Re: [PATCH v3] alloc_tag: fix undetected compressed tag overflow when profiling is disabled

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  5 Aug 2026 17:06:33 +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.
> 
> This silently corrupts /proc/allocinfo: allocated pages get attributed
> to the wrong call sites, so the statistics it reports are wrong.
> 
> 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.

AI review points at a cpuple of possible things, one pre-existing:
	https://sashiko.dev/#/patchset/20260805090633.141001-1-hao.ge@linux.dev

"Does this unintentionally result in a denial of service for module
loading, preventing critical drivers from loading when they otherwise
could have just disabled profiling and gracefully continued?" sounds
pretty obscure and I doubt if we care?