linux-kernel - Re: [patch 00/13] preempt: Make preempt count unconditional

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Date:   Tue, 29 Sep 2020 11:00:03 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
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        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 00/13] preempt: Make preempt count unconditional

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