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Date: Thu, 19 Apr 2018 12:47:51 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: David Miller <davem@davemloft.net>, linux-mm@kvack.org,
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        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 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.