[PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Xie Yongji]

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(). 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@infradead.org>
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
---
 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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>
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Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

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?

> 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@infradead.org>
> Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> ---
>  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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

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?

> 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@infradead.org>
> Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> ---
>  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

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Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Max Gurtovoy]

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.

This commit looks good to me, thanks Xie Yongji.

Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>

>> 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@infradead.org>
>> Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
>> ---
>>   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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

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.

> This commit looks good to me, thanks Xie Yongji.
> 
> Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> 
> > > 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@infradead.org>
> > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > ---
> > >   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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

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.

> This commit looks good to me, thanks Xie Yongji.
> 
> Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> 
> > > 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@infradead.org>
> > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > ---
> > >   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

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Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Max Gurtovoy]

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.

Probably no such devices, but we need to be prepared.

>
>> This commit looks good to me, thanks Xie Yongji.
>>
>> Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
>>
>>>> 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@infradead.org>
>>>> Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
>>>> ---
>>>>    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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

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.
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@nvidia.com>
> > > 
> > > > > 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@infradead.org>
> > > > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > > > ---
> > > > >    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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

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.
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@nvidia.com>
> > > 
> > > > > 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@infradead.org>
> > > > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > > > ---
> > > > >    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

_______________________________________________
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Virtualization@lists.linux-foundation.org
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Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Max Gurtovoy]

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.

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@nvidia.com>
>>>>
>>>>>> 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@infradead.org>
>>>>>> Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
>>>>>> ---
>>>>>>     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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

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.


> 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@nvidia.com>
> > > > > 
> > > > > > > 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@infradead.org>
> > > > > > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > > > > > ---
> > > > > > >     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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

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.


> 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@nvidia.com>
> > > > > 
> > > > > > > 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@infradead.org>
> > > > > > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > > > > > ---
> > > > > > >     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

_______________________________________________
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Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Max Gurtovoy]

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@nvidia.com>
>>>>>>
>>>>>>>> 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@infradead.org>
>>>>>>>> Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
>>>>>>>> ---
>>>>>>>>      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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 04:27:15PM +0200, Max Gurtovoy wrote:
> 
> 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.

Well we already have min_not_zero there, presumably for some reason.

> Do you have such device to test ?

Xie Yongji mentioned he does.

> > 
> > > 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@nvidia.com>
> > > > > > > 
> > > > > > > > > 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@infradead.org>
> > > > > > > > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > > > > > > > ---
> > > > > > > > >      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

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 04:27:15PM +0200, Max Gurtovoy wrote:
> 
> 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.

Well we already have min_not_zero there, presumably for some reason.

> Do you have such device to test ?

Xie Yongji mentioned he does.

> > 
> > > 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@nvidia.com>
> > > > > > > 
> > > > > > > > > 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@infradead.org>
> > > > > > > > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > > > > > > > ---
> > > > > > > > >      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

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Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Yongji Xie]

On Wed, Mar 2, 2022 at 9:33 PM Michael S. Tsirkin <mst@redhat.com> 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.
> I would expect discard to follow max_segments restrictions then.
>

I think one point is whether we want to allow the corner case that the
device reports a larger value for max_discard_segments than
max_segments. For example, queue size is 256, max_segments is 128 - 2,
max_discard_segments is 256 - 2.

Thanks,
Yongji

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 09:53:17PM +0800, Yongji Xie wrote:
> On Wed, Mar 2, 2022 at 9:33 PM Michael S. Tsirkin <mst@redhat.com> 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.
> > I would expect discard to follow max_segments restrictions then.
> >
> 
> I think one point is whether we want to allow the corner case that the
> device reports a larger value for max_discard_segments than
> max_segments. For example, queue size is 256, max_segments is 128 - 2,
> max_discard_segments is 256 - 2.
> 
> Thanks,
> Yongji

So if device specifies that, then I guess it's fine and from that POV
the patch is correct.  But I think the issue is when device specifies 0
which we interpret as 256 with no basis in hardware.

-- 
MST

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 09:53:17PM +0800, Yongji Xie wrote:
> On Wed, Mar 2, 2022 at 9:33 PM Michael S. Tsirkin <mst@redhat.com> 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.
> > I would expect discard to follow max_segments restrictions then.
> >
> 
> I think one point is whether we want to allow the corner case that the
> device reports a larger value for max_discard_segments than
> max_segments. For example, queue size is 256, max_segments is 128 - 2,
> max_discard_segments is 256 - 2.
> 
> Thanks,
> Yongji

So if device specifies that, then I guess it's fine and from that POV
the patch is correct.  But I think the issue is when device specifies 0
which we interpret as 256 with no basis in hardware.

-- 
MST

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Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Yongji Xie]

On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin <mst@redhat.com> 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?
>

One example: the device doesn't specify the value of max_discard_seg
in the config space, then the virtio-blk driver will use
MAX_DISCARD_SEGMENTS (256) by default. Then we're able to trigger the
BUG_ON() if the seg_max is less than 256.

While the spec didn't say what should happen if max_discard_seg
exceeds seg_max, it also doesn't explicitly prohibit this
configuration. So I think we should at least not panic the kernel in
this case.

Thanks,
Yongji

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 06:46:03PM +0800, Yongji Xie wrote:
> On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin <mst@redhat.com> 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?
> >
> 
> One example: the device doesn't specify the value of max_discard_seg
> in the config space, then the virtio-blk driver will use
> MAX_DISCARD_SEGMENTS (256) by default. Then we're able to trigger the
> BUG_ON() if the seg_max is less than 256.
> 
> While the spec didn't say what should happen if max_discard_seg
> exceeds seg_max, it also doesn't explicitly prohibit this
> configuration. So I think we should at least not panic the kernel in
> this case.
> 
> Thanks,
> Yongji

Oh that last one sounds like a bug, I think it should be
min(MAX_DISCARD_SEGMENTS, seg_max)

When max_discard_seg and seg_max both exist, that's a different question. We can
- do min(max_discard_seg, seg_max)
- fail probe
- clear the relevant feature flag

I feel we need a better plan than submitting an invalid request to device.

-- 
MST

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 06:46:03PM +0800, Yongji Xie wrote:
> On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin <mst@redhat.com> 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?
> >
> 
> One example: the device doesn't specify the value of max_discard_seg
> in the config space, then the virtio-blk driver will use
> MAX_DISCARD_SEGMENTS (256) by default. Then we're able to trigger the
> BUG_ON() if the seg_max is less than 256.
> 
> While the spec didn't say what should happen if max_discard_seg
> exceeds seg_max, it also doesn't explicitly prohibit this
> configuration. So I think we should at least not panic the kernel in
> this case.
> 
> Thanks,
> Yongji

Oh that last one sounds like a bug, I think it should be
min(MAX_DISCARD_SEGMENTS, seg_max)

When max_discard_seg and seg_max both exist, that's a different question. We can
- do min(max_discard_seg, seg_max)
- fail probe
- clear the relevant feature flag

I feel we need a better plan than submitting an invalid request to device.

-- 
MST

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Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Max Gurtovoy]

On 3/2/2022 3:15 PM, Michael S. Tsirkin wrote:
> On Wed, Mar 02, 2022 at 06:46:03PM +0800, Yongji Xie wrote:
>> On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin <mst@redhat.com> 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?
>>>
>> One example: the device doesn't specify the value of max_discard_seg
>> in the config space, then the virtio-blk driver will use
>> MAX_DISCARD_SEGMENTS (256) by default. Then we're able to trigger the
>> BUG_ON() if the seg_max is less than 256.
>>
>> While the spec didn't say what should happen if max_discard_seg
>> exceeds seg_max, it also doesn't explicitly prohibit this
>> configuration. So I think we should at least not panic the kernel in
>> this case.
>>
>> Thanks,
>> Yongji
> Oh that last one sounds like a bug, I think it should be
> min(MAX_DISCARD_SEGMENTS, seg_max)
>
> When max_discard_seg and seg_max both exist, that's a different question. We can
> - do min(max_discard_seg, seg_max)
> - fail probe
> - clear the relevant feature flag
>
> I feel we need a better plan than submitting an invalid request to device.

We should cover only for a buggy devices.

The situation that max_discard_seg > seg_max should be fine.

Thus the bellow can be added to this patch:

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c443cd64fc9b..3e372b97fe10 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -926,8 +926,8 @@ static int virtblk_probe(struct virtio_device *vdev)
                 virtio_cread(vdev, struct virtio_blk_config, 
max_discard_seg,
                              &v);
                 blk_queue_max_discard_segments(q,
-                                              min_not_zero(v,
- MAX_DISCARD_SEGMENTS));
+                                              min_t(u32, (v ? v : 
sg_elems),
+ MAX_DISCARD_SEGMENTS));

                 blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
         }

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Yongji Xie]

On Wed, Mar 2, 2022 at 11:05 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>
>
> On 3/2/2022 3:15 PM, Michael S. Tsirkin wrote:
> > On Wed, Mar 02, 2022 at 06:46:03PM +0800, Yongji Xie wrote:
> >> On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin <mst@redhat.com> 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?
> >>>
> >> One example: the device doesn't specify the value of max_discard_seg
> >> in the config space, then the virtio-blk driver will use
> >> MAX_DISCARD_SEGMENTS (256) by default. Then we're able to trigger the
> >> BUG_ON() if the seg_max is less than 256.
> >>
> >> While the spec didn't say what should happen if max_discard_seg
> >> exceeds seg_max, it also doesn't explicitly prohibit this
> >> configuration. So I think we should at least not panic the kernel in
> >> this case.
> >>
> >> Thanks,
> >> Yongji
> > Oh that last one sounds like a bug, I think it should be
> > min(MAX_DISCARD_SEGMENTS, seg_max)
> >
> > When max_discard_seg and seg_max both exist, that's a different question. We can
> > - do min(max_discard_seg, seg_max)
> > - fail probe
> > - clear the relevant feature flag
> >
> > I feel we need a better plan than submitting an invalid request to device.
>
> We should cover only for a buggy devices.
>
> The situation that max_discard_seg > seg_max should be fine.
>
> Thus the bellow can be added to this patch:
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index c443cd64fc9b..3e372b97fe10 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -926,8 +926,8 @@ static int virtblk_probe(struct virtio_device *vdev)
>                  virtio_cread(vdev, struct virtio_blk_config,
> max_discard_seg,
>                               &v);
>                  blk_queue_max_discard_segments(q,
> -                                              min_not_zero(v,
> - MAX_DISCARD_SEGMENTS));
> +                                              min_t(u32, (v ? v :
> sg_elems),
> + MAX_DISCARD_SEGMENTS));
>
>                  blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
>          }
>
>

LGTM, I can add this in v3.

Thanks,
Yongji

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

On Thu, Mar 03, 2022 at 11:31:35AM +0800, Yongji Xie wrote:
> On Wed, Mar 2, 2022 at 11:05 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
> >
> >
> > On 3/2/2022 3:15 PM, Michael S. Tsirkin wrote:
> > > On Wed, Mar 02, 2022 at 06:46:03PM +0800, Yongji Xie wrote:
> > >> On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin <mst@redhat.com> 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?
> > >>>
> > >> One example: the device doesn't specify the value of max_discard_seg
> > >> in the config space, then the virtio-blk driver will use
> > >> MAX_DISCARD_SEGMENTS (256) by default. Then we're able to trigger the
> > >> BUG_ON() if the seg_max is less than 256.
> > >>
> > >> While the spec didn't say what should happen if max_discard_seg
> > >> exceeds seg_max, it also doesn't explicitly prohibit this
> > >> configuration. So I think we should at least not panic the kernel in
> > >> this case.
> > >>
> > >> Thanks,
> > >> Yongji
> > > Oh that last one sounds like a bug, I think it should be
> > > min(MAX_DISCARD_SEGMENTS, seg_max)
> > >
> > > When max_discard_seg and seg_max both exist, that's a different question. We can
> > > - do min(max_discard_seg, seg_max)
> > > - fail probe
> > > - clear the relevant feature flag
> > >
> > > I feel we need a better plan than submitting an invalid request to device.
> >
> > We should cover only for a buggy devices.
> >
> > The situation that max_discard_seg > seg_max should be fine.
> >
> > Thus the bellow can be added to this patch:
> >
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index c443cd64fc9b..3e372b97fe10 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -926,8 +926,8 @@ static int virtblk_probe(struct virtio_device *vdev)
> >                  virtio_cread(vdev, struct virtio_blk_config,
> > max_discard_seg,
> >                               &v);
> >                  blk_queue_max_discard_segments(q,
> > -                                              min_not_zero(v,
> > - MAX_DISCARD_SEGMENTS));
> > +                                              min_t(u32, (v ? v :
> > sg_elems),
> > + MAX_DISCARD_SEGMENTS));
> >
> >                  blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
> >          }
> >
> >
> 
> LGTM, I can add this in v3.
> 
> Thanks,
> Yongji

Except the logic is convoluted then.  I would instead add

	/* max_seg == 0 is out of spec but we always handled it */
	if (!v)
		v = sg_elems;


-- 
MST

Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Michael S. Tsirkin]

On Thu, Mar 03, 2022 at 11:31:35AM +0800, Yongji Xie wrote:
> On Wed, Mar 2, 2022 at 11:05 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
> >
> >
> > On 3/2/2022 3:15 PM, Michael S. Tsirkin wrote:
> > > On Wed, Mar 02, 2022 at 06:46:03PM +0800, Yongji Xie wrote:
> > >> On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin <mst@redhat.com> 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?
> > >>>
> > >> One example: the device doesn't specify the value of max_discard_seg
> > >> in the config space, then the virtio-blk driver will use
> > >> MAX_DISCARD_SEGMENTS (256) by default. Then we're able to trigger the
> > >> BUG_ON() if the seg_max is less than 256.
> > >>
> > >> While the spec didn't say what should happen if max_discard_seg
> > >> exceeds seg_max, it also doesn't explicitly prohibit this
> > >> configuration. So I think we should at least not panic the kernel in
> > >> this case.
> > >>
> > >> Thanks,
> > >> Yongji
> > > Oh that last one sounds like a bug, I think it should be
> > > min(MAX_DISCARD_SEGMENTS, seg_max)
> > >
> > > When max_discard_seg and seg_max both exist, that's a different question. We can
> > > - do min(max_discard_seg, seg_max)
> > > - fail probe
> > > - clear the relevant feature flag
> > >
> > > I feel we need a better plan than submitting an invalid request to device.
> >
> > We should cover only for a buggy devices.
> >
> > The situation that max_discard_seg > seg_max should be fine.
> >
> > Thus the bellow can be added to this patch:
> >
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index c443cd64fc9b..3e372b97fe10 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -926,8 +926,8 @@ static int virtblk_probe(struct virtio_device *vdev)
> >                  virtio_cread(vdev, struct virtio_blk_config,
> > max_discard_seg,
> >                               &v);
> >                  blk_queue_max_discard_segments(q,
> > -                                              min_not_zero(v,
> > - MAX_DISCARD_SEGMENTS));
> > +                                              min_t(u32, (v ? v :
> > sg_elems),
> > + MAX_DISCARD_SEGMENTS));
> >
> >                  blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
> >          }
> >
> >
> 
> LGTM, I can add this in v3.
> 
> Thanks,
> Yongji

Except the logic is convoluted then.  I would instead add

	/* max_seg == 0 is out of spec but we always handled it */
	if (!v)
		v = sg_elems;


-- 
MST

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Re: [PATCH v2] virtio-blk: Remove BUG_ON() in virtio_queue_rq()

[Yongji Xie]

On Thu, Mar 3, 2022 at 3:22 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Mar 03, 2022 at 11:31:35AM +0800, Yongji Xie wrote:
> > On Wed, Mar 2, 2022 at 11:05 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
> > >
> > >
> > > On 3/2/2022 3:15 PM, Michael S. Tsirkin wrote:
> > > > On Wed, Mar 02, 2022 at 06:46:03PM +0800, Yongji Xie wrote:
> > > >> On Tue, Mar 1, 2022 at 11:43 PM Michael S. Tsirkin <mst@redhat.com> 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?
> > > >>>
> > > >> One example: the device doesn't specify the value of max_discard_seg
> > > >> in the config space, then the virtio-blk driver will use
> > > >> MAX_DISCARD_SEGMENTS (256) by default. Then we're able to trigger the
> > > >> BUG_ON() if the seg_max is less than 256.
> > > >>
> > > >> While the spec didn't say what should happen if max_discard_seg
> > > >> exceeds seg_max, it also doesn't explicitly prohibit this
> > > >> configuration. So I think we should at least not panic the kernel in
> > > >> this case.
> > > >>
> > > >> Thanks,
> > > >> Yongji
> > > > Oh that last one sounds like a bug, I think it should be
> > > > min(MAX_DISCARD_SEGMENTS, seg_max)
> > > >
> > > > When max_discard_seg and seg_max both exist, that's a different question. We can
> > > > - do min(max_discard_seg, seg_max)
> > > > - fail probe
> > > > - clear the relevant feature flag
> > > >
> > > > I feel we need a better plan than submitting an invalid request to device.
> > >
> > > We should cover only for a buggy devices.
> > >
> > > The situation that max_discard_seg > seg_max should be fine.
> > >
> > > Thus the bellow can be added to this patch:
> > >
> > > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > > index c443cd64fc9b..3e372b97fe10 100644
> > > --- a/drivers/block/virtio_blk.c
> > > +++ b/drivers/block/virtio_blk.c
> > > @@ -926,8 +926,8 @@ static int virtblk_probe(struct virtio_device *vdev)
> > >                  virtio_cread(vdev, struct virtio_blk_config,
> > > max_discard_seg,
> > >                               &v);
> > >                  blk_queue_max_discard_segments(q,
> > > -                                              min_not_zero(v,
> > > - MAX_DISCARD_SEGMENTS));
> > > +                                              min_t(u32, (v ? v :
> > > sg_elems),
> > > + MAX_DISCARD_SEGMENTS));
> > >
> > >                  blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
> > >          }
> > >
> > >
> >
> > LGTM, I can add this in v3.
> >
> > Thanks,
> > Yongji
>
> Except the logic is convoluted then.  I would instead add
>
>         /* max_seg == 0 is out of spec but we always handled it */
>         if (!v)
>                 v = sg_elems;

Got it.

Thanks,
Yongji