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

[Baolin Wang]

On 8/1/2023 10:36 AM, Kemeng Shi wrote:
> on 8/1/2023 10:18 AM, Kemeng Shi wrote:
> > 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!
> More information of aligment of section. For powerpc arch, we have SECTION_SIZE_BITS
> with 24 while PAGE_SHIFT could be configured to 18.
> Pageblock order is (18 + MAX_ORDER) which coule be 28 and is > SECTION_SZIE_BITS 24,

The maximum pageblock order is MAX_ORDER. But after thinking more, I 
think return the start pfn or end pfn of a section is okay, and it 
should be aligned to a pageblock order IIUC.

So I think your change is good:
+ block_start_pfn = max(next_pfn, low_pfn);

But in skip_offline_sections_reverse(), we should still return the last 
pfn of the online section.