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Date:   Tue, 24 Apr 2018 13:28:49 -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 13:00:11, Mikulas Patocka wrote:
> > 
> > 
> > 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.
> 
> Wouldn't it be equally trivial to simply enable the fault injection? You
> would get additional failure paths testing as a bonus.

The RHEL and Fedora debugging kernels are compiled with fault injection. 
But the fault-injection framework will do nothing unless it is enabled by 
a kernel parameter or debugfs write.

Most users don't know about the fault injection kernel parameters or 
debugfs files and won't enabled it. We need a CONFIG_ option to enable it 
by default in the debugging kernels (and we could add a kernel parameter 
to override the default, fine-tune the fallback probability etc.)

Mikulas