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>>> On Wed, Sep 10, 2025 at 09:22:40AM +0000, Wei Yang wrote: >>>> Commit 56ae0bb349b4 ("mm: compaction: convert to use a folio in >>>> isolate_migratepages_block()") converts api from page to folio. But th= e >>>> low_pfn advance for hugetlb page seems wrong when low_pfn doesn't poin= t >>>> to head page. >>>> >>>> Originally, if page is a hugetlb tail page, compound_nr() return 1, >>>> which means low_pfn only advance one in next iteration. After the >>>> change, low_pfn would advance more than the hugetlb range, since >>>> folio_nr_pages() always return total number of the large page. This >>>> results in skipping some range to isolate and then to migrate. >>>> >>>> The worst case for alloc_contig is it does all the isolation and >>>> migration, but finally find some range is still not isolated. And then >>>> undo all the work and try a new range. >>>> >>>> Advance low_pfn to the end of hugetlb. >>>> >>>> Signed-off-by: Wei Yang >>>> Fixes: 56ae0bb349b4 ("mm: compaction: convert to use a folio in isolat= e_migratepages_block()") This behavior seems to be introduced by commit 369fa227c219 ("mm: make alloc_contig_range handle free hugetlb pages=E2=80=9D). The related change = is: + if (PageHuge(page) && cc->alloc_contig) { + ret =3D isolate_or_dissolve_huge_page(page); + + /* + * Fail isolation in case isolate_or_dissolve_huge_= page() + * reports an error. In case of -ENOMEM, abort righ= t away. + */ + if (ret < 0) { + /* Do not report -EBUSY down the chain */ + if (ret =3D=3D -EBUSY) + ret =3D 0; + low_pfn +=3D (1UL << compound_order(page)) = - 1; + goto isolate_fail; + } + + /* + * Ok, the hugepage was dissolved. Now these pages = are + * Buddy and cannot be re-allocated because they ar= e + * isolated. Fall-through as the check below handle= s + * Buddy pages. + */ + } + >>>> Cc: Kefeng Wang >>>> Cc: Oscar Salvador >>> >>> Forgot to cc stable. >>> >>> Cc: >> >> Is there any bug report to justify the backport? Since it is more likely >> to be a performance issue instead of a correctness issue. >> >>> >>>> --- >>>> mm/compaction.c | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/mm/compaction.c b/mm/compaction.c >>>> index bf021b31c7ec..1e8f8eca318c 100644 >>>> --- a/mm/compaction.c >>>> +++ b/mm/compaction.c >>>> @@ -989,7 +989,7 @@ isolate_migratepages_block(struct compact_control = *cc, unsigned long low_pfn, >>>> * Hugepage was successfully isolated and placed >>>> * on the cc->migratepages list. >>>> */ >>>> - low_pfn +=3D folio_nr_pages(folio) - 1; >>>> + low_pfn +=3D folio_nr_pages(folio) - folio_page_idx(folio, page) = - 1; >>> >>> One question is why we advance compound_nr() in original version. >>> >>> Yes, there are several places advancing compound_nr(), but it seems to = iterate >>> on the same large page and do the same thing and advance 1 again. >>> >>> Not sure which part story I missed. >> >> isolate_migratepages_block() starts from the beginning of a pageblock. >> How likely the code hit in the middle of a hugetlb? >> > > In addition, there are two other =E2=80=9Clow_pfn +=3D (1UL << order) - 1= =E2=80=9D > in the if (PageHuge(page)), why not change them too if you think > page can point to the middle of a hugetlb? > > Best Regards, > Yan, Zi Best Regards, Yan, Zi