Re: [PATCH v4 3/8] mm/page_alloc: snapshot the preferred zone against shrink rebuilds

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 10/28/26 22:19, Gregory Price (Meta) wrote:
> get_page_from_freelist() and alloc_pages_bulk_noprof() reread
> ac->preferred_zoneref->zone at every use. Offlining a zone's last block
> rebuilds the zonelists in place, and the shrunk list's NULL terminator
> can land on the preferred slot mid-walk:
> 
>   CPU 0                             CPU 1
>   get_page_from_freelist(GFP_KERNEL | __GFP_THISNODE)
>     zone = NORMAL (slot 1)
>                                     offline_pages()
>                                       build_all_zonelists()
>                                         [MOVABLE, NORMAL, NULL]
>                                         -> [NORMAL, NULL]
>     rmqueue(zonelist_zone(slot 1), zone)
>       zone_statistics(NULL, zone)
> 
>   BUG: kernel NULL pointer dereference, address: 0000000000000060
>   RIP: 0010:get_page_from_freelist+0x6a5/0x15f0
>   Call Trace:
>    __alloc_frozen_pages_noprof+0x125/0x330
> 
> Read the slot once. Zones are never freed, so a stale zone is safe, and
> a NULL slot fails the same way as walking off the end.
> 
> Fixes: 11cd8638c37f ("mm, page_alloc: remove stop_machine from build_all_zonelists")
> Cc: stable@vger.kernel.org
> Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>