[PATCH v3] mm: thp: fix flags for pmd migration when split

From: Peter Xu
Date: Thu Dec 13 2018 - 00:15:24 EST

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.

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: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
CC: linux-mm@xxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
- fix it up for young/write/dirty bits too [Konstantin]
- fetch write correctly for migration entry; drop macro [Konstantin]
mm/huge_memory.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f2d19e4fe854..aebade83cec9 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2145,23 +2145,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
old_pmd = pmdp_invalidate(vma, haddr, pmd);

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
+ 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.