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 16:01:14 +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 2:08 PM, Kemeng Shi wrote:
> 
> 
> on 8/1/2023 11:53 AM, Baolin Wang wrote:
>>
>>
>> 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.
>>
> Right, I mixed up the unit.
>> 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.
>>
> Sure, then we should assign block_start_pfn with following change. Is this good to you?
> -                block_start_pfn = max(pageblock_start_pfn(next_pfn),
> +		 block_start_pfn = max(pageblock_end_pfn(next_pfn),
>                                low_pfn);

The last pfn of a section is already section aligned, so I think no need 
to call pageblock_end_pfn(), just like your original change is okay to me.
block_start_pfn = max(next_pfn, low_pfn);