Re: [PATCH 1/2] mm/page_alloc: detect zonelist rebuilds from memory hot-add

[Vlastimil Babka (SUSE)]

On 10/15/26 00:24, Gregory Price (Meta) wrote:
> The zonelist_update_seq retry was limited to CONFIG_MEMORY_HOTREMOVE,
> presumably because only hot-remove takes zones out of a zonelist.
> 
> However, build_zonerefs_node() rewrites the zonelists in place, so
> onlining the first block of a zone shifts the entries that follow it
> in every zonelist containing that zone.  This can cause a spurious
> allocation failure for a __GFP_THISNODE caller.
> 
> For a node with only ZONE_NORMAL, onlining its first ZONE_MOVABLE block
> rewrites the node's ZONELIST_NOFALLBACK list as:
> 
>   [NORMAL, NULL] -> [MOVABLE, NULL] -> [MOVABLE, NORMAL, NULL]
> 
> A concurrent GFP_KERNEL | __GFP_THISNODE allocation can observe the
> intermediate list, skip ZONE_MOVABLE, and find no eligible zone. With
> CONFIG_MEMORY_HOTREMOVE=n, check_retry_zonelist() returns 0 and the
> allocation fails even though the node has free ZONE_NORMAL memory.
> 
> Gate the sequence checks on CONFIG_MEMORY_HOTPLUG, which covers every
> configuration that rebuilds zonelists after boot.
> 
> Fixes: 3d36424b3b58 ("mm/page_alloc: fix race condition between build_all_zonelists and page allocation")
> Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>

> ---
>  mm/page_alloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7682aecc2c075..81db19f2ab551 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4425,7 +4425,7 @@ static DEFINE_SEQLOCK(zonelist_update_seq);
>  
>  static unsigned int zonelist_iter_begin(void)
>  {
> -	if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
>  		return read_seqbegin(&zonelist_update_seq);
>  
>  	return 0;
> @@ -4433,7 +4433,7 @@ static unsigned int zonelist_iter_begin(void)
>  
>  static unsigned int check_retry_zonelist(unsigned int seq)
>  {
> -	if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
>  		return read_seqretry(&zonelist_update_seq, seq);
>  
>  	return seq;