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

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.

> +               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.

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