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

[Kemeng Shi <shikemeng@huaweicloud.com>]

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!

-- 
Best wishes
Kemeng Shi