Linux Virtualization - Apr 2018 - [PATCH v3] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG

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.
-- 
Michal Hocko
SUSE Labs

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

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