Re: [PATCH 7/9] md: use kvmalloc rather than opencoded variant

[Mikulas Patocka]

On Mon, 30 Jan 2017, Michal Hocko wrote:

> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> copy_params uses kmalloc with vmalloc fallback. We already have a helper
> for that - kvmalloc. This caller requires GFP_NOIO semantic so it hasn't
> been converted with many others by previous patches. All we need to
> achieve this semantic is to use the scope memalloc_noio_{save,restore}
> around kvmalloc.
> 
> Cc: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  drivers/md/dm-ioctl.c | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> index a5a9b17f0f7f..dbf5b981f7d7 100644
> --- a/drivers/md/dm-ioctl.c
> +++ b/drivers/md/dm-ioctl.c
> @@ -1698,6 +1698,7 @@ static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl *param_kern
>  	struct dm_ioctl *dmi;
>  	int secure_data;
>  	const size_t minimum_data_size = offsetof(struct dm_ioctl, data);
> +	unsigned noio_flag;
>  
>  	if (copy_from_user(param_kernel, user, minimum_data_size))
>  		return -EFAULT;
> @@ -1720,15 +1721,9 @@ static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl *param_kern
>  	 * Use kmalloc() rather than vmalloc() when we can.
>  	 */
>  	dmi = NULL;
> -	if (param_kernel->data_size <= KMALLOC_MAX_SIZE)
> -		dmi = kmalloc(param_kernel->data_size, GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
> -
> -	if (!dmi) {
> -		unsigned noio_flag;
> -		noio_flag = memalloc_noio_save();
> -		dmi = __vmalloc(param_kernel->data_size, GFP_NOIO | __GFP_HIGH | __GFP_HIGHMEM, PAGE_KERNEL);
> -		memalloc_noio_restore(noio_flag);
> -	}
> +	noio_flag = memalloc_noio_save();
> +	dmi = kvmalloc(param_kernel->data_size, GFP_KERNEL);
> +	memalloc_noio_restore(noio_flag);
>  
>  	if (!dmi) {
>  		if (secure_data && clear_user(user, param_kernel->data_size))
> -- 
> 2.11.0

I would push these memalloc_noio_save/memalloc_noio_restore calls to 
kvmalloc, so that the othe callers can use them too.

Something like
	if ((flags & (__GFP_IO | __GFP_FS)) != (__GFP_IO | __GFP_FS))
		noio_flag = memalloc_noio_save();
	ptr = __vmalloc_node_flags(size, node, flags);
	if ((flags & (__GFP_IO | __GFP_FS)) != (__GFP_IO | __GFP_FS))
		memalloc_noio_restore(noio_flag)

Or perhaps even better - push memalloc_noio_save/memalloc_noio_restore 
directly to __vmalloc, so that __vmalloc respects the gfp flags properly - 
note that there are 14 places in the kernel where __vmalloc is called with 
GFP_NOFS and they are all buggy because __vmalloc doesn't respect the 
GFP_NOFS flag.

Mikulas