Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 9 Oct 2018 16:04:21 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry |
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On Tue, Oct 09, 2018 at 09:28:58AM +0530, 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() > 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 ?
I guess it's just a design choice. Any reason why arm64 cannot do the same?
> Nonetheless pmd_present() seems to be a better check to distinguish > between mapped and (non-mapped non-present) migration entries without > any ambiguity.
Can we instead reverse order of check:
if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) { pvmw->ptl = pmd_lock(mm, pvmw->pmd); if (!pmd_present(*pvmw->pmd)) { ... } else if (likely(pmd_trans_huge(*pvmw->pmd))) { ... } else { ... } ...
This should cover both imeplementations of pmd_trans_huge().
-- Kirill A. Shutemov
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