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

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

On Thu,  7 Apr 2022 14:06:37 +0200 Juergen Gross <jgross@xxxxxxxx> wrote:

> Since commit 6aa303defb74 ("mm, vmscan: only allocate and reclaim from
> zones with pages managed by the buddy allocator")

Six years ago!

> 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.
> 
> 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

Some details, please.  Is this really serious enough to warrant
backporting?  Is some new workload/usage pattern causing people to hit
this?