This patch is a pure code refinement without any functional changes.
try_to_unmap_one() is shared by try_to_unmap() and try_to_munlock(). As for
unmap, if try_to_unmap_one() return true, it means the pte has been teared
down and mapcount dec. Apparently the current code
if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
is_zone_device_page(page) && !is_device_private_page(page))
return true;
conflicts with this logic.
Further more, as for zone_device, migration can only happen on
is_device_private_page(page). For other zone_device, memmap_init_zone_device()
raises an extra _refcount on all zone pages. This extra _refcount will block
migration. So in try_to_unmap_one(), it can just return false for other zone
device.
The reason why original code happen to work one !private zone_device.
-1. if page mapped, then try_to_unmap_one()->page_remove_rmap() is skipped, and
finally try_to_unmap(){ return !page_mapcount(page) ? true : false;} will
return false.
-2. if page not mapped, the extra _refcount will prevent the migration.
Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
Cc: Balbir Singh <bsingharora@xxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Aneesh Kumar <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
To: linux-mm@xxxxxxxxx
---
v1 -> v2: improve commit log and note in code
mm/rmap.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index b838647..723af4f 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1358,6 +1358,10 @@ void page_remove_rmap(struct page *page, bool compound)
/*
* @arg: enum ttu_flags will be passed to this argument
+ *
+ * For munlock, return true if @page is not mlocked by @vma without killing pte
+ * For unmap, return true after tearing down pte.
+ * For both cases, return false if rmap_walk should be stopped.
*/
static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
unsigned long address, void *arg)
@@ -1380,7 +1384,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
is_zone_device_page(page) && !is_device_private_page(page))
- return true;
+ return false;
if (flags & TTU_SPLIT_HUGE_PMD) {
split_huge_pmd_address(vma, address,
@@ -1487,7 +1491,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
if (IS_ENABLED(CONFIG_MIGRATION) &&
(flags & TTU_MIGRATION) &&
- is_zone_device_page(page)) {
+ is_device_private_page(page)) {
swp_entry_t entry;
pte_t swp_pte;
--
2.7.5
