[PATCH v2] mm: thp: fix flags for pmd migration when split

[Peter Xu <peterx@redhat.com>]

When splitting a huge migrating PMD, we'll transfer all the existing
PMD bits and apply them again onto the small PTEs.  However we are
fetching the bits unconditionally via pmd_soft_dirty(), pmd_write()
or pmd_yound() while actually they don't make sense at all when it's
a migration entry.  Fix them up by make it conditional.

Note that if my understanding is correct about the problem then if
without the patch there is chance to lose some of the dirty bits in
the migrating pmd pages (on x86_64 we're fetching bit 11 which is part
of swap offset instead of bit 2) and it could potentially corrupt the
memory of an userspace program which depends on the dirty bit.

CC: Andrea Arcangeli <aarcange@redhat.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
CC: Matthew Wilcox <willy@infradead.org>
CC: Michal Hocko <mhocko@suse.com>
CC: Dave Jiang <dave.jiang@intel.com>
CC: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
CC: Souptick Joarder <jrdr.linux@gmail.com>
CC: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
CC: linux-mm@kvack.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Peter Xu <peterx@redhat.com>
---
v2:
- fix it up for young/write/dirty bits too [Konstantin]
---
 mm/huge_memory.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f2d19e4fe854..b00941b3d342 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2157,11 +2157,16 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		page = pmd_page(old_pmd);
 	VM_BUG_ON_PAGE(!page_count(page), page);
 	page_ref_add(page, HPAGE_PMD_NR - 1);
-	if (pmd_dirty(old_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);
+		young = write = false;
+	} else {
+		if (pmd_dirty(old_pmd))
+			SetPageDirty(page);
+		write = pmd_write(old_pmd);
+		young = pmd_young(old_pmd);
+		soft_dirty = pmd_soft_dirty(old_pmd);
+	}
 
 	/*
 	 * Withdraw the table only after we mark the pmd entry invalid.
-- 
2.17.1