Re: [PATCH] codetag: debug: Handle existing CODETAG_EMPTY in mark_objexts_empty for slabobj_ext

[Hao Ge <hao.ge@linux.dev>]

Hi Suren


On 2025/10/28 03:44, Suren Baghdasaryan wrote:
> On Mon, Oct 27, 2025 at 1:53 AM Hao Ge <hao.ge@linux.dev> wrote:
>> From: Hao Ge <gehao@kylinos.cn>
>>
>> Even though obj_exts was created with the __GFP_NO_OBJ_EXT flag,
>> objects in the same slab may have their extensions allocated via
>> alloc_slab_obj_exts, and handle_failed_objexts_alloc may be called
>> within alloc_slab_obj_exts to set their codetag to CODETAG_EMPTY.
>>
>> Therefore, both NULL and CODETAG_EMPTY are valid for the codetag of
>> slabobj_ext, as we do not need to re-set it to CODETAG_EMPTY if it
>> is already CODETAG_EMPTY. It also resolves the warning triggered when
>> the codetag is CODETAG_EMPTY during slab freeing.
> I'm not sure what scenario leads to handle_failed_objexts_alloc() and
> mark_objexts_empty() being used against the same codetag reference.
> Could you please explain the exact scenario you hit?
>
> handle_failed_objexts_alloc() assigns CODETAG_EMPTY to the elements of
> the obj_exts vector while mark_objexts_empty() assigns CODETAG_EMPTY
> to the obj_ext of the obj_exts vector itself. In what case do these
> two calls operate on the same reference?

This issue also occurred during our memory stress testing.

I apologize for the incorrect description in my commit message:

The possibility of its occurrence should be as follows:

When a slab allocates a slabobj_ext, the slab to which this slabobj_ext 
belongs may have already allocated its own slabobj_ext

and called handle_failed_objexts_alloc. That is to say, the codetag of 
this slabobj_ext has been set to CODETAG_EMPTY.

To quickly detect this WARN, I modified the code 
from:WARN_ON(slab_exts[offs].ref.ct) to WARN_ON(slab_exts[offs].ref.ct 
== 1);

We then obtained this message:

[21630.898561] ------------[ cut here ]------------
[21630.898596] kernel BUG at mm/slub.c:2050!
[21630.898611] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
[21630.900372] Modules linked in: squashfs isofs vfio_iommu_type1 
vhost_vsock vfio vhost_net vmw_vsock_virtio_transport_common vhost tap 
vhost_iotlb iommufd vsock binfmt_misc nfsv3 nfs_acl nfs lockd grace 
netfs tls rds dns_resolver tun brd overlay ntfs3 exfat btrfs 
blake2b_generic xor xor_neon raid6_pq loop sctp ip6_udp_tunnel 
udp_tunnel nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib 
nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct 
nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 
nf_tables rfkill ip_set sunrpc vfat fat joydev sg sch_fq_codel nfnetlink 
virtio_gpu sr_mod cdrom drm_client_lib virtio_dma_buf drm_shmem_helper 
drm_kms_helper drm ghash_ce backlight virtio_net virtio_blk virtio_scsi 
net_failover virtio_console failover virtio_mmio dm_mirror 
dm_region_hash dm_log dm_multipath dm_mod fuse i2c_dev virtio_pci 
virtio_pci_legacy_dev virtio_pci_modern_dev virtio virtio_ring autofs4 
aes_neon_bs aes_ce_blk [last unloaded: hwpoison_inject]
[21630.909177] CPU: 3 UID: 0 PID: 3787 Comm: kylin-process-m Kdump: 
loaded Tainted: G        W           6.18.0-rc1+ #74 PREEMPT(voluntary)
[21630.910495] Tainted: [W]=WARN
[21630.910867] Hardware name: QEMU KVM Virtual Machine, BIOS unknown 
2/2/2022
[21630.911625] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS 
BTYPE=--)
[21630.912392] pc : __free_slab+0x228/0x250
[21630.912868] lr : __free_slab+0x18c/0x250[21630.913334] sp : 
ffff8000a02f73e0
[21630.913830] x29: ffff8000a02f73e0 x28: fffffdffc43fc800 x27: 
ffff0000c0011c40
[21630.914677] x26: ffff0000c000cac0 x25: ffff00010fe5e5f0 x24: 
ffff000102199b40
[21630.915469] x23: 0000000000000003 x22: 0000000000000003 x21: 
ffff0000c0011c40
[21630.916259] x20: fffffdffc4086600 x19: fffffdffc43fc800 x18: 
0000000000000000
[21630.917048] x17: 0000000000000000 x16: 0000000000000000 x15: 
0000000000000000
[21630.917837] x14: 0000000000000000 x13: 0000000000000000 x12: 
ffff70001405ee66
[21630.918640] x11: 1ffff0001405ee65 x10: ffff70001405ee65 x9 : 
ffff800080a295dc
[21630.919442] x8 : ffff8000a02f7330 x7 : 0000000000000000 x6 : 
0000000000003000
[21630.920232] x5 : 0000000024924925 x4 : 0000000000000001 x3 : 
0000000000000007
[21630.921021] x2 : 0000000000001b40 x1 : 000000000000001f x0 : 
0000000000000001
[21630.921810] Call trace:
[21630.922130]  __free_slab+0x228/0x250 (P)
[21630.922669]  free_slab+0x38/0x118
[21630.923079]  free_to_partial_list+0x1d4/0x340
[21630.923591]  __slab_free+0x24c/0x348
[21630.924024]  ___cache_free+0xf0/0x110
[21630.924468]  qlist_free_all+0x78/0x130
[21630.924922]  kasan_quarantine_reduce+0x114/0x148
[21630.925525]  __kasan_slab_alloc+0x7c/0xb0
[21630.926006]  kmem_cache_alloc_noprof+0x164/0x5c8
[21630.926699]  __alloc_object+0x44/0x1f8
[21630.927153]  __create_object+0x34/0xc8
[21630.927604]  kmemleak_alloc+0xb8/0xd8
[21630.928052]  kmem_cache_alloc_noprof+0x368/0x5c8
[21630.928606]  getname_flags.part.0+0xa4/0x610
[21630.929112]  getname_flags+0x80/0xd8
[21630.929557]  vfs_fstatat+0xc8/0xe0
[21630.929975]  __do_sys_newfstatat+0xa0/0x100
[21630.930469]  __arm64_sys_newfstatat+0x90/0xd8
[21630.931046]  invoke_syscall+0xd4/0x258
[21630.931685]  el0_svc_common.constprop.0+0xb4/0x240
[21630.932467]  do_el0_svc+0x48/0x68
[21630.932972]  el0_svc+0x40/0xe0
[21630.933472]  el0t_64_sync_handler+0xa0/0xe8
[21630.934151]  el0t_64_sync+0x1ac/0x1b0
[21630.934923] Code: aa1803e0 97ffef2b a9446bf9 17ffff9c (d4210000)
[21630.936461] SMP: stopping secondary CPUs
[21630.939550] Starting crashdump kernel...
[21630.940108] Bye!

>> Fixes: 09c46563ff6d ("codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations")
>> Signed-off-by: Hao Ge <gehao@kylinos.cn>
>> ---
>>   mm/slub.c | 12 +++++++++++-
>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index d4367f25b20d..cda8f75b72e7 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2046,7 +2046,17 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
>>          if (slab_exts) {
>>                  unsigned int offs = obj_to_index(obj_exts_slab->slab_cache,
>>                                                   obj_exts_slab, obj_exts);
>> -               /* codetag should be NULL */
>> +
>> +               /*
>> +                * codetag should be either NULL or CODETAG_EMPTY.
>> +                * When the same slab calls handle_failed_objexts_alloc,
>> +                * it will set us to CODETAG_EMPTY.
>> +                *
>> +                * If codetag is already CODETAG_EMPTY, no action is needed here.
>> +                */
>> +               if (unlikely(is_codetag_empty(&slab_exts[offs].ref)))
>> +                       return;
>> +
>>                  WARN_ON(slab_exts[offs].ref.ct);
>>                  set_codetag_empty(&slab_exts[offs].ref);
>>          }
>> --
>> 2.25.1
>>