Messages in this thread | | | Date | Wed, 2 Aug 2023 09:11:25 +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 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@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(). >> >> > 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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