Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry

[Aneesh Kumar K.V]

On 10/12/18 1:32 PM, Anshuman Khandual wrote:
> On 10/09/2018 06:48 PM, Will Deacon wrote:
> > On Tue, Oct 09, 2018 at 04:04:21PM +0300, Kirill A. Shutemov wrote:
> > > 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@xxxxxxx>
> > > > ---
> > > > 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?
> >
> > Anshuman, would it work to:
> >
> > #define pmd_trans_huge(pmd)     (pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))
> yeah this works but some how does not seem like the right thing to do
> but can be the very last option.


There can be other code paths that makes that assumption. I ended up 
doing the below for pmd_trans_huge on ppc64.

/*
 * Only returns true for a THP. False for pmd migration entry.
 * We also need to return true when we come across a pte that
 * in between a thp split. While splitting THP, we mark the pmd
 * invalid (pmdp_invalidate()) before we set it with pte page
 * address. A pmd_trans_huge() check against a pmd entry during that time
 * should return true.
 * We should not call this on a hugetlb entry. We should check for HugeTLB
 * entry using vma->vm_flags
 * The page table walk rule is explained in Documentation/vm/transhuge.rst
 */
static inline int pmd_trans_huge(pmd_t pmd)
{
	if (!pmd_present(pmd))
		return false;

	if (radix_enabled())
		return radix__pmd_trans_huge(pmd);
	return hash__pmd_trans_huge(pmd);
}

-aneesh