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Subject: Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM
To:     Mikulas Patocka <mpatocka@redhat.com>,
        David Miller <davem@davemloft.net>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org
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        bhutchings@solarflare.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, mst@redhat.com, jasowang@redhat.com,
        virtualization@lists.linux-foundation.org, dm-devel@redhat.com,
        Laura Abbott <labbott@redhat.com>
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From:   Vlastimil Babka <vbabka@suse.cz>
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Date:   Thu, 19 Apr 2018 20:28:30 +0200
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On 04/19/2018 06:12 PM, Mikulas Patocka wrote:
> From: Mikulas Patocka <mpatocka@redhat.com>
> Subject: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM
> 
> 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.
> 
> 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.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Hmm AFAIK Fedora uses CONFIG_DEBUG_VM in their kernels. Sure you want to
impose this on all users? Seems too much for DEBUG_VM to me. Maybe it
should be hidden under some error injection config?

> ---
>  mm/util.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> Index: linux-2.6/mm/util.c
> ===================================================================
> --- 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

Did you verify that vmalloc does the right thing for sub-page sizes?
Shouldn't those be exempted?

>  	return __vmalloc_node_flags_caller(size, node, flags,
>  			__builtin_return_address(0));
>