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Subject: Re: [PATCH v2 4/7] mm/compaction: simplify pfn iteration in
 isolate_freepages_range
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org, baolin.wang@linux.alibaba.com,
        david@redhat.com, willy@infradead.org
References: <20230826153617.4019189-1-shikemeng@huaweicloud.com>
 <20230826153617.4019189-5-shikemeng@huaweicloud.com>
 <20230829150153.j4lw36s7yx5be7al@techsingularity.net>
From:   Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <fd145323-6418-2a8a-484b-20f83f42e2d1@huaweicloud.com>
Date:   Wed, 30 Aug 2023 15:02:19 +0800
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on 8/29/2023 11:01 PM, Mel Gorman wrote:
> On Sat, Aug 26, 2023 at 11:36:14PM +0800, 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.
>>
>> The pfn + isolated always points to start of next pageblock as:
>> In case isolated buddy page has order higher than pageblock:
>> 1. page in buddy page is aligned with it's order
>> 2. order of page is higher than pageblock order
>> Then page is aligned with pageblock order. So pfn of page and isolated
>> pages count are both aligned pageblock order. So pfn + isolated is
>> pageblock order aligned.
>>
>> In case isolated buddy page has order lower than pageblock:
>> Buddy page with order N contains two order N - 1 pages as following:
>> |        order N        |
>> |order N - 1|order N - 1|
>> So buddy pages with order N - 1 will never cross boudary of order N.
>> Similar, buddy pages with order N - 2 will never cross boudary of order
>> N - 1 and so on. Then any pages with order less than pageblock order
>> will never crosa boudary of pageblock.
>>
>> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> While I don't think the patch is wrong, I also don't think it
> meaningfully simplifies the code or optimises enough to be justified.
> Even though a branch is eliminated, the whole path is not cheap.
> 
OK, I will drop this in next version if you insistant.