linux-kernel - Re: [PATCH] virtio: Do not drop __GFP_HIGH in alloc

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Date:	Tue, 1 Dec 2015 16:17:05 +0000
From:	Will Deacon <will.deacon@....com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] virtio: Do not drop __GFP_HIGH in alloc_indirect

On Tue, Dec 01, 2015 at 03:32:49PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> b92b1b89a33c ("virtio: force vring descriptors to be allocated from
> lowmem") tried to exclude highmem pages for descriptors so it cleared
> __GFP_HIGHMEM from a given gfp mask. The patch also cleared __GFP_HIGH
> which doesn't make much sense for this fix because __GFP_HIGH only
> controls access to memory reserves and it doesn't have any influence
> on the zone selection. Some of the call paths use GFP_ATOMIC and
> dropping __GFP_HIGH will reduce their changes for success because the
> lack of access to memory reserves.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
> Hi,
> I have stumbled over this code while looking at other issue [1]. I think
> that using __GFP_HIGH simply got there because of its confusing name. It
> doesn't have anything to do with the highmem zone.
> 
> The patch is based on the current linux-next. 
> 
> I think that clearing __GFP_HIGHMEM is bogus in the current code because
> all code paths either use GFP_KERNEL or GFP_ATOMIC and those do not fall
> back to the highmem zone but I have kept it because clearing the flag
> cannot be harmful.
> 
> [1] http://lkml.kernel.org/r/87h9k4kzcv.fsf%40yhuang-dev.intel.com
> 
>  drivers/virtio/virtio_ring.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Thanks for cleaning this up:

Acked-by: Will Deacon <will.deacon@....com>

Will

> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 14e7ce9b3e96..734de927c89d 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -110,7 +110,7 @@ static struct vring_desc *alloc_indirect(struct virtqueue *_vq,
>  	 * otherwise virt_to_phys will give us bogus addresses in the
>  	 * virtqueue.
>  	 */
> -	gfp &= ~(__GFP_HIGHMEM | __GFP_HIGH);
> +	gfp &= ~__GFP_HIGHMEM;
>  
>  	desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
>  	if (!desc)
> -- 
> 2.6.2
> 
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