Re: [PATCH v3] alloc_tag: expose profiling and compression states

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

On 2026/8/4 10:15, Hao Ge wrote:
> Hi Abhishek
>
>
> Subject says "profiling and compression states", but this
>
> patch only adds the compression sysctl. The profiling state was
>
> already exposed via mem_profiling and is untouched here.
>
> I think we should make this more precise, like:
>
> alloc_tag: expose boot-time compression configuration
>
>
> On 2026/8/4 05:54, Abhishek Bapat wrote:
>> Currently, userspace has limited visibility into the exact active
>> runtime state of memory allocation profiling and its page extension
>> compression ('sysctl.vm.mem_profiling={0|1|never}[,compressed]').
>
>
> Profiling state is already readable via mem_profiling. The gap is
>
> only compression. Also, as we discussed, this sysctl reports
>
> what the user requested at boot rather than the actual runtime state.
>
>
>> While reading the sysctl provides basic on/off status, it is currently
>> impossible for userspace to natively determine whether page-tag
>> compression was successfully enabled without scraping dmesg boot logs.
>>
>> Resolve this ambiguity by exposing the active compression state by
>> adding a new read-only sysctl `vm.mem_profiling_compressed` to output 
>> the
>> state.
>>
>> v3 change:
>> - Added documentation about the behaviour details of the new sysctl.
>>
>> v2 change:
>> - Moved from displaying the state in /proc/allocinfo to a new read-only
>> sysctl.
>
>
> As this is a standalone patch, please put the v2/v3 changelog below 
> the --- line
>
> instead of inside the commit message.
>
>
> Thanks
>
> Best Regards
>
> Hao
>
>
>> Signed-off-by: Abhishek Bapat <abhishekbapat@google.com>
>> ---
>>   Documentation/mm/allocation-profiling.rst | 13 +++++++++++++
>>   mm/alloc_tag.c                            |  6 ++++++
>>   2 files changed, 19 insertions(+)
>>
>> diff --git a/Documentation/mm/allocation-profiling.rst 
>> b/Documentation/mm/allocation-profiling.rst
>> index c3a28467955f..3ad1e9aacb9a 100644
>> --- a/Documentation/mm/allocation-profiling.rst
>> +++ b/Documentation/mm/allocation-profiling.rst
>> @@ -43,6 +43,19 @@ sysctl:
>>     warnings produced by allocations made while profiling is disabled 
>> and freed
>>     when it's enabled.
>>   +  /proc/sys/vm/mem_profiling_compressed
>> +
>> +  1: Page extension compression is enabled.
>> +
>> +  0: Page extension compression is disabled.
>> +
>> +  This control is read-only and reflects the compression status 
>> initialized at boot.
>> +  Note that, unlike `mem_profiling`, which represents the current 
>> state of profiling,
>> +  `mem_profiling_compressed` represents the state configured at boot 
>> time.


Sorry, I forgot to mention this earlier.

I wonder if we could remove this section:


>> Turning off
>> +  profiling at runtime will implicitly make this sysctl effectively 
>> dormant. However, if
>> +  profiling is toggled off and then toggled on again, it will resume 
>> with compression
>> +  still enabled as long as the value of `mem_profiling_compressed` 
>> is 1.


mem_profiling_compressed only selects the tag storage format (page flags 
vs page_ext).

mem_profiling controls whether allocations are tagged at runtime.

The two are independent: toggling mem_profiling on/off has no effect on

compression. Only shutdown_mem_profiling() tears it down, but by then

the entire allocation profiling subsystem is disabled anyway. There is

no resume path.


>> +
>>   Runtime info:
>>     /proc/allocinfo
>>   diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
>> index 52aece27b00e..877068241f06 100644
>> --- a/mm/alloc_tag.c
>> +++ b/mm/alloc_tag.c
>> @@ -1303,6 +1303,12 @@ static const struct ctl_table 
>> memory_allocation_profiling_sysctls[] = {
>>           .mode        = 0644,
>>           .proc_handler    = proc_mem_profiling_handler,
>>       },
>> +    {
>> +        .procname    = "mem_profiling_compressed",
>> +        .data        = &mem_profiling_compressed,
>> +        .mode        = 0444,
>> +        .proc_handler    = proc_do_static_key,
>> +    },
>>   };
>>     static void __init sysctl_init(void)
>>
>> base-commit: 94f9b3980dd446b56acf1dfed649e9b32a9f3813