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SubjectRe: [PATCH 1/2] virtio-blk: fix hw_queue stopped on arbitrary error
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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@linux.ibm.com>
> Cc: Jens Axboe <axboe@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;
>

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