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Date:   Tue, 24 Apr 2018 13:00:11 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Michal Hocko <mhocko@kernel.org>
cc:     Matthew Wilcox <willy@infradead.org>,
        David Miller <davem@davemloft.net>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        eric.dumazet@gmail.com, edumazet@google.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        mst@redhat.com, jasowang@redhat.com,
        virtualization@lists.linux-foundation.org, dm-devel@redhat.com,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v3] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG
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On Tue, 24 Apr 2018, Michal Hocko wrote:

> On Tue 24-04-18 11:50:30, Mikulas Patocka wrote:
> > 
> > 
> > On Tue, 24 Apr 2018, Michal Hocko wrote:
> > 
> > > On Mon 23-04-18 20:06:16, Mikulas Patocka wrote:
> > > [...]
> > > > @@ -404,6 +405,12 @@ void *kvmalloc_node(size_t size, gfp_t f
> > > >  	 */
> > > >  	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
> > > >  
> > > > +#ifdef CONFIG_DEBUG_SG
> > > > +	/* Catch bugs when the caller uses DMA API on the result of kvmalloc. */
> > > > +	if (!(prandom_u32_max(2) & 1))
> > > > +		goto do_vmalloc;
> > > > +#endif
> > > 
> > > I really do not think there is anything DEBUG_SG specific here. Why you
> > > simply do not follow should_failslab path or even reuse the function?
> > 
> > CONFIG_DEBUG_SG is enabled by default in RHEL and Fedora debug kernel (if 
> > you don't like CONFIG_DEBUG_SG, pick any other option that is enabled 
> > there).
> 
> Are you telling me that you are shaping a debugging functionality basing
> on what RHEL has enabled? And you call me evil. This is just rediculous.
> 
> > Fail-injection framework is if off by default and it must be explicitly 
> > enabled and configured by the user - and most users won't enable it.
> 
> It can be enabled easily. And if you care enough for your debugging
> kernel then just make it enabled unconditionally.

So, should we add a new option CONFIG_KVMALLOC_FALLBACK_DEFAULT? I'm not 
quite sure if 3 lines of debugging code need an extra option, but if you 
don't want to reuse any existing debug option, it may be possible. Adding 
it to the RHEL debug kernel would be trivial.

Mikulas