linux-kernel - Re: [PATCH 4/9] mm/compaction: simplify pfn iteration in isolate_freepages

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Date:   Tue, 15 Aug 2023 18:07:58 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Kemeng Shi <shikemeng@...weicloud.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, david@...hat.com
Subject: Re: [PATCH 4/9] mm/compaction: simplify pfn iteration in
 isolate_freepages_range



On 8/15/2023 5:32 PM, Kemeng Shi wrote:
> 
> 
> 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

I mean, you changed to:
1) pfn += isolated;
2) block_start_pfn = pfn;
3) block_end_pfn = pfn + pageblock_nr_pages;

But in 1) pfn + isolated can go outside of the currnet pageblock if 
isolating a high-order page, for example, located in the middle of the 
next pageblock. So that the block_start_pfn can point to the middle of 
the next pageblock, not the start position. Meanwhile after 3), the 
block_end_pfn can point another pageblock. Or I missed something else?

> 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@...weicloud.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,
>>