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

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Date:   Thu,  6 Dec 2018 16:46:04 +0800
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     peterx@...hat.com, Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        linux-mm@...ck.org
Subject: [PATCH] mm: thp: fix soft dirty for migration when split

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@...hat.com>
CC: Andrew Morton <akpm@...ux-foundation.org>
CC: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC: Matthew Wilcox <willy@...radead.org>
CC: Michal Hocko <mhocko@...e.com>
CC: Dave Jiang <dave.jiang@...el.com>
CC: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
CC: Souptick Joarder <jrdr.linux@...il.com>
CC: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
CC: linux-mm@...ck.org
CC: linux-kernel@...r.kernel.org
Signed-off-by: Peter Xu <peterx@...hat.com>
---

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