Re: [PATCH v2] mm/page_alloc: mark pages freed by __free_pages_core() as not tagged

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Oct 22, 2026 at 08:27:52PM +0800, Zenghui Yu wrote:
> From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>
> 
> Onlining hotplugged memory (e.g., a dev_dax device bound to kmem) with
> CONFIG_MEM_ALLOC_PROFILING_DEBUG triggers:
> 
>   alloc_tag was not set
>   WARNING: ./include/linux/alloc_tag.h:166 at __pgalloc_tag_sub+0x1fc/0x254
>   Call trace:
>    __pgalloc_tag_sub+0x1fc/0x254 (P)
>    __free_pages_ok+0x14c/0x6ac
>    __free_pages_core+0xa8/0x188
>    generic_online_page+0x14/0x20
>    online_pages+0x220/0x3b0
>    memory_subsys_online+0xfc/0x198
>    device_online+0x58/0xe0
>    state_store+0xf0/0xf8
> 
> Pages entering the buddy through __free_pages_core() have never been
> allocated, so their allocation tag references are NULL.  The free path,
> __free_pages_prepare() -> pgalloc_tag_sub(), expects a valid tag or a
> CODETAG_EMPTY mark and warns otherwise.  Paths feeding never-allocated
> pages into the buddy are therefore expected to mark them with
> clear_page_tag_ref() first, like init_cma_reserved_pageblock() and
> free_reserved_pages() do; of __free_pages_core()'s callers only
> memblock_free_pages() clears the tag ref itself, and the hotplug
> onlining path was missed.
> 
> Since all __free_pages_core() callers free never-allocated pages, mark the
> head page as not tagged in __free_pages_core() itself instead of relying on
> each caller, and drop the now-redundant clearing from
> memblock_free_pages().
> 
> Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>

Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>