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

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Date:   Tue, 1 Aug 2023 11:53:45 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Kemeng Shi <shikemeng@...weicloud.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 10:36 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@...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,

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.