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

[Kemeng Shi]

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@xxxxxxxxxxxxxxx>
>>>>>>>>> ---
>>>>>>>>>      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
if it does not bother you.


-- 
Best wishes
Kemeng Shi