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

On Mon, Feb 15, 2021 at 5:47 PM Michal Hocko [off-list ref] wrote:

On Sat 13-02-21 01:01:59, Muchun Song 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.

Is there any actual problem though? Can you describe the specific
scenario please? Swap cache charge life time is a bit tricky and I have
to confess I have to relearn it every time I need to understand it. The
patch would be much more easier to review if the changelog was much more
specific.

I copied the reply to Shakeel here. :-)

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.

quoted

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

--
Michal Hocko
SUSE Labs