From: Vlastimil Babka <vba...@suse.cz>

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.

[a...@linux-foundation.org: comment fixes, per Minchan]
[iamjoonsoo....@lge.com: cleanups]
Signed-off-by: Vlastimil Babka <vba...@suse.cz>
Cc: Minchan Kim <minc...@kernel.org>
Cc: Mel Gorman <mgor...@suse.de>
Cc: Joonsoo Kim <iamjoonsoo....@lge.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnier...@samsung.com>
Cc: Michal Nazarewicz <min...@mina86.com>
Cc: Naoya Horiguchi <n-horigu...@ah.jp.nec.com>
Cc: Christoph Lameter <c...@linux.com>
Cc: Rik van Riel <r...@redhat.com>
Cc: Dongjun Shin <d.j.s...@samsung.com>
Cc: Sunghwan Yun <sunghwan....@samsung.com>
Signed-off-by: Joonsoo Kim <iamjoonsoo....@lge.com>
Signed-off-by: Andrew Morton <a...@linux-foundation.org>
Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
(cherry picked from commit c96b9e508f3d06ddb601dcc9792d62c044ab359e)
Signed-off-by: Andrey Ryabinin <aryabi...@virtuozzo.com>
---
 mm/compaction.c | 56 +++++++++++++++++++++++++++-----------------------------
 1 file changed, 27 insertions(+), 29 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index ea44b4bf85d9..0a2d4eded2e0 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -669,7 +669,10 @@ 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;
 
@@ -677,32 +680,33 @@ 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
@@ -711,7 +715,7 @@ static void isolate_freepages(struct zone *zone,
                 */
                cond_resched();
 
-               if (!pfn_valid(pfn))
+               if (!pfn_valid(block_start_pfn))
                        continue;
 
                /*
@@ -721,7 +725,7 @@ 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;
 
@@ -734,14 +738,8 @@ 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;
 
                /*
@@ -749,9 +747,9 @@ 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;
                }
        }
 
@@ -762,10 +760,10 @@ 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;
 }
 
-- 
2.13.6

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