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

[Michal Hocko]

On Thu 19-04-18 12:12:38, Mikulas Patocka wrote:
[...]
> From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> 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.

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.

> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

Nacked-by: Michal Hocko <mhocko@xxxxxxxx>

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

-- 
Michal Hocko
SUSE Labs