[PATCH 3/4] mm: Charge active memcg when no mm is set

Dan Schatzberg
2020-08-24T15:43:37Z

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.dan@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Chris Down <chris@chrisdown.name>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
---
mm/memcontrol.c | 11 ++++++++---
mm/shmem.c | 5 +++--
2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b2468c80085d..79c70eef3ec3 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..77c908730be4 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1695,7 +1695,8 @@ 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 mem_cgroup *memcg;
struct page *page;
swp_entry_t swap;
int error;
@@ -1809,7 +1810,7 @@ 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