Re: [PATCH] mm, page_alloc: do not break __GFP_THISNODE by zonelist reset

[Michal Hocko]

On Fri 25-05-18 12:43:00, Andrew Morton wrote:
> On Fri, 25 May 2018 15:08:53 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote:
> 
> > we might consider this for 4.17 although I don't know if there's anything
> > currently broken. Stable backports should be more important, but will have to
> > be reviewed carefully, as the code went through many changes.
> > BTW I think that also the ac->preferred_zoneref reset is currently useless if
> > we don't also reset ac->nodemask from a mempolicy to NULL first (which we
> > probably should for the OOM victims etc?), but I would leave that for a
> > separate patch.
> 
> Confused.  If nothing is currently broken then why is a backport
> needed?  Presumably because we expect breakage in the future?  Can you
> expand on this?

__GFP_THISNODE is documented to _use_ the given node. Allocating from a
different one is a bug. Maybe the current code can cope with that or at
least doesn't blow up in an obvious way but the bug is still there.

I am still not sure what to do about the zonelist reset. It still seems
like an echo from the past but using numa_node_id for __GFP_THISNODE is
a clear bug because our task could have been migrated to a cpu on a
different than requested node.

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

-- 
Michal Hocko
SUSE Labs