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

[Baolin Wang]

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);