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:
- alloc tagging is disabled because mem profiling is disabled (!alloc_tag_cttype)
- alloc tagging is enabled, but not yet initialized (!alloc_tag_cttype)
- 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.bba8... Closes: https://urldefense.com/v3/__https://lore.kernel.org/oe-lkp/202506131711.5b41... 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.bba8... 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:
- alloc_tag_cttype is not ready yet or mem profiling is disabled.
- 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.)
- alloc tags do not exist.
-- Cheers, Harry / Hyeonggon
-- Cheers, Harry / Hyeonggon