[PATCH] mm: thp: fix soft dirty for migration when split

[Peter Xu]

When splitting a huge migrating PMD, we'll transfer the soft dirty bit
from the huge page to the small pages.  However we're possibly using a
wrong data since when fetching the bit we're using pmd_soft_dirty()
upon a migration entry.  Fix it up.

CC: Andrea Arcangeli <aarcange@xxxxxxxxxx>
CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
CC: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
CC: Matthew Wilcox <willy@xxxxxxxxxxxxx>
CC: Michal Hocko <mhocko@xxxxxxxx>
CC: Dave Jiang <dave.jiang@xxxxxxxxx>
CC: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
CC: Souptick Joarder <jrdr.linux@xxxxxxxxx>
CC: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
CC: linux-mm@xxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---

I noticed this during code reading.  Only compile tested.  I'm sending
a patch directly for review comments since it's relatively
straightforward and not easy to test.  Please have a look, thanks.
---
 mm/huge_memory.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f2d19e4fe854..fb0787c3dd3b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2161,7 +2161,10 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		SetPageDirty(page);
 	write = pmd_write(old_pmd);
 	young = pmd_young(old_pmd);
-	soft_dirty = pmd_soft_dirty(old_pmd);
+	if (unlikely(pmd_migration))
+		soft_dirty = pmd_swp_soft_dirty(old_pmd);
+	else
+		soft_dirty = pmd_soft_dirty(old_pmd);
 
 	/*
 	 * Withdraw the table only after we mark the pmd entry invalid.
-- 
2.17.1