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    SubjectRe: [PATCH 1/8] mm/compaction: avoid missing last page block in section after skip offline sections
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    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@huaweicloud.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
    >>>> @@ -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.

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