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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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@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.
>>>>
>>> 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().