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Date:   Tue, 29 Sep 2020 11:00:03 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [patch 00/13] preempt: Make preempt count unconditional
Message-ID: <20200929090003.GG438822@phenom.ffwll.local>
Mail-Followup-To: Michal Hocko <mhocko@suse.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Brian Cain <bcain@codeaurora.org>,
        Richard Weinberger <richard@nod.at>,
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        alpha <linux-alpha@vger.kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
References: <871rj4owfn.fsf@nanos.tec.linutronix.de>
 <CAHk-=wj0eUuVQ=hRFZv_nY7g5ZLt7Fy3K7SMJL0ZCzniPtsbbg@mail.gmail.com>
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 <CAHk-=wht7kAeyR5xEW2ORj7m0hibVxZ3t+2ie8vNHLQfdbN2_g@mail.gmail.com>
 <CAKMK7uHAk9-Vy2cof0ws=DrcD52GHiCDiyHbjLd19CgpBU2rKQ@mail.gmail.com>
 <20200916152956.GV29330@paulmck-ThinkPad-P72>
 <CAKMK7uGFyfhEyt=jmdk2jDO-hq0_Pf0ck+cKSELHjr2U3rPuYQ@mail.gmail.com>
 <20200916205840.GD29330@paulmck-ThinkPad-P72>
 <CAKMK7uHL2dMv80b8uBXr=BqHD2TQeODQQM1MGYhAfCYbX7sLrA@mail.gmail.com>
 <20200929081938.GC22035@dhcp22.suse.cz>
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On Tue, Sep 29, 2020 at 10:19:38AM +0200, Michal Hocko wrote:
> On Wed 16-09-20 23:43:02, Daniel Vetter wrote:
> > I can
> > then figure out whether it's better to risk not spotting issues with
> > call_rcu vs slapping a memalloc_noio_save/restore around all these
> > critical section which force-degrades any allocation to GFP_ATOMIC at
> 
> did you mean memalloc_noreclaim_* here?

Yeah I picked the wrong one of that family of functions.

> > most, but has the risk that we run into code that assumes "GFP_KERNEL
> > never fails for small stuff" and has a decidedly less tested fallback
> > path than rcu code.
> 
> Even if the above then please note that memalloc_noreclaim_* or
> PF_MEMALLOC should be used with an extreme care. Essentially only for
> internal memory reclaimers. It grants access to _all_ the available
> memory so any abuse can be detrimental to the overall system operation.
> Allocation failure in this mode means that we are out of memory and any
> code relying on such an allocation has to carefuly consider failure.
> This is not a random allocation mode.

Agreed, that's why I don't like having these kind of automagic critical
sections. It's a bit a shotgun approach. Paul said that the code would
handle failures, but the problem is that it applies everywhere.

Anyway my understanding is that call_rcu will be reworked and gain a pile
of tricks so that these problems for the callchains leading to call_rcu
all disappear.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch