netdev - Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG

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Message-ID: <20180423151015.GT17484@dhcp22.suse.cz>
Date:   Mon, 23 Apr 2018 09:10:15 -0600
From:   Michal Hocko <mhocko@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        David Miller <davem@...emloft.net>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        eric.dumazet@...il.com, edumazet@...gle.com,
        bhutchings@...arflare.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, mst@...hat.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org, dm-devel@...hat.com,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM

On Mon 23-04-18 10:24:02, Mikulas Patocka wrote:
[...]

I am not going to comment on your continuous accusations. We can discuss
patches but general rants make very limited sense.
 
> > > > I already said that we can change it from CONFIG_DEBUG_VM to 
> > > > CONFIG_DEBUG_SG - or to whatever other option you may want, just to make 
> > > > sure that it is enabled in distro debug kernels by default.
> > > 
> > > Yes, and I think that's the right idea.  So send a v2 and ignore the
> > > replies that are clearly relating to an earlier version of the patch.
> > > Not everybody reads every mail in the thread before responding to one they
> > > find interesting.  Yes, ideally, one would, but sometimes one doesn't.
> > 
> > And look here. This is yet another ad-hoc idea. We have many users of
> > kvmalloc which have no relation to SG, yet you are going to control
> > their behavior by CONFIG_DEBUG_SG? No way! (yeah evil again)
> 
> Why aren't you constructive and pick up pick up the CONFIG flag?

Because config doesn't make that much of a sense. You do not want a
permanent vmalloc fallback unconditionally. Debugging option which
changes the behavior completely is not useful IMNHO. Besides that who is
going to enable it?

> > Really, we have a fault injection framework and this sounds like
> > something to hook in there.
> 
> The testing people won't set it up. They install the "kernel-debug" 
> package and run the tests in it.
> 
> If you introduce a hidden option that no one knows about, no one will use 
> it.

then make sure people know about it. Fuzzers already do test fault
injections.
-- 
Michal Hocko
SUSE Labs