Re: [v2 PATCH 1/2] mm: khugepaged: allow page allocation fallback to eligible nodes

[Yang Shi <shy828301@gmail.com>]

On Fri, Nov 4, 2022 at 1:32 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 03-11-22 14:36:40, Yang Shi wrote:
> [...]
> > So use nodemask to record the nodes which have the same hit record, the
> > hugepage allocation could fallback to those nodes.  And remove
> > __GFP_THISNODE since it does disallow fallback.  And if nodemask is
> > empty (no node is set), it means there is one single node has the most
> > hist record, the nodemask approach actually behaves like __GFP_THISNODE.
> >
> > Reported-by: syzbot+0044b22d177870ee974f@syzkaller.appspotmail.com
> > Suggested-by: Zach O'Keefe <zokeefe@google.com>
> > Suggested-by: Michal Hocko <mhocko@suse.com>
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  mm/khugepaged.c | 32 ++++++++++++++------------------
> >  1 file changed, 14 insertions(+), 18 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index ea0d186bc9d4..572ce7dbf4b0 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -97,8 +97,8 @@ struct collapse_control {
> >       /* Num pages scanned per node */
> >       u32 node_load[MAX_NUMNODES];
> >
> > -     /* Last target selected in hpage_collapse_find_target_node() */
> > -     int last_target_node;
> > +     /* nodemask for allocation fallback */
> > +     nodemask_t alloc_nmask;
>
> This will eat another 1k on the stack on most configurations
> (NODE_SHIFT=10). Along with 4k of node_load this is quite a lot even
> on shallow call chains like madvise resp. khugepaged.  I would just
> add a follow up patch which changes both node_load and alloc_nmask to
> dynamically allocated objects.

The collapse_control is allocated by kmalloc dynamically for
MADV_COLLAPSE path, and defined as a global variable for khugepaged
(khugepaged_collapse_control). So it is not on stack.

>
> Other than that LGTM. I thought we want to keep __GFP_THISNODE but after
> a closer look it seems that this flag is not really compatible with
> nodemask after all. node_zonelist() will simply return a trivial zone
> list for a single (preferred node) so no fallback to other nodes is

Yes, exactly.

> possible. My bad to not realize it earlier.

It is fine, never mind.

> --
> Michal Hocko
> SUSE Labs