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

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Message-Id: <1448980369-27130-1-git-send-email-mhocko@kernel.org>
Date:	Tue,  1 Dec 2015 15:32:49 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Will Deacon <will.deacon@....com>,
	virtualization@...ts.linux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: [PATCH] virtio: Do not drop __GFP_HIGH in alloc_indirect

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(-)

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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