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SubjectRe: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM


On Fri, 20 Apr 2018, Matthew Wilcox wrote:

> On Fri, Apr 20, 2018 at 04:54:53PM -0400, Mikulas Patocka wrote:
> > On Fri, 20 Apr 2018, Michal Hocko wrote:
> > > No way. This is just wrong! First of all, you will explode most likely
> > > on many allocations of small sizes. Second, CONFIG_DEBUG_VM tends to be
> > > enabled quite often.
> >
> > You're an evil person who doesn't want to fix bugs.
>
> Steady on. There's no need for that. Michal isn't evil. Please
> apologise.

I see this attitude from Michal again and again.

He didn't want to fix vmalloc(GFP_NOIO), he didn't want to fix alloc_pages
sleeping when __GFP_NORETRY is used. So what should I say? Fix them and
you won't be evil :-)

(he could also fix the oom killer, so that it is triggered when
free_memory+cache+free_swap goes beyond a threshold and not when you loop
too long in the allocator)

> > You refused to fix vmalloc(GFP_NOIO) misbehavior a year ago (did you make
> > some progress with it since that time?) and you refuse to fix kvmalloc
> > misuses.
>
> I understand you're frustrated, but this is not the way to get the problems
> fixed.
>
> > I tried this patch on text-only virtual machine and /proc/vmallocinfo
> > shows 614kB more memory. I tried it on a desktop machine with the chrome
> > browser open and /proc/vmallocinfo space is increased by 7MB. So no - this
> > won't exhaust memory and kill the machine.
>
> This is good data, thank you for providing it.
>
> > Arguing that this increases memory consumption is as bogus as arguing that
> > CONFIG_LOCKDEP increses memory consumption. No one is forcing you to
> > enable CONFIG_LOCKDEP and no one is forcing you to enable this kvmalloc
> > test too.
>
> I think there's a real problem which is that CONFIG_DEBUG_VM is too broad.
> It inserts code in a *lot* of places, some of which is quite expensive.
> We would do better to split it into more granular pieces ... although
> an explosion of configuration options isn't great either. Maybe just
> CONFIG_DEBUG_VM and CONFIG_DEBUG_VM_EXPENSIVE.
>
> Michal may be wrong, but he's not evil.

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.

Mikulas

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