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

[Juergen Gross <jgross@xxxxxxxx>]

On 08.04.22 00:44, Andrew Morton wrote:
> 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?
Yes. There was a report that QubesOS (based on Xen) is hitting this
problem. Xen has switched to use the zone device functionality in
kernel 5.9 and QubesOS wants to use memory hotplugging for guests in
order to be able to start a guest with minimal memory and expand it
as needed. This was the report leading to the patch.


Juergen

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