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

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Date:   Tue,  9 Oct 2018 09:28:58 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     kirill.shutemov@...ux.intel.com, akpm@...ux-foundation.org,
        mhocko@...e.com, zi.yan@...rutgers.edu, will.deacon@....com
Subject: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry

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 ? Nonetheless
pmd_present() seems to be a better check to distinguish between mapped
and (non-mapped non-present) migration entries without any ambiguity.

 mm/page_vma_mapped.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index ae3c2a3..b384396 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -161,7 +161,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 	pmde = READ_ONCE(*pvmw->pmd);
 	if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) {
 		pvmw->ptl = pmd_lock(mm, pvmw->pmd);
-		if (likely(pmd_trans_huge(*pvmw->pmd))) {
+		if (likely(pmd_trans_huge(*pvmw->pmd) &&
+					pmd_present(*pvmw->pmd))) {
 			if (pvmw->flags & PVMW_MIGRATION)
 				return not_found(pvmw);
 			if (pmd_page(*pvmw->pmd) != page)
-- 
2.7.4