v4:
  - Take rcu_read_lock() when memcg is being accessed as suggested by
    Michal.
  - Make print_page_owner_memcg() return the new offset into the buffer
    and put CONFIG_MEMCG block inside as suggested by Mike.
  - Directly use TASK_COMM_LEN as length of name buffer as suggested by
    Roman.

 v3:
  - Add unlikely() to patch 1 and clarify that -1 will not be returned.
  - Use a helper function to print out memcg information in patch 3.
  - Add a new patch 4 to store task command name in page_owner
    structure.

 v2:
  - Remove the SNPRINTF() macro as suggested by Ira and use scnprintf()
    instead to remove some buffer overrun checks.
  - Add a patch to optimize vscnprintf with a size parameter of 0.

While debugging the constant increase in percpu memory consumption on
a system that spawned large number of containers, it was found that a
lot of offline mem_cgroup structures remained in place without being
freed. Further investigation indicated that those mem_cgroup structures
were pinned by some pages.

In order to find out what those pages are, the existing page_owner
debugging tool is extended to show memory cgroup information and whether
those memcgs are offline or not. With the enhanced page_owner tool,
the following is a typical page that pinned the mem_cgroup structure
in my test case:

Page allocated via order 0, mask 0x1100cca(GFP_HIGHUSER_MOVABLE), pid 162970 (podman), ts 1097761405537 ns, free_ts 1097760838089 ns
PFN 1925700 type Movable Block 3761 type Movable Flags 0x17ffffc00c001c(uptodate|dirty|lru|reclaim|swapbacked|node=0|zone=2|lastcpupid=0x1fffff)
 prep_new_page+0xac/0xe0
 get_page_from_freelist+0x1327/0x14d0
 __alloc_pages+0x191/0x340
 alloc_pages_vma+0x84/0x250
 shmem_alloc_page+0x3f/0x90
 shmem_alloc_and_acct_page+0x76/0x1c0
 shmem_getpage_gfp+0x281/0x940
 shmem_write_begin+0x36/0xe0
 generic_perform_write+0xed/0x1d0
 __generic_file_write_iter+0xdc/0x1b0
 generic_file_write_iter+0x5d/0xb0
 new_sync_write+0x11f/0x1b0
 vfs_write+0x1ba/0x2a0
 ksys_write+0x59/0xd0
 do_syscall_64+0x37/0x80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
Charged to offline memcg libpod-conmon-15e4f9c758422306b73b2dd99f9d50a5ea53cbb16b4a13a2c2308a4253cc0ec8.

So the page was not freed because it was part of a shmem segment. That
is useful information that can help users to diagnose similar problems.

With cgroup v1, /proc/cgroups can be read to find out the total number
of memory cgroups (online + offline). With cgroup v2, the cgroup.stat of
the root cgroup can be read to find the number of dying cgroups (most
likely pinned by dying memcgs).

The page_owner feature is not supposed to be enabled for production
system due to its memory overhead. However, if it is suspected that
dying memcgs are increasing over time, a test environment with page_owner
enabled can then be set up with appropriate workload for further analysis
on what may be causing the increasing number of dying memcgs.

Waiman Long (4):
  lib/vsprintf: Avoid redundant work with 0 size
  mm/page_owner: Use scnprintf() to avoid excessive buffer overrun check
  mm/page_owner: Print memcg information
  mm/page_owner: Record task command name

 lib/vsprintf.c  |  8 +++---
 mm/page_owner.c | 70 ++++++++++++++++++++++++++++++++++++++-----------
 2 files changed, 60 insertions(+), 18 deletions(-)

-- 
2.27.0

On Wed, Feb 02, 2022 at 03:30:32PM -0500, Waiman Long wrote:
>  v4:
>   - Take rcu_read_lock() when memcg is being accessed as suggested by
>     Michal.
>   - Make print_page_owner_memcg() return the new offset into the buffer
>     and put CONFIG_MEMCG block inside as suggested by Mike.
>   - Directly use TASK_COMM_LEN as length of name buffer as suggested by
>     Roman.
> 
>  v3:
>   - Add unlikely() to patch 1 and clarify that -1 will not be returned.
>   - Use a helper function to print out memcg information in patch 3.
>   - Add a new patch 4 to store task command name in page_owner
>     structure.
> 
>  v2:
>   - Remove the SNPRINTF() macro as suggested by Ira and use scnprintf()
>     instead to remove some buffer overrun checks.
>   - Add a patch to optimize vscnprintf with a size parameter of 0.
> 
> While debugging the constant increase in percpu memory consumption on
> a system that spawned large number of containers, it was found that a
> lot of offline mem_cgroup structures remained in place without being
> freed. Further investigation indicated that those mem_cgroup structures
> were pinned by some pages.
> 
> In order to find out what those pages are, the existing page_owner
> debugging tool is extended to show memory cgroup information and whether
> those memcgs are offline or not. With the enhanced page_owner tool,
> the following is a typical page that pinned the mem_cgroup structure
> in my test case:
> 
> Page allocated via order 0, mask 0x1100cca(GFP_HIGHUSER_MOVABLE), pid 162970 (podman), ts 1097761405537 ns, free_ts 1097760838089 ns
> PFN 1925700 type Movable Block 3761 type Movable Flags 0x17ffffc00c001c(uptodate|dirty|lru|reclaim|swapbacked|node=0|zone=2|lastcpupid=0x1fffff)
>  prep_new_page+0xac/0xe0
>  get_page_from_freelist+0x1327/0x14d0
>  __alloc_pages+0x191/0x340
>  alloc_pages_vma+0x84/0x250
>  shmem_alloc_page+0x3f/0x90
>  shmem_alloc_and_acct_page+0x76/0x1c0
>  shmem_getpage_gfp+0x281/0x940
>  shmem_write_begin+0x36/0xe0
>  generic_perform_write+0xed/0x1d0
>  __generic_file_write_iter+0xdc/0x1b0
>  generic_file_write_iter+0x5d/0xb0
>  new_sync_write+0x11f/0x1b0
>  vfs_write+0x1ba/0x2a0
>  ksys_write+0x59/0xd0
>  do_syscall_64+0x37/0x80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> Charged to offline memcg libpod-conmon-15e4f9c758422306b73b2dd99f9d50a5ea53cbb16b4a13a2c2308a4253cc0ec8.
> 
> So the page was not freed because it was part of a shmem segment. That
> is useful information that can help users to diagnose similar problems.
> 
> With cgroup v1, /proc/cgroups can be read to find out the total number
> of memory cgroups (online + offline). With cgroup v2, the cgroup.stat of
> the root cgroup can be read to find the number of dying cgroups (most
> likely pinned by dying memcgs).
> 
> The page_owner feature is not supposed to be enabled for production
> system due to its memory overhead. However, if it is suspected that
> dying memcgs are increasing over time, a test environment with page_owner
> enabled can then be set up with appropriate workload for further analysis
> on what may be causing the increasing number of dying memcgs.
> 
> Waiman Long (4):
>   lib/vsprintf: Avoid redundant work with 0 size
>   mm/page_owner: Use scnprintf() to avoid excessive buffer overrun check
>   mm/page_owner: Print memcg information
>   mm/page_owner: Record task command name
> 
>  lib/vsprintf.c  |  8 +++---
>  mm/page_owner.c | 70 ++++++++++++++++++++++++++++++++++++++-----------
>  2 files changed, 60 insertions(+), 18 deletions(-)
> 
> -- 
> 2.27.0
>

Thank you, Waiman.

Acked-by: Rafael Aquini <aquini@redhat.com>