Re: [PATCH v3] lib/alloc_tag: do not acquire non-existent lock in alloc_tag_top_users()y

["David Wang" <00107082@163.com>]

At 2025-06-24 17:09:54, "Harry Yoo" <harry.yoo@oracle.com> wrote:
>On Tue, Jun 24, 2025 at 04:21:23PM +0800, David Wang wrote:
>> At 2025-06-24 15:25:13, "Harry Yoo" <harry.yoo@oracle.com> wrote:
>> >alloc_tag_top_users() attempts to lock alloc_tag_cttype->mod_lock
>> >even when the alloc_tag_cttype is not allocated because:
>> >
>> >  1) alloc tagging is disabled because mem profiling is disabled
>> >     (!alloc_tag_cttype)
>> >  2) alloc tagging is enabled, but not yet initialized (!alloc_tag_cttype)
>> >  3) alloc tagging is enabled, but failed initialization
>> >     (!alloc_tag_cttype or IS_ERR(alloc_tag_cttype))
>> >
>> >In all cases, alloc_tag_cttype is not allocated, and therefore
>> >alloc_tag_top_users() should not attempt to acquire the semaphore.
>> >
>> >This leads to a crash on memory allocation failure by attempting to
>> >acquire a non-existent semaphore:
>> >
>> >  Oops: general protection fault, probably for non-canonical address 0xdffffc000000001b: 0000 [#3] SMP KASAN NOPTI
>> >  KASAN: null-ptr-deref in range [0x00000000000000d8-0x00000000000000df]
>> >  CPU: 2 UID: 0 PID: 1 Comm: systemd Tainted: G      D             6.16.0-rc2 #1 VOLUNTARY
>> >  Tainted: [D]=DIE
>> >  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>> >  RIP: 0010:down_read_trylock+0xaa/0x3b0
>> >  Code: d0 7c 08 84 d2 0f 85 a0 02 00 00 8b 0d df 31 dd 04 85 c9 75 29 48 b8 00 00 00 00 00 fc ff df 48 8d 6b 68 48 89 ea 48 c1 ea 03 <80> 3c 02 00 0f 85 88 02 00 00 48 3b 5b 68 0f 85 53 01 00 00 65 ff
>> >  RSP: 0000:ffff8881002ce9b8 EFLAGS: 00010016
>> >  RAX: dffffc0000000000 RBX: 0000000000000070 RCX: 0000000000000000
>> >  RDX: 000000000000001b RSI: 000000000000000a RDI: 0000000000000070
>> >  RBP: 00000000000000d8 R08: 0000000000000001 R09: ffffed107dde49d1
>> >  R10: ffff8883eef24e8b R11: ffff8881002cec20 R12: 1ffff11020059d37
>> >  R13: 00000000003fff7b R14: ffff8881002cec20 R15: dffffc0000000000
>> >  FS:  00007f963f21d940(0000) GS:ffff888458ca6000(0000) knlGS:0000000000000000
>> >  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> >  CR2: 00007f963f5edf71 CR3: 000000010672c000 CR4: 0000000000350ef0
>> >  Call Trace:
>> >   <TASK>
>> >   codetag_trylock_module_list+0xd/0x20
>> >   alloc_tag_top_users+0x369/0x4b0
>> >   __show_mem+0x1cd/0x6e0
>> >   warn_alloc+0x2b1/0x390
>> >   __alloc_frozen_pages_noprof+0x12b9/0x21a0
>> >   alloc_pages_mpol+0x135/0x3e0
>> >   alloc_slab_page+0x82/0xe0
>> >   new_slab+0x212/0x240
>> >   ___slab_alloc+0x82a/0xe00
>> >   </TASK>
>> >
>> >As David Wang points out, this issue became easier to trigger after commit
>> >780138b12381 ("alloc_tag: check mem_profiling_support in alloc_tag_init").
>> >
>> >Before the commit, the issue occurred only when it failed to allocate
>> >and initialize alloc_tag_cttype or if a memory allocation fails before
>> >alloc_tag_init() is called. After the commit, it can be easily triggered
>> >when memory profiling is compiled but disabled at boot.
>> >
>> >To properly determine whether alloc_tag_init() has been called and
>> >its data structures initialized, verify that alloc_tag_cttype is a valid
>> >pointer before acquiring the semaphore. If the variable is NULL or an error
>> >value, it has not been properly initialized. In such a case, just skip
>> >and do not attempt to acquire the semaphore.
>> >
>> >Reported-by: kernel test robot <oliver.sang@intel.com>
>> >Closes: https://urldefense.com/v3/__https://lore.kernel.org/oe-lkp/202506181351.bba867dd-lkp@intel.com__;!!ACWV5N9M2RV99hQ!PxJNKp4Bj6h0XIWpRXcmFeIz51jORtRRAo1j23ZnRgvTm0E0Mp5l6UrLNCkiHww6AVWOSfbDDdBwKgJ9_Q$ 
>> >Closes: https://urldefense.com/v3/__https://lore.kernel.org/oe-lkp/202506131711.5b41931c-lkp@intel.com__;!!ACWV5N9M2RV99hQ!PxJNKp4Bj6h0XIWpRXcmFeIz51jORtRRAo1j23ZnRgvTm0E0Mp5l6UrLNCkiHww6AVWOSfbDDdC-7OiUsg$ 
>> >Fixes: 780138b12381 ("alloc_tag: check mem_profiling_support in alloc_tag_init")
>> >Fixes: 1438d349d16b ("lib: add memory allocations report in show_mem()")
>> >Cc: stable@vger.kernel.org
>> >Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
>> >---
>> >
>> >@Suren: I did not add another pr_warn() because every error path in
>> >alloc_tag_init() already has pr_err().
>> >
>> >v2 -> v3:
>> >- Added another Closes: tag (David)
>> >- Moved the condition into a standalone if block for better readability
>> >  (Suren)
>> >- Typo fix (Suren)
>> >
>> > lib/alloc_tag.c | 3 +++
>> > 1 file changed, 3 insertions(+)
>> >
>> >diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
>> >index 41ccfb035b7b..e9b33848700a 100644
>> >--- a/lib/alloc_tag.c
>> >+++ b/lib/alloc_tag.c
>> >@@ -127,6 +127,9 @@ size_t alloc_tag_top_users(struct codetag_bytes *tags, size_t count, bool can_sl
>> > 	struct codetag_bytes n;
>> > 	unsigned int i, nr = 0;
>> > 
>> >+	if (IS_ERR_OR_NULL(alloc_tag_cttype))
>> 
>> Should a warning  added here? indicating  codetag module not ready yet and the memory failure happened during boot:
>>  if (mem_profiling_support) pr_warn("...
>
>I think you're saying we need to print a warning when alloc tagging
>can't provide "top users".

I just meant printing a warning when show_mem is needed before codetag module initialized, 
as reported in https://lore.kernel.org/oe-lkp/202506181351.bba867dd-lkp@intel.com/
where mem_profiling_support is 1, but alloc_tag_cttype is still NULL.
This can tell we do have a memory failure during boot before codetag_init, even with memory profiling activated.

>
>And there can be three different reasons why it can't provide them:
>
>1) alloc_tag_cttype is not ready yet or mem profiling is disabled.
>2) the context can't sleep and trylock failed.

This case is not just about warning, it is a bug if possible.

>3) alloc tags do not exist.
>

>-- 
>Cheers,
>Harry / Hyeonggon