Messages in this thread | | | From | "Zi Yan" <> | Subject | Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry | Date | Tue, 09 Oct 2018 09:58:36 -0400 |
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cc: Naoya Horiguchi (who proposed to use !_PAGE_PRESENT && !_PAGE_PSE for x86 PMD migration entry check)
On 8 Oct 2018, at 23:58, Anshuman Khandual wrote:
> A normal mapped THP page at PMD level should be correctly differentiated > from a PMD migration entry while walking the page table. A mapped THP would > additionally check positive for pmd_present() along with pmd_trans_huge() > as compared to a PMD migration entry. This just adds a new conditional test > differentiating the two while walking the page table. > > Fixes: 616b8371539a6 ("mm: thp: enable thp migration in generic path") > Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> > --- > On X86, pmd_trans_huge() and is_pmd_migration_entry() are always mutually > exclusive which makes the current conditional block work for both mapped > and migration entries. This is not same with arm64 where pmd_trans_huge()
!pmd_present() && pmd_trans_huge() is used to represent THPs under splitting, since _PAGE_PRESENT is cleared during THP splitting but _PAGE_PSE is not. See the comment in pmd_present() for x86, in arch/x86/include/asm/pgtable.h
> returns positive for both mapped and migration entries. Could some one > please explain why pmd_trans_huge() has to return false for migration > entries which just install swap bits and its still a PMD ? Nonetheless > pmd_present() seems to be a better check to distinguish between mapped > and (non-mapped non-present) migration entries without any ambiguity.
If arm64 does it differently, I just wonder how THP splitting is handled in arm64.
-- Best Regards Yan Zi [unhandled content-type:application/pgp-signature] | |