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

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Date:   Fri, 4 Nov 2022 20:55:50 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     zokeefe@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 1/2] mm: khugepaged: allow page allocation fallback to
 eligible nodes

On Fri 04-11-22 10:37:39, Yang Shi wrote:
> On Fri, Nov 4, 2022 at 1:32 AM Michal Hocko <mhocko@...e.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@...kaller.appspotmail.com
> > > Suggested-by: Zach O'Keefe <zokeefe@...gle.com>
> > > Suggested-by: Michal Hocko <mhocko@...e.com>
> > > Signed-off-by: Yang Shi <shy828301@...il.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.

Dang, I must have been blind because I _think_ I have seen it as a local
stack defined. Maybe I just implicitly put that to the same bucket as
othe $foo_control (e.g. scan_control, oom_control etc) which leave on the
stack usually. Sorry about the confusion. Sorry for the noise.

Acked-by: Michal Hocko <mhocko@...e.com>
-- 
Michal Hocko
SUSE Labs