Re: [PATCH 4/8] mm/compaction: remove stale fast_find_block flag in isolate_migratepages

From: Baolin Wang
Date: Tue Aug 01 2023 - 04:15:23 EST

On 8/1/2023 11:48 AM, Kemeng Shi wrote:

on 8/1/2023 11:34 AM, Baolin Wang wrote:

On 8/1/2023 11:24 AM, Kemeng Shi wrote:

on 8/1/2023 10:42 AM, Baolin Wang wrote:

On 7/29/2023 1:10 AM, Kemeng Shi wrote:
In old code, we set skip to found page block in fast_find_migrateblock. So
we use fast_find_block to avoid skip found page block from
In 90ed667c03fe5 ("Revert "Revert "mm/compaction: fix set skip in
fast_find_migrateblock"""), we remove skip set in fast_find_migrateblock,
then fast_find_block is useless.

Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
   mm/compaction.c | 12 +-----------
   1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index ad535f880c70..09c36251c613 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1949,7 +1949,6 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
       const isolate_mode_t isolate_mode =
           (sysctl_compact_unevictable_allowed ? ISOLATE_UNEVICTABLE : 0) |
           (cc->mode != MIGRATE_SYNC ? ISOLATE_ASYNC_MIGRATE : 0);
-    bool fast_find_block;
        * Start at where we last stopped, or beginning of the zone as
@@ -1961,13 +1960,6 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
       if (block_start_pfn < cc->zone->zone_start_pfn)
           block_start_pfn = cc->zone->zone_start_pfn;
   -    /*
-     * fast_find_migrateblock marks a pageblock skipped so to avoid
-     * the isolation_suitable check below, check whether the fast
-     * search was successful.
-     */
-    fast_find_block = low_pfn != cc->migrate_pfn && !cc->fast_search_fail;
       /* Only scan within a pageblock boundary */
       block_end_pfn = pageblock_end_pfn(low_pfn);
   @@ -1976,7 +1968,6 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
        * Do not cross the free scanner.
       for (; block_end_pfn <= cc->free_pfn;
-            fast_find_block = false,
               cc->migrate_pfn = low_pfn = block_end_pfn,
               block_start_pfn = block_end_pfn,
               block_end_pfn += pageblock_nr_pages) {
@@ -2007,8 +1998,7 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
            * before making it "skip" so other compaction instances do
            * not scan the same block.
-        if (pageblock_aligned(low_pfn) &&
-            !fast_find_block && !isolation_suitable(cc, page))
+        if (pageblock_aligned(low_pfn) && !isolation_suitable(cc, page))

I do not think so. If the pageblock is found by fast_find_migrateblock(), that means it definitely has not been set the skip flag, so there is not need to call isolation_suitable() if fast_find_block is true, right?

Actually, found pageblock could be set skip as:
1. other compactor could mark this pageblock as skip after zone lock is realeased
in fast_find_migrateblock.

Yes, but your patch also can not close this race window, that means it can also be set skip flag after the isolation_suitable() validation by other compactors.

Yes, I think it's still worth to remove a lot of fast_find_block relevant check and reduce
code complexity with one redundant isolation_suitable which may skip some block with luck.
2. fast_find_migrateblock may uses pfn from reinit_migrate_pfn which is previously found
and sacnned. It could be fully sacnned and marked skip after it's first return from

Right, but now the 'fast_find_block' is false, and we will call isolation_suitable() to validate the skip flag.

Right, sorry for missing that.

But it's ok to keep the fast_find_block if you insist and I will just correct the stale

Yes, I still prefer to keep the fast_find_block, since I did not see this patch can fix any real issue and might have a side effect for fast-find-pageblock(?).

comment that "fast_find_migrateblock marks a pageblock skipped ..." in next version.

Sure, please do it.