mm/compaction: cleanup isolate_freepages()

isolate_freepages() is currently somewhat hard to follow thanks to many
looks like it is related to the 'low_pfn' variable, but in fact it is not.

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.

[akpm@linux-foundation.org: comment fixes, per Minchan]
[iamjoonsoo.kim@lge.com: cleanups]
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>
Cc: Dongjun Shin <d.j.shin@samsung.com>
Cc: Sunghwan Yun <sunghwan.yun@samsung.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 27 insertions, 29 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index 95f7531458f7..6010aabde28c 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -665,7 +665,10 @@ 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 block_start_pfn;  /* start of current pageblock */
+        unsigned long block_end_pfn;    /* end of current pageblock */
+        unsigned long low_pfn;       /* lowest pfn scanner is able to scan */
+        unsigned long next_free_pfn; /* start pfn for scaning at next round */
         int nr_freepages = cc->nr_freepages;
         struct list_head *freelist = &cc->freepages;
 
@@ -673,32 +676,33 @@ static void isolate_freepages(struct zone *zone,
          * Initialise the free scanner. The starting point is where we last
          * successfully isolated from, zone-cached value, or the end of the
          * zone when isolating for the first time. We need this aligned to
-         * the pageblock boundary, because we do pfn -= pageblock_nr_pages
-         * in the for loop.
+         * the pageblock boundary, because we do
+         * block_start_pfn -= pageblock_nr_pages in the for loop.
+         * For ending point, take care when isolating in last pageblock of a
+         * a zone which ends in the middle of a pageblock.
          * The low boundary is the end of the pageblock the migration scanner
          * is using.
          */
-        pfn = cc->free_pfn & ~(pageblock_nr_pages-1);
+        block_start_pfn = cc->free_pfn & ~(pageblock_nr_pages-1);
+        block_end_pfn = min(block_start_pfn + pageblock_nr_pages,
+                                                zone_end_pfn(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.
+         * If no pages are isolated, the block_start_pfn < low_pfn check
+         * will kick in.
          */
-        high_pfn = min(low_pfn, pfn);
-
-        z_end_pfn = zone_end_pfn(zone);
+        next_free_pfn = 0;
 
         /*
          * Isolate free pages until enough are available to migrate the
          * pages on cc->migratepages. We stop searching if the migrate
          * and free page scanners meet or enough free pages are isolated.
          */
-        for (; pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
-                                        pfn -= pageblock_nr_pages) {
+        for (; block_start_pfn >= low_pfn && cc->nr_migratepages > nr_freepages;
+                                block_end_pfn = block_start_pfn,
+                                block_start_pfn -= pageblock_nr_pages) {
                 unsigned long isolated;
-                unsigned long end_pfn;
 
                 /*
                  * This can iterate a massively long zone without finding any
@@ -707,7 +711,7 @@ static void isolate_freepages(struct zone *zone,
                  */
                 cond_resched();
 
-                if (!pfn_valid(pfn))
+                if (!pfn_valid(block_start_pfn))
                         continue;
 
                 /*
@@ -717,7 +721,7 @@ static void isolate_freepages(struct zone *zone,
                  * i.e. it's possible that all pages within a zones range of
                  * pages do not belong to a single zone.
                  */
-                page = pfn_to_page(pfn);
+                page = pfn_to_page(block_start_pfn);
                 if (page_zone(page) != zone)
                         continue;
 
@@ -730,14 +734,8 @@ static void isolate_freepages(struct zone *zone,
                         continue;
 
                 /* Found a block suitable for isolating free pages from */
-
-                /*
-                 * Take care when isolating in last pageblock of a zone which
-                 * ends in the middle of a pageblock.
-                 */
-                end_pfn = min(pfn + pageblock_nr_pages, z_end_pfn);
-                isolated = isolate_freepages_block(cc, pfn, end_pfn,
-                                                   freelist, false);
+                isolated = isolate_freepages_block(cc, block_start_pfn,
+                                        block_end_pfn, freelist, false);
                 nr_freepages += isolated;
 
                 /*
@@ -745,9 +743,9 @@ static void isolate_freepages(struct zone *zone,
                  * looking for free pages, the search will restart here as
                  * page migration may have returned some pages to the allocator
                  */
-                if (isolated) {
+                if (isolated && next_free_pfn == 0) {
                         cc->finished_update_free = true;
-                        high_pfn = max(high_pfn, pfn);
+                        next_free_pfn = block_start_pfn;
                 }
         }
 
@@ -758,10 +756,10 @@ static void isolate_freepages(struct zone *zone,
          * If we crossed the migrate scanner, we want to keep it that way
          * 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;
+        if (block_start_pfn < low_pfn)
+                next_free_pfn = cc->migrate_pfn;
+
+        cc->free_pfn = next_free_pfn;
         cc->nr_freepages = nr_freepages;
 }