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 18:59:52, "Harry Yoo" <harry.yoo@oracle.com> wrote:
>On Tue, Jun 24, 2025 at 05:30:02PM +0800, David Wang wrote:
>> 
>> 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://urldefense.com/v3/__https://lore.kernel.org/oe-lkp/202506181351.bba867dd-lkp@intel.com/__;!!ACWV5N9M2RV99hQ!J2waTUro8owaYlpAZJ6fnrHZvcGMbY6qAO5QvvIGZzUv-ryWjCjhO-maTOolfpPvPSr6CpqOgkRalCwJow$ 
>> 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.
>
>Ok. You didn't mean that.
>
>But still I think it's better to handle all cases and print distinct
>warnings, rather than handling only the specific case where memory profiling
>is enabled but not yet initialized.
>
>Users will want to know why allocation information is not available,
>and there can be multiple reasons including the one you mentioned.
>
>What do you think?

I am not sure.... 
I think most cases you mentioned is just a pr_info,  those are expected behavior or designed that way.
But when  mem_profiling_support==1 && alloc_tag_cttype==NULL, this is an unexpected behavior, which is a pr_warn.



>
>> >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.
>
>Why do you think it is a bug? If trylock fails, alloc_tag_top_users()
>returns 0 and do nothing.

Oh, I was wrong, I thought this is about failure to acquire a lock. (not trylock.)



>
>> >3) alloc tags do not exist.
>> >
>> 
>> >-- 
>> >Cheers,
>> >Harry / Hyeonggon
>
>-- 
>Cheers,
>Harry / Hyeonggon