linux-kernel - Re: [PATCH 4/8] mm/compaction: remove stale fast_find_block flag in isolate

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Message-ID: <ab91a0e6-8a19-148c-1e93-ca7a51f5c459@linux.alibaba.com>
Date:   Tue, 1 Aug 2023 11:34:57 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Kemeng Shi <shikemeng@...weicloud.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        mgorman@...hsingularity.net, willy@...radead.org, david@...hat.com
Subject: Re: [PATCH 4/8] mm/compaction: remove stale fast_find_block flag in
 isolate_migratepages



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@...weicloud.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.

> 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.

> fast_find_migrateblock and it should be skipped.
> Thanks!