linux-kernel - Re: [PATCH] virtio-blk: fix minimum number of S/G elements

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Message-Id: <201005261733.29460.rusty@rustcorp.com.au>
Date:	Wed, 26 May 2010 17:33:28 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Hellwig <hch@....de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio-blk: fix minimum number of S/G elements

On Tue, 25 May 2010 09:47:54 pm Christoph Hellwig wrote:
> 
> We need at least one S/G element to operate properly, as does the block
> layer which increments it to one anyway.  We hit this due to a qemu
> bug which advertises a sg_elements of 0 under some circumstances.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

Seems reasonable: we might as well ignore it if device is obviously talking
crap.

However, the result is awkward (we assign err then don't use it), and assumes
that sg_elems isn't touched on error (true at the moment).

Prefer this:

Subject: virtio-blk: fix minimum number of S/G elements
Date: Tue, 25 May 2010 14:17:54 +0200
From: Christoph Hellwig <hch@....de>

We need at least one S/G element to operate properly, as does the block
layer which increments it to one anyway.  We hit this due to a qemu
bug which advertises a sg_elements of 0 under some circumstances.

Signed-off-by: Christoph Hellwig <hch@....de>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au> (tweaked logic)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -298,7 +298,9 @@ static int __devinit virtblk_probe(struc
 	err = virtio_config_val(vdev, VIRTIO_BLK_F_SEG_MAX,
 				offsetof(struct virtio_blk_config, seg_max),
 				&sg_elems);
-	if (err)
+
+	/* We need at least one SG element, whatever they say. */
+	if (err || !sg_elems)
 		sg_elems = 1;
 
 	/* We need an extra sg elements at head and tail. */
--
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