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

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Message-ID: <a4a4c935-d7c8-ffba-cf51-6eaeb88ed19c@huaweicloud.com>
Date:   Tue, 1 Aug 2023 10:36:01 +0800
From:   Kemeng Shi <shikemeng@...weicloud.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, mgorman@...hsingularity.net,
        willy@...radead.org, david@...hat.com
Subject: Re: [PATCH 1/8] mm/compaction: avoid missing last page block in
 section after skip offline sections



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@...weicloud.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,
then section start is not aligned with pageblock size. Please correct me if I miss
anything. Thanks!

-- 
Best wishes
Kemeng Shi