Linux Virtualization - Mar 2020 - [PATCH 0/2] virtio-blk: improve handling of DMA mapping failures

Two patches are handling new edge cases introduced by doing DMA mappings
(which can fail) in virtio core.

I stumbled upon this while stress testing I/O for Protected Virtual
Machines. I deliberately chose a tiny swiotlb size and have generated
load with fio. With more than one virtio-blk disk in use I experienced
hangs.

The goal of this series is to fix those hangs.

Halil Pasic (2):
  virtio-blk: fix hw_queue stopped on arbitrary error
  virtio-blk: improve virtqueue error to BLK_STS

 drivers/block/virtio_blk.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)


base-commit: 39bed42de2e7d74686a2d5a45638d6a5d7e7d473
-- 
2.17.1

Since nobody else is going to restart our hw_queue for us, the
blk_mq_start_stopped_hw_queues() is in virtblk_done() is not sufficient
necessarily sufficient to ensure that the queue will get started again.
In case of global resource outage (-ENOMEM because mapping failure,
because of swiotlb full) our virtqueue may be empty and we can get
stuck with a stopped hw_queue.

Let us not stop the queue on arbitrary errors, but only on -EONSPC which
indicates a full virtqueue, where the hw_queue is guaranteed to get
started by virtblk_done() before when it makes sense to carry on
submitting requests. Let us also remove a stale comment.

Signed-off-by: Halil Pasic <pasic at linux.ibm.com>
Cc: Jens Axboe <axboe at kernel.dk>
Fixes: f7728002c1c7 ("virtio_ring: fix return code on DMA mapping
fails")
---

I'm in doubt with regards to the Fixes tag. The thing is, virtio-blk
probably made an assumption on virtqueue_add: the failure is either
because the virtqueue is full, or the failure is fatal. In both cases it
seems acceptable to stop the queue, although the fatal case is arguable.
Since commit f7728002c1c7 it the dma mapping has failed returns -ENOMEM
and not -EIO, and thus we have a recoverable failure that ain't
virtqueue full. So I lean towards to a fixes tag that references that
commit, although it ain't broken. Alternatively one could say 'Fixes:
e467cde23818 ("Block driver using virtio.")', if the
aforementioned
assumption shouldn't have made in the first place.
---
 drivers/block/virtio_blk.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 54158766334b..adfe43f5ffe4 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -245,10 +245,12 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx
*hctx,
 	err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg, num);
 	if (err) {
 		virtqueue_kick(vblk->vqs[qid].vq);
-		blk_mq_stop_hw_queue(hctx);
+		/* Don't stop the queue if -ENOMEM: we may have failed to
+		 * bounce the buffer due to global resource outage.
+		 */
+		if (err == -ENOSPC)
+			blk_mq_stop_hw_queue(hctx);
 		spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
-		/* Out of mem doesn't actually happen, since we fall back
-		 * to direct descriptors */
 		if (err == -ENOMEM || err == -ENOSPC)
 			return BLK_STS_DEV_RESOURCE;
 		return BLK_STS_IOERR;
-- 
2.17.1

Let's change the mapping between virtqueue_add errors to BLK_STS
statuses, so that -ENOSPC, which indicates virtqueue full is still
mapped to BLK_STS_DEV_RESOURCE, but -ENOMEM which indicates non-device
specific resource outage is mapped to BLK_STS_RESOURCE.

Signed-off-by: Halil Pasic <pasic at linux.ibm.com>
---
See comment about BLK_STS_DEV_RESOURCE in include/linux/blk_types.h
---
 drivers/block/virtio_blk.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index adfe43f5ffe4..0736248999b0 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -251,9 +251,14 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx
*hctx,
 		if (err == -ENOSPC)
 			blk_mq_stop_hw_queue(hctx);
 		spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
-		if (err == -ENOMEM || err == -ENOSPC)
+		switch (err) {
+		case -ENOSPC:
 			return BLK_STS_DEV_RESOURCE;
-		return BLK_STS_IOERR;
+		case -ENOMEM:
+			return BLK_STS_RESOURCE;
+		default:
+			return BLK_STS_IOERR;
+		}
 	}
 
 	if (bd->last && virtqueue_kick_prepare(vblk->vqs[qid].vq))
-- 
2.17.1

Hi Halil,

When swiotlb full is hit for virtio_blk, there is below warning for once (the
warning is not by this patch set). Is this expected or just false positive?

[   54.767257] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 16 bytes),
total 32768 (slots), used 258 (slots)
[   54.767260] virtio-pci 0000:00:04.0: overflow 0x0000000075770110+16 of DMA
mask ffffffffffffffff bus limit 0
[   54.769192] ------------[ cut here ]------------
[   54.769200] WARNING: CPU: 3 PID: 102 at kernel/dma/direct.c:35
report_addr+0x71/0x77
[   54.769200] Modules linked in:
[   54.769203] CPU: 3 PID: 102 Comm: kworker/u8:2 Not tainted 5.6.0-rc1+ #2
[   54.769204] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
[   54.769208] Workqueue: writeback wb_workfn (flush-253:0)
[   54.769211] RIP: 0010:report_addr+0x71/0x77
... ...
[   54.769226] Call Trace:
[   54.769241]  dma_direct_map_page+0xc9/0xe0
[   54.769245]  virtqueue_add+0x172/0xaa0
[   54.769248]  virtqueue_add_sgs+0x85/0xa0
[   54.769251]  virtio_queue_rq+0x292/0x480
[   54.769255]  __blk_mq_try_issue_directly+0x13e/0x1f0
[   54.769257]  blk_mq_request_issue_directly+0x48/0xa0
[   54.769259]  blk_mq_try_issue_list_directly+0x3c/0xb0
[   54.769261]  blk_mq_sched_insert_requests+0xb6/0x100
[   54.769263]  blk_mq_flush_plug_list+0x146/0x210
[   54.769264]  blk_flush_plug_list+0xba/0xe0
[   54.769266]  blk_mq_make_request+0x331/0x5b0
[   54.769268]  generic_make_request+0x10d/0x2e0
[   54.769270]  submit_bio+0x69/0x130
[   54.769273]  ext4_io_submit+0x44/0x50
[   54.769275]  ext4_writepages+0x56f/0xd30
[   54.769278]  ? cpumask_next_and+0x19/0x20
[   54.769280]  ? find_busiest_group+0x11a/0xa40
[   54.769283]  do_writepages+0x15/0x70
[   54.769288]  __writeback_single_inode+0x38/0x330
[   54.769290]  writeback_sb_inodes+0x219/0x4c0
[   54.769292]  __writeback_inodes_wb+0x82/0xb0
[   54.769293]  wb_writeback+0x267/0x300
[   54.769295]  wb_workfn+0x1aa/0x430
[   54.769298]  process_one_work+0x156/0x360
[   54.769299]  worker_thread+0x41/0x3b0
[   54.769300]  kthread+0xf3/0x130
[   54.769302]  ? process_one_work+0x360/0x360
[   54.769303]  ? kthread_bind+0x10/0x10
[   54.769305]  ret_from_fork+0x35/0x40
[   54.769307] ---[ end trace 923a87a9ce0e777a ]---

Thank you very much!

Dongli Zhang

On 2/13/20 4:37 AM, Halil Pasic wrote:
> Since nobody else is going to restart our hw_queue for us, the
> blk_mq_start_stopped_hw_queues() is in virtblk_done() is not sufficient
> necessarily sufficient to ensure that the queue will get started again.
> In case of global resource outage (-ENOMEM because mapping failure,
> because of swiotlb full) our virtqueue may be empty and we can get
> stuck with a stopped hw_queue.
> 
> Let us not stop the queue on arbitrary errors, but only on -EONSPC which
> indicates a full virtqueue, where the hw_queue is guaranteed to get
> started by virtblk_done() before when it makes sense to carry on
> submitting requests. Let us also remove a stale comment.
> 
> Signed-off-by: Halil Pasic <pasic at linux.ibm.com>
> Cc: Jens Axboe <axboe at kernel.dk>
> Fixes: f7728002c1c7 ("virtio_ring: fix return code on DMA mapping
fails")
> ---
> 
> I'm in doubt with regards to the Fixes tag. The thing is, virtio-blk
> probably made an assumption on virtqueue_add: the failure is either
> because the virtqueue is full, or the failure is fatal. In both cases it
> seems acceptable to stop the queue, although the fatal case is arguable.
> Since commit f7728002c1c7 it the dma mapping has failed returns -ENOMEM
> and not -EIO, and thus we have a recoverable failure that ain't
> virtqueue full. So I lean towards to a fixes tag that references that
> commit, although it ain't broken. Alternatively one could say
'Fixes:
> e467cde23818 ("Block driver using virtio.")', if the
aforementioned
> assumption shouldn't have made in the first place.
> ---
>  drivers/block/virtio_blk.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 54158766334b..adfe43f5ffe4 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -245,10 +245,12 @@ static blk_status_t virtio_queue_rq(struct
blk_mq_hw_ctx *hctx,
>  	err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg, num);
>  	if (err) {
>  		virtqueue_kick(vblk->vqs[qid].vq);
> -		blk_mq_stop_hw_queue(hctx);
> +		/* Don't stop the queue if -ENOMEM: we may have failed to
> +		 * bounce the buffer due to global resource outage.
> +		 */
> +		if (err == -ENOSPC)
> +			blk_mq_stop_hw_queue(hctx);
>  		spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
> -		/* Out of mem doesn't actually happen, since we fall back
> -		 * to direct descriptors */
>  		if (err == -ENOMEM || err == -ENOSPC)
>  			return BLK_STS_DEV_RESOURCE;
>  		return BLK_STS_IOERR;
>

On Thu, Feb 13, 2020 at 8:38 PM Halil Pasic <pasic at linux.ibm.com>
wrote:
>
> Since nobody else is going to restart our hw_queue for us, the
> blk_mq_start_stopped_hw_queues() is in virtblk_done() is not sufficient
> necessarily sufficient to ensure that the queue will get started again.
> In case of global resource outage (-ENOMEM because mapping failure,
> because of swiotlb full) our virtqueue may be empty and we can get
> stuck with a stopped hw_queue.
>
> Let us not stop the queue on arbitrary errors, but only on -EONSPC which
> indicates a full virtqueue, where the hw_queue is guaranteed to get
> started by virtblk_done() before when it makes sense to carry on
> submitting requests. Let us also remove a stale comment.
The generic solution may be to stop queue only when there is any
in-flight request
not completed.

Checking -ENOMEM may not be enough, given -EIO can be returned from
virtqueue_add()
too in case of dma map failure.

Thanks,

On Thu, Feb 13, 2020 at 01:37:26PM +0100, Halil Pasic
wrote:
> Two patches are handling new edge cases introduced by doing DMA mappings
> (which can fail) in virtio core.
> 
> I stumbled upon this while stress testing I/O for Protected Virtual
> Machines. I deliberately chose a tiny swiotlb size and have generated
> load with fio. With more than one virtio-blk disk in use I experienced
> hangs.
> 
> The goal of this series is to fix those hangs.
> 
> Halil Pasic (2):
>   virtio-blk: fix hw_queue stopped on arbitrary error
>   virtio-blk: improve virtqueue error to BLK_STS
> 
>  drivers/block/virtio_blk.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> 
> base-commit: 39bed42de2e7d74686a2d5a45638d6a5d7e7d473
> -- 
> 2.17.1
> 
Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
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On Thu, 13 Feb 2020 13:37:26 +0100
Halil Pasic <pasic at linux.ibm.com> wrote:
> Two patches are handling new edge cases introduced by doing DMA mappings
> (which can fail) in virtio core.
> 
> I stumbled upon this while stress testing I/O for Protected Virtual
> Machines. I deliberately chose a tiny swiotlb size and have generated
> load with fio. With more than one virtio-blk disk in use I experienced
> hangs.
> 
> The goal of this series is to fix those hangs.
> 
> Halil Pasic (2):
>   virtio-blk: fix hw_queue stopped on arbitrary error
>   virtio-blk: improve virtqueue error to BLK_STS
> 
>  drivers/block/virtio_blk.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> 
> base-commit: 39bed42de2e7d74686a2d5a45638d6a5d7e7d473
ping

Hi Michael, hi Jason,

I got some favorable reviews for this, but AFAIK I got nothing form the
maintainers yet.

Is some of you going to pick these?

Regards,
Halil

On Tue, Mar 03, 2020 at 03:12:52PM +0100, Halil Pasic
wrote:
> On Thu, 13 Feb 2020 13:37:26 +0100
> Halil Pasic <pasic at linux.ibm.com> wrote:
> 
> > Two patches are handling new edge cases introduced by doing DMA
mappings
> > (which can fail) in virtio core.
> > 
> > I stumbled upon this while stress testing I/O for Protected Virtual
> > Machines. I deliberately chose a tiny swiotlb size and have generated
> > load with fio. With more than one virtio-blk disk in use I experienced
> > hangs.
> > 
> > The goal of this series is to fix those hangs.
> > 
> > Halil Pasic (2):
> >   virtio-blk: fix hw_queue stopped on arbitrary error
> >   virtio-blk: improve virtqueue error to BLK_STS
> > 
> >  drivers/block/virtio_blk.c | 17 ++++++++++++-----
> >  1 file changed, 12 insertions(+), 5 deletions(-)
> > 
> > 
> > base-commit: 39bed42de2e7d74686a2d5a45638d6a5d7e7d473
> 
> ping
> 
> Hi Michael, hi Jason,
> 
> I got some favorable reviews for this, but AFAIK I got nothing form the
> maintainers yet.
> 
> Is some of you going to pick these?
> 
> Regards,
> Halil
I've queued this, thanks!