From: Roman Gushchin <guro@xxxxxx>
Subject: dcache: account external names as indirectly reclaimable memory
I received a report about suspicious growth of unreclaimable slabs on some
machines. I've found that it happens on machines with low memory
pressure, and these unreclaimable slabs are external names attached to
dentries.
External names are allocated using generic kmalloc() function, so they are
accounted as unreclaimable. But they are held by dentries, which are
reclaimable, and they will be reclaimed under the memory pressure.
In particular, this breaks MemAvailable calculation, as it doesn't take
unreclaimable slabs into account. This leads to a silly situation, when a
machine is almost idle, has no memory pressure and therefore has a big
dentry cache. And the resulting MemAvailable is too low to start a new
workload.
To address the issue, the NR_INDIRECTLY_RECLAIMABLE_BYTES counter is used
to track the amount of memory, consumed by external names. The counter is
increased in the dentry allocation path, if an external name structure is
allocated; and it's decreased in the dentry freeing path.
To reproduce the problem I've used the following Python script:
import os
for iter in range (0, 10000000):
try:
name = ("/some_long_name_%d" % iter) + "_" * 220
os.stat(name)
except Exception:
pass
Without this patch:
$ cat /proc/meminfo | grep MemAvailable
MemAvailable: 7811688 kB
$ python indirect.py
$ cat /proc/meminfo | grep MemAvailable
MemAvailable: 2753052 kB
With the patch:
$ cat /proc/meminfo | grep MemAvailable
MemAvailable: 7809516 kB
$ python indirect.py
$ cat /proc/meminfo | grep MemAvailable
MemAvailable: 7749144 kB
[guro@xxxxxx: fix indirectly reclaimable memory accounting for CONFIG_SLOB]
Link: http://lkml.kernel.org/r/20180312194140.19517-1-guro@xxxxxx
[guro@xxxxxx: fix indirectly reclaimable memory accounting]
Link: http://lkml.kernel.org/r/20180313125701.7955-1-guro@xxxxxx
Link: http://lkml.kernel.org/r/20180305133743.12746-5-guro@xxxxxx
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/dcache.c | 39 ++++++++++++++++++++++++++++++---------
1 file changed, 30 insertions(+), 9 deletions(-)
diff -puN fs/dcache.c~dcache-account-external-names-as-indirectly-reclaimable-memory fs/dcache.c
--- a/fs/dcache.c~dcache-account-external-names-as-indirectly-reclaimable-memory
+++ a/fs/dcache.c
@@ -257,11 +257,25 @@ static void __d_free(struct rcu_head *he
kmem_cache_free(dentry_cache, dentry);
}
+static void __d_free_external_name(struct rcu_head *head)
+{
+ struct external_name *name = container_of(head, struct external_name,
+ u.head);
+
+ mod_node_page_state(page_pgdat(virt_to_page(name)),
+ NR_INDIRECTLY_RECLAIMABLE_BYTES,
+ -ksize(name));
+
+ kfree(name);
+}
+
static void __d_free_external(struct rcu_head *head)
{
struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
- kfree(external_name(dentry));
- kmem_cache_free(dentry_cache, dentry);
+
+ __d_free_external_name(&external_name(dentry)->u.head);
+
+ kmem_cache_free(dentry_cache, dentry);
}
static inline int dname_external(const struct dentry *dentry)
@@ -291,7 +305,7 @@ void release_dentry_name_snapshot(struct
struct external_name *p;
p = container_of(name->name, struct external_name, name[0]);
if (unlikely(atomic_dec_and_test(&p->u.count)))
- kfree_rcu(p, u.head);
+ call_rcu(&p->u.head, __d_free_external_name);
}
}
EXPORT_SYMBOL(release_dentry_name_snapshot);
@@ -1617,6 +1631,7 @@ EXPORT_SYMBOL(d_invalidate);
struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
{
+ struct external_name *ext = NULL;
struct dentry *dentry;
char *dname;
int err;
@@ -1637,14 +1652,14 @@ struct dentry *__d_alloc(struct super_bl
dname = dentry->d_iname;
} else if (name->len > DNAME_INLINE_LEN-1) {
size_t size = offsetof(struct external_name, name[1]);
- struct external_name *p = kmalloc(size + name->len,
- GFP_KERNEL_ACCOUNT);
- if (!p) {
+
+ ext = kmalloc(size + name->len, GFP_KERNEL_ACCOUNT);
+ if (!ext) {
kmem_cache_free(dentry_cache, dentry);
return NULL;
}
- atomic_set(&p->u.count, 1);
- dname = p->name;
+ atomic_set(&ext->u.count, 1);
+ dname = ext->name;
} else {
dname = dentry->d_iname;
}
@@ -1683,6 +1698,12 @@ struct dentry *__d_alloc(struct super_bl
}
}
+ if (unlikely(ext)) {
+ pg_data_t *pgdat = page_pgdat(virt_to_page(ext));
+ mod_node_page_state(pgdat, NR_INDIRECTLY_RECLAIMABLE_BYTES,
+ ksize(ext));
+ }
+
this_cpu_inc(nr_dentry);
return dentry;
@@ -2770,7 +2791,7 @@ static void copy_name(struct dentry *den
dentry->d_name.hash_len = target->d_name.hash_len;
}
if (old_name && likely(atomic_dec_and_test(&old_name->u.count)))
- kfree_rcu(old_name, u.head);
+ call_rcu(&old_name->u.head, __d_free_external_name);
}
/*
_
--
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