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mm, memcg: reconsider kmem.limit_in_bytes deprecation

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This reverts commits 86327e8eb9 ("memcg: drop kmem.limit_in_bytes") and
partially reverts 58056f7750 ("memcg, kmem: further deprecate
kmem.limit_in_bytes") which have incrementally removed support for the
kernel memory accounting hard limit.  Unfortunately it has turned out that
there is still userspace depending on the existence of
memory.kmem.limit_in_bytes [1].  The underlying functionality is not
really required but the non-existent file just confuses the userspace
which fails in the result.  The patch to fix this on the userspace side
has been submitted but it is hard to predict how it will propagate through
the maze of 3rd party consumers of the software.

Now, reverting alone 86327e8eb9 is not an option because there is
another set of userspace which cannot cope with ENOTSUPP returned when
writing to the file.  Therefore we have to go and revisit 58056f7750 as
well.  There are two ways to go ahead.  Either we give up on the
deprecation and fully revert 58056f7750 as well or we can keep
kmem.limit_in_bytes but make the write a noop and warn about the fact. 
This should work for both known breaking workloads which depend on the
existence but do not depend on the hard limit enforcement.

Note to backporters to stable trees.  a8c49af3be ("memcg: add per-memcg
total kernel memory stat") introduced in 4.18 has added memcg_account_kmem
so the accounting is not done by obj_cgroup_charge_pages directly for v1
anymore.  Prior kernels need to add it explicitly (thanks to Johannes for
pointing this out).

[akpm@linux-foundation.org: fix build - remove unused local]
Link: http://lkml.kernel.org/r/20230920081101.GA12096@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net [1]
Link: https://lkml.kernel.org/r/ZRE5VJozPZt9bRPy@dhcp22.suse.cz
Fixes: 86327e8eb9 ("memcg: drop kmem.limit_in_bytes")
Fixes: 58056f7750 ("memcg, kmem: further deprecate kmem.limit_in_bytes")
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Michal Hocko 2023-09-21 09:38:29 +02:00 committed by Andrew Morton
parent ca56489c2f
commit 4597648fdd
2 changed files with 20 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
Documentation/admin-guide/cgroup-v1/memory.rst
View File

@ -92,6 +92,13 @@ Brief summary of control files.
memory.oom_control set/show oom controls.
memory.numa_stat show the number of memory usage per numa
node
memory.kmem.limit_in_bytes Deprecated knob to set and read the kernel
memory hard limit. Kernel hard limit is not
supported since 5.16. Writing any value to
do file will not have any effect same as if
nokmem kernel parameter was specified.
Kernel memory is still charged and reported
by memory.kmem.usage_in_bytes.
memory.kmem.usage_in_bytes show current kernel memory allocation
memory.kmem.failcnt show the number of kernel memory usage
hits limits

13
mm/memcontrol.c
View File

@ -3867,6 +3867,13 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
case _MEMSWAP:
ret = mem_cgroup_resize_max(memcg, nr_pages, true);
break;
case _KMEM:
pr_warn_once("kmem.limit_in_bytes is deprecated and will be removed. "
"Writing any value to this file has no effect. "
"Please report your usecase to linux-mm@kvack.org if you "
"depend on this functionality.\n");
ret = 0;
break;
case _TCP:
ret = memcg_update_tcp_max(memcg, nr_pages);
break;
@ -5077,6 +5084,12 @@ static struct cftype mem_cgroup_legacy_files[] = {
.seq_show = memcg_numa_stat_show,
},
#endif
{
.name = "kmem.limit_in_bytes",
.private = MEMFILE_PRIVATE(_KMEM, RES_LIMIT),
.write = mem_cgroup_write,
.read_u64 = mem_cgroup_read_u64,
},
{
.name = "kmem.usage_in_bytes",
.private = MEMFILE_PRIVATE(_KMEM, RES_USAGE),