On 3/2/2022 4:15 PM, Michael S. Tsirkin wrote:
On Wed, Mar 02, 2022 at 03:45:10PM +0200, Max Gurtovoy wrote:
On 3/2/2022 3:33 PM, Michael S. Tsirkin wrote:
On Wed, Mar 02, 2022 at 03:24:51PM +0200, Max Gurtovoy wrote:
On 3/2/2022 3:17 PM, Michael S. Tsirkin wrote:
On Wed, Mar 02, 2022 at 11:51:27AM +0200, Max Gurtovoy wrote:
On 3/1/2022 5:43 PM, Michael S. Tsirkin wrote:
On Mon, Feb 28, 2022 at 02:57:20PM +0800, Xie Yongji wrote:
Currently we have a BUG_ON() to make sure the number of sg
list does not exceed queue_max_segments() in virtio_queue_rq().
However, the block layer uses queue_max_discard_segments()
instead of queue_max_segments() to limit the sg list for
discard requests. So the BUG_ON() might be triggered if
virtio-blk device reports a larger value for max discard
segment than queue_max_segments().
Hmm the spec does not say what should happen if max_discard_seg
exceeds seg_max. Is this the config you have in mind? how do you
create it?
I don't think it's hard to create it. Just change some registers in the
device.
But with the dynamic sgl allocation that I added recently, there is no
problem with this scenario.
Well the problem is device says it can't handle such large descriptors,
I guess it works anyway, but it seems scary.
I don't follow.
The only problem this patch solves is when a virtio blk device reports
larger value for max_discard_segments than max_segments.
No, the peroblem reported is when virtio blk device reports
max_segments < 256 but not max_discard_segments.
You mean the code will work in case device report max_discard_segments >
max_segments ?
I don't think so.
I think it's like this:
if (virtio_has_feature(vdev, VIRTIO_BLK_F_DISCARD)) {
....
virtio_cread(vdev, struct virtio_blk_config, max_discard_seg,
&v);
blk_queue_max_discard_segments(q,
min_not_zero(v,
MAX_DISCARD_SEGMENTS));
}
so, IIUC the case is of a device that sets max_discard_seg to 0.
Which is kind of broken, but we handled this since 2018 so I guess
we'll need to keep doing that.
A device can't state VIRTIO_BLK_F_DISCARD and set max_discard_seg to 0.
If so, it's a broken device and we can add a quirk for it.
Do you have such device to test ?
This is exactly what Xie Yongji mention in the commit message and what I was
seeing.
But the code will work if VIRTIO_BLK_F_DISCARD is not supported by the
device (even if max_segments < 256) , since blk layer set
queue_max_discard_segments = 1 in the initialization.
And the virtio-blk driver won't change it unless VIRTIO_BLK_F_DISCARD is
supported.
I would expect discard to follow max_segments restrictions then.
Probably no such devices, but we need to be prepared.
Right, question is how to handle this.
This commit looks good to me, thanks Xie Yongji.
Reviewed-by: Max Gurtovoy <mgurtovoy@xxxxxxxxxx>
To fix it, let's simply
remove the BUG_ON() which has become unnecessary after commit
02746e26c39e("virtio-blk: avoid preallocating big SGL for data").
And the unused vblk->sg_elems can also be removed together.
Fixes: 1f23816b8eb8 ("virtio_blk: add discard and write zeroes support")
Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Signed-off-by: Xie Yongji <xieyongji@xxxxxxxxxxxxx>
---
drivers/block/virtio_blk.c | 10 +---------
1 file changed, 1 insertion(+), 9 deletions(-)
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c443cd64fc9b..a43eb1813cec 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -76,9 +76,6 @@ struct virtio_blk {
*/
refcount_t refs;
- /* What host tells us, plus 2 for header & tailer. */
- unsigned int sg_elems;
-
/* Ida index - used to track minor number allocations. */
int index;
@@ -322,8 +319,6 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
blk_status_t status;
int err;
- BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);
-
status = virtblk_setup_cmd(vblk->vdev, req, vbr);
if (unlikely(status))
return status;
@@ -783,8 +778,6 @@ static int virtblk_probe(struct virtio_device *vdev)
/* Prevent integer overflows and honor max vq size */
sg_elems = min_t(u32, sg_elems, VIRTIO_BLK_MAX_SG_ELEMS - 2);
- /* We need extra sg elements at head and tail. */
- sg_elems += 2;
vdev->priv = vblk = kmalloc(sizeof(*vblk), GFP_KERNEL);
if (!vblk) {
err = -ENOMEM;
@@ -796,7 +789,6 @@ static int virtblk_probe(struct virtio_device *vdev)
mutex_init(&vblk->vdev_mutex);
vblk->vdev = vdev;
- vblk->sg_elems = sg_elems;
INIT_WORK(&vblk->config_work, virtblk_config_changed_work);
@@ -853,7 +845,7 @@ static int virtblk_probe(struct virtio_device *vdev)
set_disk_ro(vblk->disk, 1);
/* We can handle whatever the host told us to handle. */
- blk_queue_max_segments(q, vblk->sg_elems-2);
+ blk_queue_max_segments(q, sg_elems);
/* No real sector limit. */
blk_queue_max_hw_sectors(q, -1U);
--
2.20.1
