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

From: Dan Schatzberg
Date: Mon Apr 20 2020 - 18:41:50 EST


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@xxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Acked-by: Chris Down <chris@xxxxxxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
---
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 5beea03dd58a..af68d1d7b456 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6435,7 +6435,8 @@ 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.
@@ -6479,8 +6480,12 @@ 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 d722eb830317..8c8ffc35a957 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1618,7 +1618,7 @@ 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 mem_cgroup *memcg;
struct page *page;
swp_entry_t swap;
@@ -1753,7 +1753,7 @@ 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