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

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

On 2026/8/5 03:43, Andrew Morton wrote:
> On Tue,  4 Aug 2026 20:20:38 +0800 Hao Ge <hao.ge@linux.dev> 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.
> 
> Thanks.  Sashiko pointed at a few things, one pre-existing.
> 	https://sashiko.dev/#/patchset/20260804122038.190270-1-hao.ge@linux.dev
> 
> Is there any sanity in there?
Hi Andrew

Thanks for pointing these out.
I went through all the Sashiko feedback from v1 and v2.
Most of what it flagged is pre-existing and I'll fix it separately.
The only point that actually touches this patch is the -ENOMEM failing 
the first load. Between wasting a few KB of tag memory to let the load 
succeed and returning -ENOMEM so the user just reloads the module once,
I prefer the latter: a re-insmod works because the codetag section is 
skipped once profiling is off, whereas those mapped tag pages would 
never be used or reclaimed.

Thanks
Best Regards
Hao