[PATCH 2/3] mm: Charge active memcg when no mm is set

[Dan Schatzberg]

memalloc_use_memcg() worked for kernel allocations but was silently
ignored for user pages.

This patch establishes a precedence order for who gets charged:

1. If there is a memcg associated with the page already, that memcg is
   charged. This happens during swapin.

2. If an explicit mm is passed, mm->memcg is charged. This happens
   during page faults, which can be triggered in remote VMs (eg gup).

3. Otherwise consult the current process context. If it has configured
   a current->active_memcg, use that. Otherwise, current->mm->memcg.

Previously, if a NULL mm was passed to mem_cgroup_try_charge (case 3) it
would always charge the root cgroup. Now it looks up the current
active_memcg first (falling back to charging the root cgroup if not
set).

Signed-off-by: Dan Schatzberg <schatzberg....@gmail.com>
Acked-by: Johannes Weiner <han...@cmpxchg.org>
Acked-by: Tejun Heo <t...@kernel.org>
Acked-by: Chris Down <ch...@chrisdown.name>
Reviewed-by: Shakeel Butt <shake...@google.com>
---
 mm/memcontrol.c | 11 ++++++++---
 mm/shmem.c      |  4 ++--
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e27ac6d79a32..877778b792da 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6676,7 +6676,8 @@ void mem_cgroup_calculate_protection(struct mem_cgroup 
*root,
  * @gfp_mask: reclaim mode
  *
  * 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. Otherwise, an error code is returned.
  */
@@ -6712,8 +6713,12 @@ int mem_cgroup_charge(struct page *page, struct 
mm_struct *mm, gfp_t gfp_mask)
                rcu_read_unlock();
        }
 
-       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);
        if (ret)
diff --git a/mm/shmem.c b/mm/shmem.c
index 271548ca20f3..1139f52ac4ee 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1695,7 +1695,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t 
index,
 {
        struct address_space *mapping = inode->i_mapping;
        struct shmem_inode_info *info = SHMEM_I(inode);
-       struct mm_struct *charge_mm = vma ? vma->vm_mm : current->mm;
+       struct mm_struct *charge_mm = vma ? vma->vm_mm : NULL;
        struct page *page;
        swp_entry_t swap;
        int error;
@@ -1809,7 +1809,7 @@ 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)) {
-- 
2.24.1