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

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Date:   Fri, 4 Nov 2022 20:51:17 +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 2/2] mm: don't warn if the node is offlined

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?
-- 
Michal Hocko
SUSE Labs