mm: Charge active memcg when no mm is set

PATCH Chris Down 2 participants 2 messages Feb 7, 2020 – Feb 19, 2020 2 messages not in the archive

Subsystems (3)

Share & tools

A message that is not in the archive (sent off-list or not yet received): [email protected]

A message that is not in the archive (sent off-list or not yet received): 8e41630b9d1c5d00f92a00f998285fa6003af5eb.1581088326.git.dschatzberg@fb.com

Chris Down

author Acked-by

2020-02-07 21:18 UTC #2 lore raw

Quoted 1 line · Dan Schatzberg

This is a dependency for 3/3

This can be omitted, since "3" won't mean anything in the change history (and patch series are generally considered as a unit unless there are explicit requests to split them out).

Quoted 8 lines

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.

Quoted 8 lines

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.
Signed-off-by Dan Schatzberg <[email protected]>
Acked-by Johannes Weiner <[email protected]>

Thanks, this seems reasonable. One (minor and optional) suggestion would be to make the title more clear that this is a change in try_charge/memalloc_use_memcg behaviour overall rather than a charge site, since this wasn't what I expected to find when I saw the patch title :-)

I only have one other question about behaviour when there is no active_memcg and mm/memcg in try_charge are NULL below, but assuming that's been checked:

Acked-by Chris Down <[email protected]>

Quoted 21 lines

---
mm/memcontrol.c | 11 ++++++++---
mm/shmem.c | 2 +-

2 files changed, 9 insertions(+), 4 deletions(-)

mm/memcontrol.c +2 -1

index f7da3ff135ed..69935d166bdb 100644
@@ -6812,7 +6812,8 @@ 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.

@@ -6856,8 +6857,12 @@ int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
 		}

mm/memcontrol.c int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm, · line 6865

Quoted 10 lines

mm/memcontrol.c +6 -2

 	}
 
-	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);
+	}

Just to do due diligence, did we double check whether this results in any unintentional shift in accounting for those passing in both mm and memcg as NULL with no current->active_memcg set, since previously we never even tried to consult current->mm and always used root_mem_cgroup in get_mem_cgroup_from_mm?

It's entirely possible that this results in exactly the same outcome as before just by different means, but with the number of try_charge callsites I'm not totally certain of that.

Quoted 19 lines

mm/memcontrol.c +0 -0

 	ret = try_charge(memcg, gfp_mask, nr_pages, false);

mm/shmem.c +1 -2

index ca74ede9e40b..70aabd9aba1a 100644
@@ -1748,7 +1748,7 @@ 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

Johannes Weiner

maintainer

2020-02-19 19:37 UTC #3 lore raw

Quoted 4 lines · Chris Down

Quoted 4 lines

(quoted diff) +0 -0

@@ -6856,8 +6857,12 @@ int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
 		}
 	}

Quoted 14 lines

Review int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm, · line 6865
Quoted 8 lines
(quoted diff) +6 -2
-	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);
+	}
Just to do due diligence, did we double check whether this results in any unintentional shift in accounting for those passing in both mm and memcg as NULL with no current->active_memcg set, since previously we never even tried to consult current->mm and always used root_mem_cgroup in get_mem_cgroup_from_mm?

Excellent question on a subtle issue.

But nobody actually passes NULL. They either pass current->mm (or a destination mm) in syscalls, or vma->vm_mm in page faults.

The only times we end up with NULL is when kernel threads do something and have !current->mm. We redirect those to root_mem_cgroup.

So this patch doesn't change those semantics.