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

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Message-ID: <20181009130421.bmus632ocurn275u@kshutemo-mobl1>
Date:   Tue, 9 Oct 2018 16:04:21 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kirill.shutemov@...ux.intel.com, akpm@...ux-foundation.org,
        mhocko@...e.com, zi.yan@...rutgers.edu, will.deacon@....com
Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and
 PMD migration entry

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@....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