linux-kernel - [PATCH v3 2/3] mm: Charge active memcg when no mm is set

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Date:   Thu, 20 Feb 2020 11:51:52 -0500
From:   Dan Schatzberg <schatzberg.dan@...il.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Dan Schatzberg <schatzberg.dan@...il.com>,
        Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-block@...r.kernel.org (open list:BLOCK LAYER),
        linux-kernel@...r.kernel.org (open list),
        cgroups@...r.kernel.org (open list:CONTROL GROUP (CGROUP)),
        linux-mm@...ck.org (open list:CONTROL GROUP - MEMORY RESOURCE
        CONTROLLER (MEMCG))
Subject: [PATCH v3 2/3] mm: Charge active memcg when no mm is set

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@...il.com>
Acked-by: Johannes Weiner <hannes@...xchg.org>
Acked-by: Tejun Heo <tj@...nel.org>
---
 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 6f6dc8712e39..b174aff4f069 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6317,7 +6317,8 @@ 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 *@...cgp pointing to the charged memcg.
  * Otherwise, an error code is returned.
@@ -6361,8 +6362,12 @@ 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 c8f7540ef048..7c7f5acf89d6 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