[PATCH 1/2] mm: Charge current memcg when no mm is set

[Dan Schatzberg <dschatzberg@fb.com>]

This modifies the shmem and mm charge logic so that now if there is no
mm set (as in the case of tmpfs backed loop device), we charge the
current memcg, if set.

Signed-off-by: Dan Schatzberg <dschatzberg@fb.com>
---
 mm/memcontrol.c | 11 ++++++++---
 mm/shmem.c      |  2 +-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c5b5f74cfd4d..5a6ab3183525 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6354,7 +6354,8 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
  * @compound: charge the page as compound or small page
  *
  * Try to charge @page to the memcg that @mm belongs to, reclaiming
- * pages according to @gfp_mask if necessary.
+ * pages according to @gfp_mask if necessary. If @mm is NULL, try to
+ * charge to the active memcg.
  *
  * Returns 0 on success, with *@memcgp pointing to the charged memcg.
  * Otherwise, an error code is returned.
@@ -6398,8 +6399,12 @@ int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
 		}
 	}
 
-	if (!memcg)
-		memcg = get_mem_cgroup_from_mm(mm);
+	if (!memcg) {
+		if (!mm)
+			memcg = get_mem_cgroup_from_current();
+		else
+			memcg = get_mem_cgroup_from_mm(mm);
+	}
 
 	ret = try_charge(memcg, gfp_mask, nr_pages);
 
diff --git a/mm/shmem.c b/mm/shmem.c
index 165fa6332993..014e576a617b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1766,7 +1766,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 	}
 
 	sbinfo = SHMEM_SB(inode->i_sb);
-	charge_mm = vma ? vma->vm_mm : current->mm;
+	charge_mm = vma ? vma->vm_mm : NULL;
 
 	page = find_lock_entry(mapping, index);
 	if (xa_is_value(page)) {
-- 
2.17.1