On Thu, 19 Apr 2018, Andrew Morton wrote:
> On Thu, 19 Apr 2018 17:19:20 -0400 (EDT) Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
> > > > In order to detect these bugs reliably I submit this patch that changes
> > > > kvmalloc to always use vmalloc if CONFIG_DEBUG_VM is turned on.
> > > >
> > > > ...
> > > >
> > > > --- linux-2.6.orig/mm/util.c 2018-04-18 15:46:23.000000000 +0200
> > > > +++ linux-2.6/mm/util.c 2018-04-18 16:00:43.000000000 +0200
> > > > @@ -395,6 +395,7 @@ EXPORT_SYMBOL(vm_mmap);
> > > > */
> > > > void *kvmalloc_node(size_t size, gfp_t flags, int node)
> > > > {
> > > > +#ifndef CONFIG_DEBUG_VM
> > > > gfp_t kmalloc_flags = flags;
> > > > void *ret;
> > > >
> > > > @@ -426,6 +427,7 @@ void *kvmalloc_node(size_t size, gfp_t f
> > > > */
> > > > if (ret || size <= PAGE_SIZE)
> > > > return ret;
> > > > +#endif
> > > >
> > > > return __vmalloc_node_flags_caller(size, node, flags,
> > > > __builtin_return_address(0));
> > >
> > > Well, it doesn't have to be done at compile-time, does it? We could
> > > add a knob (in debugfs, presumably) which enables this at runtime.
> > > That's far more user-friendly.
> >
> > But who will turn it on in debugfs?
>
> But who will turn it on in Kconfig? Just a handful of developers. We
So, it won't receive much testing.
I've never played with those debugfs files (because I didn't need it), and
most users also won't play with it. Having a debugfs option is like having
no option at all.
> could add SONFIG_DEBUG_SG to the list in
> Documentation/process/submit-checklist.rst, but nobody reads that.
>
> Also, a whole bunch of defconfigs set CONFIG_DEBUG_SG=y and some
> googling indicates that they aren't the only ones...
>
> > It should be default for debugging
> > kernels, so that users using them would report the error.
>
> Well. This isn't the first time we've wanted to enable expensive (or
> noisy) debugging things in debug kernels, by any means.
>
> So how could we define a debug kernel in which it's OK to enable such
> things?
Debug kernel is what distributions distribute as debug kernel - i.e. RHEL
or Fedora have debugging kernels. So it needs to be bound to an option
that is turned on in these kernels - so that any user who boots the
debugging kernel triggers the bug.
> - Could be "it's an -rc kernel". But then we'd be enabling a bunch of
> untested code when Linus cuts a release.
>
> - Could be "it's an -rc kernel with SUBLEVEL <= 5". But then we risk
> unexpected things happening when Linux cuts -rc6, which still isn't
> good.
>
> - How about "it's an -rc kernel with odd-numbered SUBLEVEL and
> SUBLEVEL <= 5". That way everybody who runs -rc1, -rc3 and -rc5 will
> have kvmalloc debugging enabled. That's potentially nasty because
> vmalloc is much slower than kmalloc. But kvmalloc() is only used for
> large and probably infrequent allocations, so it's probably OK.
>
> I wonder how we get at SUBLEVEL from within .c.
Don't bind it to rc level, bind it to some debugging configuration option.
Mikulas
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