linux-kernel - Re: [PATCH v4 2/2] vhost: add max_mem

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Date:	Thu, 16 Jul 2015 13:05:39 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	mst@...hat.com, kvm@...r.kernel.org
Subject: Re: [PATCH v4 2/2] vhost: add max_mem_regions module parameter

On Thu,  2 Jul 2015 15:08:11 +0200
Igor Mammedov <imammedo@...hat.com> wrote:

> it became possible to use a bigger amount of memory
> slots, which is used by memory hotplug for
> registering hotplugged memory.
> However QEMU crashes if it's used with more than ~60
> pc-dimm devices and vhost-net enabled since host kernel
> in module vhost-net refuses to accept more than 64
> memory regions.
> 
> Allow to tweak limit via max_mem_regions module paramemter
> with default value set to 64 slots.
Michael,

what was the reason not to rise default?
As much as I think I can't come up with one.

Making it as module option doesn't make much sense
since old userspace will crash on new kernels anyway
until admins learn that there is a module option
to rise limit.

Rising default limit on par with kvm's will help
to avoid those crashes and would allow to drop
not necessary option to reduce user confusion.

> 
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> ---
>  drivers/vhost/vhost.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 6488011..9a68e2e 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -29,8 +29,12 @@
>  
>  #include "vhost.h"
>  
> +static ushort max_mem_regions = 64;
> +module_param(max_mem_regions, ushort, 0444);
> +MODULE_PARM_DESC(max_mem_regions,
> +	"Maximum number of memory regions in memory map. (default: 64)");
> +
>  enum {
> -	VHOST_MEMORY_MAX_NREGIONS = 64,
>  	VHOST_MEMORY_F_LOG = 0x1,
>  };
>  
> @@ -696,7 +700,7 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
>  		return -EFAULT;
>  	if (mem.padding)
>  		return -EOPNOTSUPP;
> -	if (mem.nregions > VHOST_MEMORY_MAX_NREGIONS)
> +	if (mem.nregions > max_mem_regions)
>  		return -E2BIG;
>  	newmem = vhost_kvzalloc(size + mem.nregions * sizeof(*m->regions));
>  	if (!newmem)

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