Re: [PATCH v2] mm, page_alloc: fix build_zonerefs_node()

[Michal Hocko <mhocko@xxxxxxxx>]

[CC Mel]

On Thu 07-04-22 14:06:37, Juergen Gross wrote:
> Since commit 6aa303defb74 ("mm, vmscan: only allocate and reclaim from
> zones with pages managed by the buddy allocator") only zones with free
> memory are included in a built zonelist. This is problematic when e.g.
> all memory of a zone has been ballooned out when zonelists are being
> rebuilt.
> 
> The decision whether to rebuild the zonelists when onlining new memory
> is done based on populated_zone() returning 0 for the zone the memory
> will be added to. The new zone is added to the zonelists only, if it
> has free memory pages (managed_zone() returns a non-zero value) after
> the memory has been onlined. This implies, that onlining memory will
> always free the added pages to the allocator immediately, but this is
> not true in all cases: when e.g. running as a Xen guest the onlined
> new memory will be added only to the ballooned memory list, it will be
> freed only when the guest is being ballooned up afterwards.

Thanks this is much more clearer!
 
> Another problem with using managed_zone() for the decision whether a
> zone is being added to the zonelists is, that a zone with all memory
> used will in fact be removed from all zonelists in case the zonelists
> happen to be rebuilt.
> 
> Use populated_zone() when building a zonelist as it has been done
> before that commit.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 6aa303defb74 ("mm, vmscan: only allocate and reclaim from zones with 
> pages managed by the buddy allocator")
> Reported-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> V2:
> - updated commit message (Michal Hocko)
> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index bdc8f60ae462..3d0662af3289 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6128,7 +6128,7 @@ static int build_zonerefs_node(pg_data_t *pgdat, struct 
> zoneref *zonerefs)
>       do {
>               zone_type--;
>               zone = pgdat->node_zones + zone_type;
> -             if (managed_zone(zone)) {
> +             if (populated_zone(zone)) {
>                       zoneref_set_zone(zone, &zonerefs[nr_zones++]);
>                       check_highest_zone(zone_type);
>               }
> -- 
> 2.34.1

-- 
Michal Hocko
SUSE Labs