Messages in this thread | | | Date | Tue, 1 Aug 2023 17:32:48 +0800 | Subject | Re: [PATCH 1/8] mm/compaction: avoid missing last page block in section after skip offline sections | From | Baolin Wang <> |
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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().
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