Date:   Wed, 2 Aug 2023 09:11:25 +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 8:33 PM, Kemeng Shi wrote:
> 
> 
> on 8/1/2023 5:32 PM, Baolin Wang wrote:
>>
>>
>> On 8/1/2023 4:42 PM, Kemeng Shi wrote:
>>>
>>>
>>> on 8/1/2023 4:01 PM, Baolin Wang wrote:
>>>>
>>>>
>>>> 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);
>>>>
>>>>
>>> Um, if we keep "block_start_pfn = max(next_pfn, low_pfn);", should we also keep
>>> returning end of section "section_nr_to_pfn(start_nr + 1);" instead of original last
>>> pfn of the section "section_nr_to_pfn(start_nr) + PAGES_PER_SECTION - 1;" which seems
>>> not aligned.
>>> Assume SECTION_SIZE_BITS = 27, PAGE_SHIFT = 12, pageblock order = 10
>>> Last pfn of the section 0 is 0x7fff, end pfn of section 0 is 0x8000. The last pfn
>>> is not aligned.
>>> Please tell me if I misunderstand anything. Thanks!
>>
>> Ah, you are right, sorry for my bad arithmetic. Maybe we should return the end pfn (section_nr_to_pfn(start_nr) + PAGES_PER_SECTION) of the section in skip_offline_sections_reverse() with adding some comments to explain the return value like David suggested. Then we can remove the pageblock_end_pfn() in isolate_freepages().
>>
>>
> Sure, I will add comments in next version. As (section_nr_to_pfn(start_nr) + PAGES_PER_SECTION)
> is = section_nr_to_pfn(start_nr + 1), I will keep the change to skip_offline_sections_reverse

IMO, next section is confusing. We need return the end pfn of the 
current online section, and we usually get it by 
"section_nr_to_pfn(start_nr) + PAGES_PER_SECTION".

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