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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 >> --- >>   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