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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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
>>>>> 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.
>>
>> 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.