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Date:   Thu, 19 Apr 2018 17:19:20 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     David Miller <davem@davemloft.net>, linux-mm@kvack.org,
        eric.dumazet@gmail.com, edumazet@google.com,
        bhutchings@solarflare.com, netdev@vger.kernel.org,
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        virtualization@lists.linux-foundation.org, dm-devel@redhat.com,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM
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On Thu, 19 Apr 2018, Andrew Morton wrote:

> On Thu, 19 Apr 2018 12:12:38 -0400 (EDT) Mikulas Patocka <mpatocka@redhat.com> wrote:
> 
> > The kvmalloc function tries to use kmalloc and falls back to vmalloc if
> > kmalloc fails.
> > 
> > Unfortunatelly, some kernel code has bugs - it uses kvmalloc and then
> > uses DMA-API on the returned memory or frees it with kfree. Such bugs were
> > found in the virtio-net driver, dm-integrity or RHEL7 powerpc-specific
> > code.
> > 
> > These bugs are hard to reproduce because vmalloc falls back to kmalloc
> > only if memory is fragmented.
> 
> Yes, that's nasty.
> 
> > 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? It should be default for debugging 
kernels, so that users using them would report the error.

Conditioning it on CONFIG_DEBUG_SG is better than CONFIG_DEBUG_VM, it will 
print a stacktrace where the incorrect use happened.

Mikulas