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
>>>> 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@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
comment that "fast_find_migrateblock marks a pageblock skipped ..." in next version.
Thanks!
>> fast_find_migrateblock and it should be skipped.
>> Thanks!
>
--
Best wishes
Kemeng Shi
