfn

Thanks.