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memcg: make the v1 soft limit knob inert

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The v1 soft limit has been deprecated since v6.12 and nobody has reported
depending on it.  Start the removal by decoupling the interface from the
implementation: keep memory.soft_limit_in_bytes, but ignore writes to it
and always report the maximum value on read similar to what
memory.kmem.limit_in_bytes already does.

Writes are still parsed, so malformed input keeps returning -EINVAL.  The
knob now also behaves the same everywhere: it used to return -EOPNOTSUPP
on PREEMPT_RT, where soft limit reclaim has always been disabled.

This also fixes the syzbot report linked below.  Soft limit reclaim is the
only caller that runs shrink_lruvec() from kswapd against a specific
memcg, so it is the only way to reach lru_gen_shrink_lruvec() and in turn
set_mm_walk(), which warns when called from kswapd.

Link: https://lore.kernel.org/20260811203203.3456029-2-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reported-by: syzbot+12ee2725d5fde63a9c96@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6a7a6929.b50370da.49fe0.005e.GAE@google.com/
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Shakeel Butt
2026-09-01 20:26:20 -07:00
committed by Andrew Morton
parent 7b8a8ae4dd
commit a3417097fb
2 changed files with 32 additions and 60 deletions
Download Patch File Download Diff File Expand all files Collapse all files
mm/memcontrol-v1.c
+26 -17
View File
@@ -96,7 +96,6 @@ enum {
RES_LIMIT,
RES_MAX_USAGE,
RES_FAILCNT,
RES_SOFT_LIMIT,
};
#ifdef CONFIG_LOCKDEP
@@ -1888,6 +1887,30 @@ static int mem_cgroup_hierarchy_write(struct cgroup_subsys_state *css,
return -EINVAL;
}
static u64 mem_cgroup_soft_limit_read(struct cgroup_subsys_state *css,
struct cftype *cft)
{
return (u64)PAGE_COUNTER_MAX * PAGE_SIZE;
}
static ssize_t mem_cgroup_soft_limit_write(struct kernfs_open_file *of,
char *buf, size_t nbytes, loff_t off)
{
unsigned long nr_pages;
int ret;
ret = page_counter_memparse(strstrip(buf), "-1", &nr_pages);
if (ret)
return ret;
pr_warn_once("soft_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");
return nbytes;
}
static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
struct cftype *cft)
{
@@ -1924,8 +1947,6 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
return (u64)counter->watermark * PAGE_SIZE;
case RES_FAILCNT:
return counter->failcnt;
case RES_SOFT_LIMIT:
return (u64)READ_ONCE(memcg->soft_limit) * PAGE_SIZE;
default:
BUG();
}
@@ -2020,17 +2041,6 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
break;
}
break;
case RES_SOFT_LIMIT:
if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
ret = -EOPNOTSUPP;
} else {
pr_warn_once("soft_limit_in_bytes is deprecated and will be removed. "
"Please report your usecase to linux-mm@kvack.org if you "
"depend on this functionality.\n");
WRITE_ONCE(memcg->soft_limit, nr_pages);
ret = 0;
}
break;
}
return ret ?: nbytes;
}
@@ -2384,9 +2394,8 @@ struct cftype mem_cgroup_legacy_files[] = {
},
{
.name = "soft_limit_in_bytes",
.private = MEMFILE_PRIVATE(_MEM, RES_SOFT_LIMIT),
.write = mem_cgroup_write,
.read_u64 = mem_cgroup_read_u64,
.write = mem_cgroup_soft_limit_write,
.read_u64 = mem_cgroup_soft_limit_read,
},
{
.name = "failcnt",