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

[Kemeng Shi <shikemeng@huaweicloud.com>]

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
>> fast_find_migrateblock.
>> 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@huaweicloud.com>
>> ---
>>   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.
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
fast_find_migrateblock and it should be skipped.
Thanks!
-- 
Best wishes
Kemeng Shi