[PATCH] mm/rmap: fix the handling of !private device page in try_to_unmap_one()

[Pingfan Liu <kernelfans@gmail.com>]

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@gmail.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
To: linux-mm@kvack.org
---
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