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

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Message-Id: <20200420223936.6773-4-schatzberg.dan@gmail.com>
Date:   Mon, 20 Apr 2020 18:39:31 -0400
From:   Dan Schatzberg <schatzberg.dan@...il.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Dan Schatzberg <schatzberg.dan@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Chris Down <chris@...isdown.name>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        Li Zefan <lizefan@...wei.com>,
        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>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-block@...r.kernel.org (open list:BLOCK LAYER),
        linux-kernel@...r.kernel.org (open list),
        linux-fsdevel@...r.kernel.org (open list:FILESYSTEMS (VFS and
        infrastructure)),
        cgroups@...r.kernel.org (open list:CONTROL GROUP (CGROUP)),
        linux-mm@...ck.org (open list:CONTROL GROUP - MEMORY RESOURCE
        CONTROLLER (MEMCG))
Subject: [PATCH 3/4] 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>
Acked-by: Chris Down <chris@...isdown.name>
Reviewed-by: Shakeel Butt <shakeelb@...gle.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 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 *@...cgp 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