From 7fc60ad173f4d8f29abec9a1d01ce3fccdcbffe9 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Fri, 9 Jul 2021 17:22:30 +0800
Subject: [PATCH] mm, slab, slub: stop taking cpu hotplug lock
mainline inclusion
from mainline-5.12-rc1
commit 59450bbc12bee1c4e5dd25e6aa5d6a45a7bd6e81
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I3ZY8M
CVE: NA
-------------------------------------------------
SLAB has been using get/put_online_cpus() around creating, destroying and
shrinking kmem caches since 95402b382901 ("cpu-hotplug: replace
per-subsystem mutexes with get_online_cpus()") in 2008, which is supposed
to be replacing a private mutex (cache_chain_mutex, called slab_mutex
today) with system-wide mechanism, but in case of SLAB it's in fact used
in addition to the existing mutex, without explanation why.
SLUB appears to have avoided the cpu hotplug lock initially, but gained it
due to common code unification, such as 20cea9683ecc ("mm, sl[aou]b: Move
kmem_cache_create mutex handling to common code").
Regardless of the history, checking if the hotplug lock is actually needed
today suggests that it's not, and therefore it's better to avoid this
system-wide lock and the ordering this imposes wrt other locks (such as
slab_mutex).
Specifically, in SLAB we have for_each_online_cpu() in do_tune_cpucache()
protected by slab_mutex, and cpu hotplug callbacks that also take the
slab_mutex, which is also taken by the common slab function that currently
also take the hotplug lock. Thus the slab_mutex protection should be
sufficient. Also per-cpu array caches are allocated for each possible
cpu, so not affected by their online/offline state.
In SLUB we have for_each_online_cpu() in functions that show statistics
and are already unprotected today, as racing with hotplug is not harmful.
Otherwise SLUB relies on percpu allocator. The slub_cpu_dead() hotplug
callback takes the slab_mutex.
To sum up, this patch removes get/put_online_cpus() calls from slab as it
should be safe without further adjustments.
Link: https://lkml.kernel.org/r/20210113131634.3671-4-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Qian Cai <cai@redhat.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Chengyang Fan <cy.fan@huawei.com>
Reviewed-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
---
mm/slab_common.c | 10 ----------
1 file changed, 10 deletions(-)
diff --git a/mm/slab_common.c b/mm/slab_common.c
index e8519d5f3a32..79ae785f2c9f 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -307,8 +307,6 @@ kmem_cache_create_usercopy(const char *name,
const char *cache_name;
int err;
- get_online_cpus();
-
mutex_lock(&slab_mutex);
err = kmem_cache_sanity_check(name, size);
@@ -357,8 +355,6 @@ kmem_cache_create_usercopy(const char *name,
out_unlock:
mutex_unlock(&slab_mutex);
- put_online_cpus();
-
if (err) {
if (flags & SLAB_PANIC)
panic("kmem_cache_create: Failed to create slab '%s'. Error %d\n",
@@ -482,8 +478,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
if (unlikely(!s))
return;
- get_online_cpus();
-
mutex_lock(&slab_mutex);
s->refcount--;
@@ -498,8 +492,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
}
out_unlock:
mutex_unlock(&slab_mutex);
-
- put_online_cpus();
}
EXPORT_SYMBOL(kmem_cache_destroy);
@@ -516,12 +508,10 @@ int kmem_cache_shrink(struct kmem_cache *cachep)
{
int ret;
- get_online_cpus();
kasan_cache_shrink(cachep);
ret = __kmem_cache_shrink(cachep);
- put_online_cpus();
return ret;
}
EXPORT_SYMBOL(kmem_cache_shrink);
--
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