CONFIG_TEST_VMALLOC=y conflict/race with alloc_tag_init

[David Wang <00107082@163.com>]

On Wed, Jun 18, 2025 at 02:25:37PM +0800, kernel test robot wrote:
> 
> Hello,
> 
> for this change, we reported
> "[linux-next:master] [lib/test_vmalloc.c]  7fc85b92db: Mem-Info"
> in
> https://lore.kernel.org/all/202505071555.e757f1e0-lkp@intel.com/
> 
> at that time, we made some tests with x86_64 config which runs well.
> 
> now we noticed the commit is in mainline now.

> the config still has expected diff with parent:
> 
> --- /pkg/linux/x86_64-randconfig-161-20250614/gcc-12/7a73348e5d4715b5565a53f21c01ea7b54e46cbd/.config   2025-06-17 14:40:29.481052101 +0800
> +++ /pkg/linux/x86_64-randconfig-161-20250614/gcc-12/2d76e79315e403aab595d4c8830b7a46c19f0f3b/.config   2025-06-17 14:41:18.448543738 +0800
> @@ -7551,7 +7551,7 @@ CONFIG_TEST_IDA=m
>  CONFIG_TEST_MISC_MINOR=m
>  # CONFIG_TEST_LKM is not set
>  CONFIG_TEST_BITOPS=m
> -CONFIG_TEST_VMALLOC=m
> +CONFIG_TEST_VMALLOC=y
>  # CONFIG_TEST_BPF is not set
>  CONFIG_FIND_BIT_BENCHMARK=m
>  # CONFIG_TEST_FIRMWARE is not set
> 
> 
> then we noticed similar random issue with x86_64 randconfig this time.
> 
> 7a73348e5d4715b5 2d76e79315e403aab595d4c8830
> ---------------- ---------------------------
>        fail:runs  %reproduction    fail:runs
>            |             |             |
>            :199         34%          67:200   dmesg.KASAN:null-ptr-deref_in_range[#-#]
>            :199         34%          67:200   dmesg.Kernel_panic-not_syncing:Fatal_exception
>            :199         34%          67:200   dmesg.Mem-Info
>            :199         34%          67:200   dmesg.Oops:general_protection_fault,probably_for_non-canonical_address#:#[##]SMP_KASAN
>            :199         34%          67:200   dmesg.RIP:down_read_trylock
> 
> we don't have enough knowledge to understand the relationship between code
> change and the random issues. just report what we obsverved in our tests FYI.
> 

I think this is caused by a race between vmalloc_test_init and alloc_tag_init.

vmalloc_test actually depends on alloc_tag via alloc_tag_top_users, because when
memory allocation fails show_mem() would invoke alloc_tag_top_users.

With following configuration:

CONFIG_TEST_VMALLOC=y
CONFIG_MEM_ALLOC_PROFILING=y
CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT=y
CONFIG_MEM_ALLOC_PROFILING_DEBUG=y

If vmalloc_test_init starts before alloc_tag_init, show_mem() would cause
a NULL deference because alloc_tag_cttype was not init yet.

I add some debug to confirm this theory
diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
index d48b80f3f007..9b8e7501010f 100644
--- a/lib/alloc_tag.c
+++ b/lib/alloc_tag.c
@@ -133,6 +133,8 @@ size_t alloc_tag_top_users(struct codetag_bytes *tags, size_t count, bool can_sl
        struct codetag *ct;
        struct codetag_bytes n;
        unsigned int i, nr = 0;
+       pr_info("memory profiling alloc top %d: %llx\n", mem_profiling_support, (long long)alloc_tag_cttype);
+       return 0;
 
        if (can_sleep)
                codetag_lock_module_list(alloc_tag_cttype, true);
@@ -831,6 +833,7 @@ static int __init alloc_tag_init(void)
                shutdown_mem_profiling(true);
                return PTR_ERR(alloc_tag_cttype);
        }
+       pr_info("memory profiling ready %d: %llx\n", mem_profiling_support, (long long)alloc_tag_cttype);
 
        return 0;
 }

When bootup the kernel, the log shows:

$ sudo dmesg -T | grep profiling
[Fri Jun 20 17:29:35 2025] memory profiling alloc top 1: 0  <--- alloc_tag_cttype == NULL
[Fri Jun 20 17:30:24 2025] memory profiling ready 1: ffff9b1641aa06c0


vmalloc_test_init should happened after alloc_tag_init if CONFIG_TEST_VMALLOC=y,
or mem_show() should check whether alloc_tag is done initialized when calling
alloc_tag_top_users



David