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

From: Dan Schatzberg <schatzberg.dan@gmail.com>
To: unlisted-recipients:; (no To-header on input)
Cc: Dan Schatzberg <schatzberg.dan@gmail.com>,
	Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
	Li Zefan <lizefan@huawei.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>, Roman Gushchin <guro@fb.com>,
	Shakeel Butt <shakeelb@google.com>,
	Chris Down <chris@chrisdown.name>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-block@vger.kernel.org (open list:BLOCK LAYER),
	linux-kernel@vger.kernel.org (open list),
	cgroups@vger.kernel.org (open list:CONTROL GROUP (CGROUP)),
	linux-mm@kvack.org (open list:CONTROL GROUP - MEMORY RESOURCE
	CONTROLLER (MEMCG))
Subject: [PATCH v3 2/3] mm: Charge active memcg when no mm is set
Date: Mon, 24 Feb 2020 17:17:46 -0500	[thread overview]
Message-ID: <4456276af198412b2d41cd09d246cd20e0c6d22a.1582581887.git.schatzberg.dan@gmail.com> (raw)
In-Reply-To: <cover.1582581887.git.schatzberg.dan@gmail.com>

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>
Signed-off-by: Dan Schatzberg <schatzberg.dan@gmail.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 d09776cd6e10..222e4aac0c85 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6319,7 +6319,8 @@ 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.
@@ -6363,8 +6364,12 @@ 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..8664c97851f2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1631,7 +1631,7 @@ 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;
@@ -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

