During migration special page table entries are installed for each page
being migrated. These entries store the pfn and associated permissions
of ptes mapping the page being migarted.
Device-private pages use special swap pte entries to distinguish
read-only vs. writeable pages which the migration code checks when
creating migration entries. Normally this follows a fast path in
migrate_vma_collect_pmd() which correctly copies the permissions of
device-private pages over to migration entries when migrating pages back
to the CPU.
However the slow-path falls back to using try_to_migrate() which
unconditionally creates read-only migration entries for device-private
pages. This leads to unnecessary double faults on the CPU as the new
pages are always mapped read-only even when they could be mapped
writeable. Fix this by correctly copying device-private permissions in
try_to_migrate_one().
Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
Reported-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
---
mm/rmap.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index b9eb5c12f3fe..271de8118cdd 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1804,6 +1804,7 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
update_hiwater_rss(mm);
if (is_zone_device_page(page)) {
+ unsigned long pfn = page_to_pfn(page);
swp_entry_t entry;
pte_t swp_pte;
@@ -1812,8 +1813,11 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma,
* pte. do_swap_page() will wait until the migration
* pte is removed and then restart fault handling.
*/
- entry = make_readable_migration_entry(
- page_to_pfn(page));
+ entry = pte_to_swp_entry(pteval);
+ if (is_writable_device_private_entry(entry))
+ entry = make_writable_migration_entry(pfn);
+ else
+ entry = make_readable_migration_entry(pfn);
swp_pte = swp_entry_to_pte(entry);
/*
--
2.30.2
