linux-kernel - Re: [PATCH] mm/failslab: By default, do not fail allocations with direct reclaim only

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Date:   Mon, 20 May 2019 10:07:59 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Akinobu Mita <akinobu.mita@...il.com>
cc:     Nicolas Boichat <drinkcat@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Joe Perches <joe@...ches.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mm@...ck.org, Pekka Enberg <penberg@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/failslab: By default, do not fail allocations with
 direct reclaim only

On Tue, 21 May 2019, Akinobu Mita wrote:

> > When failslab was originally written, the intention of the
> > "ignore-gfp-wait" flag default value ("N") was to fail
> > GFP_ATOMIC allocations. Those were defined as (__GFP_HIGH),
> > and the code would test for __GFP_WAIT (0x10u).
> >
> > However, since then, __GFP_WAIT was replaced by __GFP_RECLAIM
> > (___GFP_DIRECT_RECLAIM|___GFP_KSWAPD_RECLAIM), and GFP_ATOMIC is
> > now defined as (__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM).
> >
> > This means that when the flag is false, almost no allocation
> > ever fails (as even GFP_ATOMIC allocations contain
> > __GFP_KSWAPD_RECLAIM).
> >
> > Restore the original intent of the code, by ignoring calls
> > that directly reclaim only (___GFP_DIRECT_RECLAIM), and thus,
> > failing GFP_ATOMIC calls again by default.
> >
> > Fixes: 71baba4b92dc1fa1 ("mm, page_alloc: rename __GFP_WAIT to __GFP_RECLAIM")
> > Signed-off-by: Nicolas Boichat <drinkcat@...omium.org>
> 
> Good catch.
> 
> Reviewed-by: Akinobu Mita <akinobu.mita@...il.com>
> 
> > ---
> >  mm/failslab.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/failslab.c b/mm/failslab.c
> > index ec5aad211c5be97..33efcb60e633c0a 100644
> > --- a/mm/failslab.c
> > +++ b/mm/failslab.c
> > @@ -23,7 +23,8 @@ bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags)
> >         if (gfpflags & __GFP_NOFAIL)
> >                 return false;
> >
> > -       if (failslab.ignore_gfp_reclaim && (gfpflags & __GFP_RECLAIM))
> > +       if (failslab.ignore_gfp_reclaim &&
> > +                       (gfpflags & ___GFP_DIRECT_RECLAIM))
> >                 return false;
> 
> Should we use __GFP_DIRECT_RECLAIM instead of ___GFP_DIRECT_RECLAIM?
> Because I found the following comment in gfp.h
> 
> /* Plain integer GFP bitmasks. Do not use this directly. */
> 

Yes, we should use the two underscore version instead of the three.

Nicolas, after that's fixed up, feel free to add Acked-by: David Rientjes 
<rientjes@...gle.com>.

Thanks!