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

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Message-ID: <Y2mpvAy2W0hhI7Yl@google.com>
Date:   Mon, 7 Nov 2022 16:58:36 -0800
From:   Zach O'Keefe <zokeefe@...gle.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     Michal Hocko <mhocko@...e.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 2/2] mm: don't warn if the node is offlined

On Nov 07 10:48, Yang Shi wrote:
> On Sun, Nov 6, 2022 at 11:55 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Fri 04-11-22 13:52:52, Yang Shi wrote:
> > > On Fri, Nov 4, 2022 at 12:51 PM Michal Hocko <mhocko@...e.com> wrote:
> > > >
> > > > On Fri 04-11-22 10:42:45, Yang Shi wrote:
> > > > > On Fri, Nov 4, 2022 at 2:56 AM Michal Hocko <mhocko@...e.com> 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.
> >
> > And it would trigger even if it did because GFP_TRANSHUGE has
> > __GFP_NOWARN
> 
> Yeah, the syzbot has panic on warn set, so kernel just panicked before
> entering the allocator.
> 

Sorry I'm late to the party here.  I think Michal's suggestion is sound --
catches instances like we saw with MADV_COLLAPSE, but no risk of panic-on-warn.
Thanks for the suggestion.

Best,
Zach

> > --
> > Michal Hocko
> > SUSE Labs