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mm, slab, slub: stop taking memory hotplug lock

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Since commit 03afc0e25f ("slab: get_online_mems for
kmem_cache_{create,destroy,shrink}") we are taking memory hotplug lock for
SLAB and SLUB when creating, destroying or shrinking a cache.  It is quite
a heavy lock and it's best to avoid it if possible, as we had several
issues with lockdep complaining about ordering in the past, see e.g.
e4f8e513c3 ("mm/slub: fix a deadlock in show_slab_objects()").

The problem scenario in 03afc0e25f (solved by the memory hotplug lock)
can be summarized as follows: while there's slab_mutex synchronizing new
kmem cache creation and SLUB's MEM_GOING_ONLINE callback
slab_mem_going_online_callback(), we may miss creation of kmem_cache_node
for the hotplugged node in the new kmem cache, because the hotplug
callback doesn't yet see the new cache, and cache creation in
init_kmem_cache_nodes() only inits kmem_cache_node for nodes in the
N_NORMAL_MEMORY nodemask, which however may not yet include the new node,
as that happens only later after the MEM_GOING_ONLINE callback.

Instead of using get/put_online_mems(), the problem can be solved by SLUB
maintaining its own nodemask of nodes for which it has allocated the
per-node kmem_cache_node structures.  This nodemask would generally mirror
the N_NORMAL_MEMORY nodemask, but would be updated only in under SLUB's
control in its memory hotplug callbacks under the slab_mutex.  This patch
adds such nodemask and its handling.

Commit 03afc0e25f mentiones "issues like [the one above]", but there
don't appear to be further issues.  All the paths (shared for SLAB and
SLUB) taking the memory hotplug locks are also taking the slab_mutex,
except kmem_cache_shrink() where 03afc0e25f replaced slab_mutex with
get/put_online_mems().

We however cannot simply restore slab_mutex in kmem_cache_shrink(), as
SLUB can enters the function from a write to sysfs 'shrink' file, thus
holding kernfs lock, and in kmem_cache_create() the kernfs lock is nested
within slab_mutex.  But on closer inspection we don't actually need to
protect kmem_cache_shrink() from hotplug callbacks: While SLUB's
__kmem_cache_shrink() does for_each_kmem_cache_node(), missing a new node
added in parallel hotplug is not fatal, and parallel hotremove does not
free kmem_cache_node's anymore after the previous patch, so use-after free
cannot happen.  The per-node shrinking itself is protected by
n->list_lock.  Same is true for SLAB, and SLOB is no-op.

SLAB also doesn't need the memory hotplug locking, which it only gained by
03afc0e25f through the shared paths in slab_common.c.  Its memory
hotplug callbacks are also protected by slab_mutex against races with
these paths.  The problem of SLUB relying on N_NORMAL_MEMORY doesn't apply
to SLAB, as its setup_kmem_cache_nodes relies on N_ONLINE, and the new
node is already set there during the MEM_GOING_ONLINE callback, so no
special care is needed for SLAB.

As such, this patch removes all get/put_online_mems() usage by the slab
subsystem.

Link: https://lkml.kernel.org/r/20210113131634.3671-3-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>
This commit is contained in:
Vlastimil Babka 2021-02-24 12:01:12 -08:00 committed by Linus Torvalds
parent 666716fd26
commit 7e1fa93def
2 changed files with 27 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
mm/slab_common.c
View File

@ -310,7 +310,6 @@ kmem_cache_create_usercopy(const char *name,
int err;
get_online_cpus();
get_online_mems();
mutex_lock(&slab_mutex);
@ -360,7 +359,6 @@ kmem_cache_create_usercopy(const char *name,
out_unlock:
mutex_unlock(&slab_mutex);
put_online_mems();
put_online_cpus();
if (err) {
@ -487,7 +485,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
return;
get_online_cpus();
get_online_mems();
mutex_lock(&slab_mutex);
@ -504,7 +501,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
out_unlock:
mutex_unlock(&slab_mutex);
put_online_mems();
put_online_cpus();
}
EXPORT_SYMBOL(kmem_cache_destroy);
@ -523,10 +519,10 @@ int kmem_cache_shrink(struct kmem_cache *cachep)
int ret;
get_online_cpus();
get_online_mems();
kasan_cache_shrink(cachep);
ret = __kmem_cache_shrink(cachep);
put_online_mems();
put_online_cpus();
return ret;
}

28
mm/slub.c
View File

@ -235,6 +235,14 @@ static inline void stat(const struct kmem_cache *s, enum stat_item si)
#endif
}
/*
* Tracks for which NUMA nodes we have kmem_cache_nodes allocated.
* Corresponds to node_state[N_NORMAL_MEMORY], but can temporarily
* differ during memory hotplug/hotremove operations.
* Protected by slab_mutex.
*/
static nodemask_t slab_nodes;
/********************************************************************
* Core slab cache functions
*******************************************************************/
@ -2678,7 +2686,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
* ignore the node constraint
*/
if (unlikely(node != NUMA_NO_NODE &&
!node_state(node, N_NORMAL_MEMORY)))
!node_isset(node, slab_nodes)))
node = NUMA_NO_NODE;
goto new_slab;
}
@ -2689,7 +2697,7 @@ redo:
* same as above but node_match() being false already
* implies node != NUMA_NO_NODE
*/
if (!node_state(node, N_NORMAL_MEMORY)) {
if (!node_isset(node, slab_nodes)) {
node = NUMA_NO_NODE;
goto redo;
} else {
@ -3592,7 +3600,7 @@ static int init_kmem_cache_nodes(struct kmem_cache *s)
{
int node;
for_each_node_state(node, N_NORMAL_MEMORY) {
for_each_node_mask(node, slab_nodes) {
struct kmem_cache_node *n;
if (slab_state == DOWN) {
@ -4286,6 +4294,7 @@ static void slab_mem_offline_callback(void *arg)
return;
mutex_lock(&slab_mutex);
node_clear(offline_node, slab_nodes);
/*
* We no longer free kmem_cache_node structures here, as it would be
* racy with all get_node() users, and infeasible to protect them with
@ -4335,6 +4344,11 @@ static int slab_mem_going_online_callback(void *arg)
init_kmem_cache_node(n);
s->node[nid] = n;
}
/*
* Any cache created after this point will also have kmem_cache_node
* initialized for the new node.
*/
node_set(nid, slab_nodes);
out:
mutex_unlock(&slab_mutex);
return ret;
@ -4415,6 +4429,7 @@ void __init kmem_cache_init(void)
{
static __initdata struct kmem_cache boot_kmem_cache,
boot_kmem_cache_node;
int node;
if (debug_guardpage_minorder())
slub_max_order = 0;
@ -4422,6 +4437,13 @@ void __init kmem_cache_init(void)
kmem_cache_node = &boot_kmem_cache_node;
kmem_cache = &boot_kmem_cache;
/*
* Initialize the nodemask for which we will allocate per node
* structures. Here we don't need taking slab_mutex yet.
*/
for_each_node_state(node, N_NORMAL_MEMORY)
node_set(node, slab_nodes);
create_boot_cache(kmem_cache_node, "kmem_cache_node",
sizeof(struct kmem_cache_node), SLAB_HWCACHE_ALIGN, 0, 0);