[PATCH 3/12] mm: page migration fix PageMlocked on migrated pages

Hugh Dickins
2015-10-19T04:52:56Z

Commit e6c509f85455 ("mm: use clear_page_mlock() in page_remove_rmap()")
in v3.7 inadvertently made mlock_migrate_page() impotent: page migration
unmaps the page from userspace before migrating, and that commit clears
PageMlocked on the final unmap, leaving mlock_migrate_page() with nothing
to do. Not a serious bug, the next attempt at reclaiming the page would
fix it up; but a betrayal of page migration's intent - the new page ought
to emerge as PageMlocked.

I don't see how to fix it for mlock_migrate_page() itself; but easily
fixed in remove_migration_pte(), by calling mlock_vma_page() when the
vma is VM_LOCKED - under pte lock as in try_to_unmap_one().

Delete mlock_migrate_page()? Not quite, it does still serve a purpose
for migrate_misplaced_transhuge_page(): where we could replace it by a
test, clear_page_mlock(), mlock_vma_page() sequence; but would that be
an improvement? mlock_migrate_page() is fairly lean, and let's make
it leaner by skipping the irq save/restore now clearly not needed.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/internal.h | 9 ++++-----
mm/migrate.c | 6 ++++--
2 files changed, 8 insertions(+), 7 deletions(-)

--- migrat.orig/mm/internal.h 2015-09-12 18:30:20.841039780 -0700
+++ migrat/mm/internal.h 2015-10-18 17:53:11.061322013 -0700
@@ -271,20 +271,19 @@ extern unsigned int munlock_vma_page(str
extern void clear_page_mlock(struct page *page);

/*
- * mlock_migrate_page - called only from migrate_page_copy() to
- * migrate the Mlocked page flag; update statistics.
+ * mlock_migrate_page - called only from migrate_misplaced_transhuge_page()
+ * (because that does not go through the full procedure of migration ptes):
+ * to migrate the Mlocked page flag; update statistics.
*/
static inline void mlock_migrate_page(struct page *newpage, struct page *page)
{
if (TestClearPageMlocked(page)) {
- unsigned long flags;
int nr_pages = hpage_nr_pages(page);

- local_irq_save(flags);
+ /* Holding pmd lock, no change in irq context: __mod is safe */
__mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
SetPageMlocked(newpage);
__mod_zone_page_state(page_zone(newpage), NR_MLOCK, nr_pages);
- local_irq_restore(flags);
}
}

--- migrat.orig/mm/migrate.c 2015-10-04 10:47:52.469445854 -0700
+++ migrat/mm/migrate.c 2015-10-18 17:53:11.062322014 -0700
@@ -171,6 +171,9 @@ static int remove_migration_pte(struct p
else
page_add_file_rmap(new);

+ if (vma->vm_flags & VM_LOCKED)
+ mlock_vma_page(new);
+
/* No need to invalidate - it was non-present before */
update_mmu_cache(vma, addr, ptep);
unlock:
@@ -537,7 +540,6 @@ void migrate_page_copy(struct page *newp
cpupid = page_cpupid_xchg_last(page, -1);
page_cpupid_xchg_last(newpage, cpupid);

- mlock_migrate_page(newpage, page);
ksm_migrate_page(newpage, page);
/*
* Please do not reorder this without considering how mm/ksm.c's
@@ -1786,7 +1788,6 @@ fail_putback:
SetPageActive(page);
if (TestClearPageUnevictable(new_page))
SetPageUnevictable(page);
- mlock_migrate_page(page, new_page);

unlock_page(new_page);
put_page(new_page); /* Free it */
@@ -1828,6 +1829,7 @@ fail_putback:
goto fail_putback;
}

+ mlock_migrate_page(new_page, page);
mem_cgroup_migrate(page, new_page, false);

page_remove_rmap(page);