[patch 2/4] mm: thp: fix flags for pmd migration when split

[patch 2/4] mm: thp: fix flags for pmd migration when split

[akpm]

From: Peter Xu <peterx@redhat.com>
Subject: mm: thp: fix flags for pmd migration when split

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.  Since at it, drop the ifdef together as not needed.

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.

Link: http://lkml.kernel.org/r/20181213051510.20306-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.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: Zi Yan <zi.yan@cs.rutgers.edu>
Cc: <stable@vger.kernel.org>	[4.14+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/huge_memory.c |   20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

--- a/mm/huge_memory.c~mm-thp-fix-flags-for-pmd-migration-when-split
+++ a/mm/huge_memory.c
@@ -2144,23 +2144,25 @@ static void __split_huge_pmd_locked(stru
 	 */
 	old_pmd = pmdp_invalidate(vma, haddr, pmd);
 
-#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
 	pmd_migration = is_pmd_migration_entry(old_pmd);
-	if (pmd_migration) {
+	if (unlikely(pmd_migration)) {
 		swp_entry_t entry;
 
 		entry = pmd_to_swp_entry(old_pmd);
 		page = pfn_to_page(swp_offset(entry));
-	} else
-#endif
+		write = is_write_migration_entry(entry);
+		young = false;
+		soft_dirty = pmd_swp_soft_dirty(old_pmd);
+	} else {
 		page = pmd_page(old_pmd);
+		if (pmd_dirty(old_pmd))
+			SetPageDirty(page);
+		write = pmd_write(old_pmd);
+		young = pmd_young(old_pmd);
+		soft_dirty = pmd_soft_dirty(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);
 
 	/*
 	 * Withdraw the table only after we mark the pmd entry invalid.
_