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SubjectRe: [PATCH v3] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG
On Tue 24-04-18 13:28:49, Mikulas Patocka wrote:
>
>
> 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.)

If it is a real issue to install the debugging kernel with the required
kernel parameter then I a config option for the default on makes sense
to me.
--
Michal Hocko
SUSE Labs

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