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Date:   Mon, 8 Apr 2019 11:59:17 +1000
From:   "Tobin C. Harding" <me@tobin.cc>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, cl@linux.com, penberg@kernel.org,
        rientjes@google.com, iamjoonsoo.kim@lge.com, tj@kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] slab: fix a crash by reading /proc/slab_allocators
Message-ID: <20190408015917.GA633@eros.localdomain>
References: <20190406225901.35465-1-cai@lca.pw>
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On Sat, Apr 06, 2019 at 06:59:01PM -0400, Qian Cai wrote:
> The commit 510ded33e075 ("slab: implement slab_root_caches list")
> changes the name of the list node within "struct kmem_cache" from
> "list" to "root_caches_node"

Are you sure? It looks to me like it adds a member to the memcg_cache_array

diff --git a/include/linux/slab.h b/include/linux/slab.h
index a0cc7a77cda2..af1a5bef80f4 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -556,6 +556,8 @@ struct memcg_cache_array {
  *             used to index child cachces during allocation and cleared
  *             early during shutdown.
  * 
+ * @root_caches_node: List node for slab_root_caches list.
+ * 
  * @children:  List of all child caches.  While the child caches are also
  *             reachable through @memcg_caches, a child cache remains on
  *             this list until it is actually destroyed.
@@ -573,6 +575,7 @@ struct memcg_cache_params {
        union { 
                struct {
                        struct memcg_cache_array __rcu *memcg_caches;
+                       struct list_head __root_caches_node;
                        struct list_head children;
                };

And then defines 'root_caches_node' to be 'memcg_params.__root_caches_node'
if we have CONFIG_MEMCG otherwise defines 'root_caches_node' to be 'list'


> but leaks_show() still use the "list"

I believe it should since 'list' is used to add to slab_caches list.

> which causes a crash when reading /proc/slab_allocators.

I was unable to reproduce this crash, I built with

# CONFIG_MEMCG is not set
CONFIG_SLAB=y
CONFIG_SLAB_MERGE_DEFAULT=y
CONFIG_SLAB_FREELIST_RANDOM=y
CONFIG_DEBUG_SLAB=y
CONFIG_DEBUG_SLAB_LEAK=y

I then booted in Qemu and successfully ran 
$ cat slab_allocators

Perhaps you could post your config?

Hope this helps,
Tobin.