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

[Kemeng Shi]

on 7/31/2023 8:01 PM, Baolin Wang wrote:
> 
> 
> On 7/29/2023 1:10 AM, 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@xxxxxxxxxxxxxxx>
>> ---
>>   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
>> @@ -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);
> 
> This is incorrect, you returned the start pfn of this section.
> 
>>       }
>>         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);
> 
> 'block_start_pfn' should be pageblock aligned. If the 'next_pfn' is not pageblock-aligned (though this is not the common case), we should skip it.
> 
> But if the 'next_pfn' is pageblock-aligned, yes, the commit f63224525309 still ignores the last pageblock, which is not right. So I think it should be:
> block_start_pfn = pageblock_aligned(next_pfn) ? : pageblock_start_pfn(next_pfn);
> block_start_pfn = max(block_start_pfn, low_pfn);
> 
Hi Baolin, thanks for reply! As skip_offline_sections_reverse is based
on skip_offline_sections. I make the assumption that section is pageblock
aligned based on that we use section start from skip_offline_sections as
block_start_fpn without align check.
If section size is not pageblock aligned in real world, the pageblock aligned
check should be added to skip_offline_sections and skip_offline_sections_reverse.
If no one is against this, I will fix this in next version. THanks!

-- 
Best wishes
Kemeng Shi