linux-kernel - Re: [PATCH 1/8] mm/compaction: avoid missing last page block in section after skip offline sections

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Message-ID: <b3b4b896-14e9-c432-602d-e9b63050aeee@huaweicloud.com>
Date:   Sat, 29 Jul 2023 10:23:38 +0800
From:   Kemeng Shi <shikemeng@...weicloud.com>
To:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        baolin.wang@...ux.alibaba.com, mgorman@...hsingularity.net,
        willy@...radead.org
Subject: Re: [PATCH 1/8] mm/compaction: avoid missing last page block in
 section after skip offline sections



on 7/28/2023 6:41 PM, David Hildenbrand wrote:
> On 28.07.23 19:10, Kemeng Shi wrote:
>> skip_offline_sections_reverse will return the last pfn in found online
>> section. Then we set block_start_pfn to start of page block which
>> contains the last pfn in section. Then we continue, move one page
>> block forward and ignore the last page block in the online section.
>> Make block_start_pfn point to first page block after online section to fix
>> this:
>> 1. make skip_offline_sections_reverse return end pfn of online section,
>> i.e. pfn of page block after online section.
>> 2. assign block_start_pfn with next_pfn.
>>
>> Fixes: f63224525309 ("mm: compaction: skip the memory hole rapidly when isolating free pages")
>> Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
>> ---
>>   mm/compaction.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 9b7a0a69e19f..ce7841363b12 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
> 
> Can we add a short comment which kind of PFN we return (first pfn of first offline section after an online section)?
> 
Hi David, thanks for the review. Sure, I will add comment to skip_offline_sections_reverse.

-- 
Best wishes
Kemeng Shi
>> @@ -259,7 +259,7 @@ static unsigned long skip_offline_sections_reverse(unsigned long start_pfn)
>>         while (start_nr-- > 0) {
>>           if (online_section_nr(start_nr))
>> -            return section_nr_to_pfn(start_nr) + PAGES_PER_SECTION - 1;
>> +            return section_nr_to_pfn(start_nr + 1);
>>       }
>>         return 0;
>> @@ -1670,8 +1670,7 @@ static void isolate_freepages(struct compact_control *cc)
>>                 next_pfn = skip_offline_sections_reverse(block_start_pfn);
>>               if (next_pfn)
>> -                block_start_pfn = max(pageblock_start_pfn(next_pfn),
>> -                              low_pfn);
>> +                block_start_pfn = max(next_pfn, low_pfn);
> 
> 
> So block_start_pfn() will now point at the first PFN of the offline section.
> 
> Confusing stuff, but I get the idea and I think it makes sense to me :)
>