Re: [v2 PATCH 2/2] mm: don't warn if the node is offlined

[Yang Shi]

On Fri, Nov 4, 2022 at 12:51 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Fri 04-11-22 10:42:45, Yang Shi wrote:
> > On Fri, Nov 4, 2022 at 2:56 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Fri 04-11-22 10:35:21, Michal Hocko wrote:
> > > [...]
> > > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > > > index ef4aea3b356e..308daafc4871 100644
> > > > --- a/include/linux/gfp.h
> > > > +++ b/include/linux/gfp.h
> > > > @@ -227,7 +227,10 @@ static inline
> > > >  struct folio *__folio_alloc_node(gfp_t gfp, unsigned int order, int nid)
> > > >  {
> > > >       VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
> > > > -     VM_WARN_ON((gfp & __GFP_THISNODE) && !node_online(nid));
> > > > +     if((gfp & __GFP_THISNODE) && !node_online(nid)) {
> > >
> > > or maybe even better
> > >         if ((gfp & (__GFP_THISNODE|__GFP_NOWARN) == __GFP_THISNODE|__GFP_NOWARN) && !node_online(nid))
> > >
> > > because it doesn't really make much sense to dump this information if
> > > the allocation failure is going to provide sufficient (and even more
> > > comprehensive) context for the failure. It looks more hairy but this can
> > > be hidden in a nice little helper shared between the two callers.
> >
> > Thanks a lot for the suggestion, printing warning if the gfp flag
> > allows sounds like a good idea to me. Will adopt it. But the check
> > should look like:
> >
> > if ((gfp & __GFP_THISNODE) && !(gfp & __GFP_NOWARN) && !node_online(nid))
>
> The idea was to warn if __GFP_NOWARN _was_ specified. Otherwise we will
> get an allocation failure splat from the page allocator and there it
> will be clear that the node doesn't have any memory associated. It is
> exactly __GFP_NOWARN case that would be a silent failure and potentially
> a buggy code (like this THP collapse path). See my point?

Aha, yeah, see your point now. I didn't see the splat from the
allocator from the bug report, then I realized it had not called into
allocator yet before the warning was triggered.

> --
> Michal Hocko
> SUSE Labs