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

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

[Andrew Morton]

On Tue,  4 Aug 2026 14:44:08 +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.

With bad effects, one assumes.

When fixing a bug please always include a complete description of the
userspace-visible runtime effects of that bug.

> 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.
> 
> We therefore remove the mem_alloc_profiling_enabled() guard.

It appears that AI review has found a pre-existing error-path cleanup
issue:

	https://sashiko.dev/#/patchset/20260804064408.105033-1-hao.ge@linux.dev