Re: [PATCH] mm/alloc_tag: account for reserved tag ids in the kernel tag check

[Suren Baghdasaryan]

On Wed, Sep 16, 2026 at 12:55 AM Hao Ge <hao.ge@xxxxxxxxx> wrote:
>
> The tag ids stored in the page flags include two reserved markers.
> Id 0 means the page has no tag and id 1 means the tag was cleared,
> so real tags start at CODETAG_ID_FIRST. The kernel-side check in
> alloc_tag_sec_init() compared kernel_tags.count alone against the
> addressable limit, so with the count at or just under the limit
> the last tag ids wrapped into those markers. Pages allocated
> through them then look the same as untagged pages on free, nothing
> is ever subtracted from the real tag and /proc/allocinfo shows
> that memory as still allocated.
>
> Add the missing CODETAG_ID_FIRST, same as tags_addressable().
>
> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Hao Ge <hao.ge@xxxxxxxxx>

Nice catch!

Acked-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> ---
>  mm/alloc_tag.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
> index cfa0fc84b68f..cf100e7a852f 100644
> --- a/mm/alloc_tag.c
> +++ b/mm/alloc_tag.c
> @@ -629,7 +629,7 @@ void __init alloc_tag_sec_init(void)
>         kernel_tags.count = last_codetag - kernel_tags.first_tag;
>
>         /* Check if kernel tags fit into page flags */
> -       if (kernel_tags.count > (1UL << NR_UNUSED_PAGEFLAG_BITS)) {
> +       if (CODETAG_ID_FIRST + kernel_tags.count > (1UL << NR_UNUSED_PAGEFLAG_BITS)) {
>                 shutdown_mem_profiling(false); /* allocinfo file does not exist yet */
>                 pr_err("%lu allocation tags cannot be references using %d available page flag bits. Memory allocation profiling is disabled!\n",
>                         kernel_tags.count, NR_UNUSED_PAGEFLAG_BITS);
> --
> 2.25.1
>