Re: [PATCH v2] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

Hi Suren


Thanks a lot for your review.


On 2026/8/5 04:08, Suren Baghdasaryan wrote:
> On Tue, Aug 4, 2026 at 5:21 AM Hao Ge <[email protected]> wrote:
>> In reserve_module_tags(), the tag overflow check is gated on
>> mem_alloc_profiling_enabled():
>>
>>      if (mem_alloc_profiling_enabled() && !tags_addressable())
>>
>> If profiling is toggled off at runtime and a module is loaded whose
>> tags exceed the compressed-mode limit, shutdown_mem_profiling() is
>> skipped. vm_module_tags_populate() still maps memory for the tags and
>> the module loads successfully, but the total tag count now exceeds what
>> NR_UNUSED_PAGEFLAG_BITS can address.
>>
>> Once profiling is re-enabled, ref_to_idx() computes each tag's index
>> as its position in the alloc_tag array. update_page_tag_ref() masks
>> it to alloc_tag_ref_mask before storing in page->flags. Indices
>> beyond the mask are truncated and idx_to_ref() resolves them to wrong
>> tags.
>>
>> mem_alloc_profiling_enabled() and mem_profiling_compressed are
>> independent. Once compressed mode is established at boot, it stays
>> active regardless of runtime toggles of mem_profiling.
>>
>> Remove the mem_alloc_profiling_enabled() guard. Also return an error
>> after shutdown_mem_profiling() to skip vm_module_tags_populate(), as
>> the mapped pages would never be reused - shutdown_mem_profiling() sets
>> mem_profiling_support to false, so no future module load enters the
>> codetag path.
>>
>> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
>> Cc: [email protected]
>> Signed-off-by: Hao Ge <[email protected]>
> Thanks for the fix, Hao!
>
>> ---
>> Changes in v2:
>> - Return error after shutdown_mem_profiling() to skip unnecessary
>>    vm_module_tags_populate()
>>    v1 link: https://lore.kernel.org/all/[email protected]/
>> ---
>>   mm/alloc_tag.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
>> index 52aece27b00e..d8c36430f1c0 100644
>> --- a/mm/alloc_tag.c
>> +++ b/mm/alloc_tag.c
>> @@ -904,10 +904,11 @@ static void *reserve_module_tags(struct module *mod, unsigned long size,
>>                  int grow_res;
>>
>>                  module_tags.size = offset + size;
>> -               if (mem_alloc_profiling_enabled() && !tags_addressable()) {
> Makes sense but how about replacing mem_alloc_profiling_enabled() with
> mem_profiling_support? Otherwise this warning will be issued multiple
> times if we are loading multiple modules.


That's a really good point, this duplicate warning issue will indeed happen.

When I thought over your point, I realized using mem_profiling_support 
creates a small race window.


Thread A（insmod A）                      Thread B (insmod B)

--------- ---------

needs_section_mem() -> true

     needs_section_mem() -> true

     (profiling still supported)

reserve_module_tags()

     overflow -> shutdown

     mem_profiling_support=false

     return -ENOSPC

     reserve_module_tags()

         mem_profiling_support==false

         -> overflow check skipped

         -> vm_module_tags_populate()

             maps unused tag pages

So I'd rather go with pr_warn_once here.


>> +               if (!tags_addressable()) {
>>                          shutdown_mem_profiling(true);
>>                          pr_warn("With module %s there are too many tags to fit in %d page flag bits. Memory allocation profiling is disabled!\n",
>>                                  mod->name, NR_UNUSED_PAGEFLAG_BITS);
>> +                       return ERR_PTR(-ENOSPC);
> This ENOSPC error will be propagated all the way up to the init_module
> syscall and it's not among the error codes currently expected (see:
> https://man7.org/linux/man-pages/man2/init_module.2.html). I suggest
> returning ENOMEM instead.


Agree, will change


Thanks

Best Regards

Hao

>>                  }
>>
>>                  grow_res = vm_module_tags_populate();
>> --
>> 2.25.1
>>