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Subject: Re: [PATCH 4/9] mm/compaction: simplify pfn iteration in
 isolate_freepages_range
To:     Baolin Wang <baolin.wang@linux.alibaba.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        mgorman@techsingularity.net, david@redhat.com
References: <20230805110711.2975149-1-shikemeng@huaweicloud.com>
 <20230805110711.2975149-5-shikemeng@huaweicloud.com>
 <ecb315f9-a5cd-4fb3-bae6-eb94a08eccb3@linux.alibaba.com>
From:   Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <43b726c1-3ea6-9acc-d4e4-c7deabcf7ecd@huaweicloud.com>
Date:   Tue, 15 Aug 2023 17:32:45 +0800
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on 8/15/2023 4:38 PM, Baolin Wang wrote:
> 
> 
> On 8/5/2023 7:07 PM, Kemeng Shi wrote:
>> We call isolate_freepages_block in strict mode, continuous pages in
>> pageblock will be isolated if isolate_freepages_block successed.
>> Then pfn + isolated will point to start of next pageblock to scan
>> no matter how many pageblocks are isolated in isolate_freepages_block.
>> Use pfn + isolated as start of next pageblock to scan to simplify the
>> iteration.
> 
> IIUC, the isolate_freepages_block() can isolate high-order free pages, which means the pfn + isolated can be larger than the block_end_pfn. So in your patch, the 'block_start_pfn' and 'block_end_pfn' can be in different pageblocks, that will break pageblock_pfn_to_page().
> 
In for update statement, we always update block_start_pfn to pfn and
update block_end_pfn to pfn + pageblock_nr_pages. So they should point
to the same pageblock. I guess you missed the change to update of
block_end_pfn. :)
>>
>> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
>> ---
>>   mm/compaction.c | 14 ++------------
>>   1 file changed, 2 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 684f6e6cd8bc..8d7d38073d30 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -733,21 +733,11 @@ isolate_freepages_range(struct compact_control *cc,
>>       block_end_pfn = pageblock_end_pfn(pfn);
>>         for (; pfn < end_pfn; pfn += isolated,
>> -                block_start_pfn = block_end_pfn,
>> -                block_end_pfn += pageblock_nr_pages) {
>> +                block_start_pfn = pfn,
>> +                block_end_pfn = pfn + pageblock_nr_pages) {
>>           /* Protect pfn from changing by isolate_freepages_block */
>>           unsigned long isolate_start_pfn = pfn;
>>   -        /*
>> -         * pfn could pass the block_end_pfn if isolated freepage
>> -         * is more than pageblock order. In this case, we adjust
>> -         * scanning range to right one.
>> -         */
>> -        if (pfn >= block_end_pfn) {
>> -            block_start_pfn = pageblock_start_pfn(pfn);
>> -            block_end_pfn = pageblock_end_pfn(pfn);
>> -        }
>> -
>>           block_end_pfn = min(block_end_pfn, end_pfn);
>>             if (!pageblock_pfn_to_page(block_start_pfn,
>