Re: [PATCH 2/2] mm/compaction: cleanup isolate_freepages()

Minchan Kim
2014-04-17T00:07:06Z

Hi Vlastimil,

Below just nitpicks.

On Tue, Apr 15, 2014 at 11:18:27AM +0200, Vlastimil Babka wrote:
> isolate_freepages() is currently somewhat hard to follow thanks to many
> different pfn variables. Especially misleading is the name 'high_pfn' which
> looks like it is related to the 'low_pfn' variable, but in fact it is not.

Indeed.

>
> This patch renames the 'high_pfn' variable to a hopefully less confusing name,
> and slightly changes its handling without a functional change. A comment made
> obsolete by recent changes is also updated.

It's clean up patch so if we do fixing, I'd like to do more.

>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> Cc: Michal Nazarewicz <mina86@mina86.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Rik van Riel <riel@redhat.com>
> ---
> mm/compaction.c | 17 ++++++++---------
> 1 file changed, 8 insertions(+), 9 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 627dc2e..169c7b2 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -671,7 +671,7 @@ static void isolate_freepages(struct zone *zone,
> struct compact_control *cc)
> {
> struct page *page;
> - unsigned long high_pfn, low_pfn, pfn, z_end_pfn;
> + unsigned long pfn, low_pfn, next_free_pfn, z_end_pfn;

Could you add comment for each variable?

unsigned long pfn; /* scanning cursor */
unsigned long low_pfn; /* lowest pfn free scanner is able to scan */
unsigned long next_free_pfn; /* start pfn for scaning at next truen */
unsigned long z_end_pfn; /* zone's end pfn */

> int nr_freepages = cc->nr_freepages;
> struct list_head *freelist = &cc->freepages;
>
> @@ -688,11 +688,10 @@ static void isolate_freepages(struct zone *zone,
> low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages);
>
> /*
> - * Take care that if the migration scanner is at the end of the zone
> - * that the free scanner does not accidentally move to the next zone
> - * in the next isolation cycle.
> + * Seed the value for max(next_free_pfn, pfn) updates. If there are
> + * none, the pfn < low_pfn check will kick in.

"none" what? I'd like to clear more.

> */
> - high_pfn = min(low_pfn, pfn);
> + next_free_pfn = 0;
>
> z_end_pfn = zone_end_pfn(zone);
>
> @@ -754,7 +753,7 @@ static void isolate_freepages(struct zone *zone,
> */
> if (isolated) {
> cc->finished_update_free = true;
> - high_pfn = max(high_pfn, pfn);
> + next_free_pfn = max(next_free_pfn, pfn);
> }
> }
>
> @@ -766,9 +765,9 @@ static void isolate_freepages(struct zone *zone,
> * so that compact_finished() may detect this
> */
> if (pfn < low_pfn)
> - cc->free_pfn = max(pfn, zone->zone_start_pfn);
> - else
> - cc->free_pfn = high_pfn;
> + next_free_pfn = max(pfn, zone->zone_start_pfn);

Why we need max operation?
IOW, what's the problem if we do (next_free_pfn = pfn)?

> +
> + cc->free_pfn = next_free_pfn;
> cc->nr_freepages = nr_freepages;
> }
>
> --
> 1.8.4.5
>
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--
Kind regards,
Minchan Kim