Re: [External] Re: [PATCH 4/4] mm: memcontrol: fix swap uncharge on cgroup v2

On Sat, Feb 13, 2021 at 2:57 AM Shakeel Butt [off-list ref] wrote:

CCing more folks.

On Fri, Feb 12, 2021 at 9:14 AM Muchun Song [off-list ref] wrote:

quoted

The swap charges the actual number of swap entries on cgroup v2.
If a swap cache page is charged successful, and then we uncharge
the swap counter. It is wrong on cgroup v2. Because the swap
entry is not freed.

Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control")
Signed-off-by: Muchun Song <redacted>

What's the user visible impact of this change?

IIUC, I think that we cannot limit the swap to memory.swap.max
on cgroup v2.

  cd /sys/fs/cgroup/
  mkdir test
  cd test
  echo 8192 > memory.max
  echo 4096 > memory.swap.max

OK. Now we limit swap to 1 page and memory to 2 pages.
Firstly, we allocate 1 page from this memory cgroup and
swap this page to swap disk. We can see:

  memory.current: 0
  memory.swap.current: 1

Then we touch this page, we will swap in and charge
the swap cache page to the memory counter and uncharge
the swap counter.

  memory.current: 1
  memory.swap.current: 0 (but actually we use a swap entry)

Then we allocate another 1 page from this memory cgroup.

  memory.current: 2
  memory.swap.current: 0 (but actually we use a swap entry)

If we swap those 2 pages to swap disk. We can charge and swap
those 2 pages successfully. Right? Maybe I am wrong.

One impact I can see is that without this patch meminfo's (SwapTotal -
SwapFree) is larger than the sum of top level memory.swap.current.
This change will reduce that gap.

BTW what about per-cpu slots_ret cache? Should we call
mem_cgroup_uncharge_swap() before putting in the cache after this
change?

quoted

---
 mm/memcontrol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c737c8f05992..be6bc5044150 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -6753,7 +6753,7 @@ int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask)
        memcg_check_events(memcg, page);
        local_irq_enable();

-       if (PageSwapCache(page)) {
+       if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && PageSwapCache(page)) {
                swp_entry_t entry = { .val = page_private(page) };
                /*
                 * The swap entry might not get freed for a long time,
--

2.11.0