Re: [PATCH v2] virtio-pmem: allocate flush bio from a driver-private bio_set

+CC more folks

> async_pmem_flush() allocates a child bio for the flush with GFP_ATOMIC.
> This runs from pmem_submit_bio(), a ->submit_bio callback that executes
> in a sleepable context, so there is no atomicity requirement here.
>
> bio_alloc() only guarantees success when __GFP_DIRECT_RECLAIM is set,
> because that is what lets it fall back to the mempool reserve. With
> GFP_ATOMIC the reclaim bit is absent, so the allocation can fail and
> return -ENOMEM whenever the fast paths (percpu cache and slab) are
> exhausted, which is common right after boot. A flush is issued from
> filesystem writeback and must not fail on a transient allocation
> shortage, otherwise the device can appear unmountable:
>
>   Buffer I/O error on dev pmem0, logical block 0, lost sync page write
>
> Switch to GFP_NOIO so __GFP_DIRECT_RECLAIM is set and the allocation can
> make forward progress. However, bio_alloc() draws from the shared
> fs_bio_set, and the incoming bio being flushed may itself have come from
> fs_bio_set; allocating a second bio from the same set while submitting
> underneath ->submit_bio can deadlock the mempool. Add a driver-private
> bio_set for the flush and allocate from it via bio_alloc_bioset(), so
> the flush bio has an independent reserve.
>
> With a dedicated mempool-backed bio_set and GFP_NOIO the allocation
> cannot fail, so drop the now-redundant NULL check.
>
> Fixes: 6e84200c0a29 ("virtio-pmem: Add virtio pmem driver")
> Suggested-by: Christoph Hellwig <[email protected]>
> Signed-off-by: Joseph Qi <[email protected]>
> ---
>  drivers/nvdimm/nd_virtio.c   | 11 ++++++-----
>  drivers/nvdimm/virtio_pmem.c | 11 ++++++++++-
>  drivers/nvdimm/virtio_pmem.h |  4 ++++
>  3 files changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
> index 4176046627beb..b4bd21edf5c1c 100644
> --- a/drivers/nvdimm/nd_virtio.c
> +++ b/drivers/nvdimm/nd_virtio.c
> @@ -110,17 +110,18 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
>  /* The asynchronous flush callback function */
>  int async_pmem_flush(struct nd_region *nd_region, struct bio *bio)
>  {
> +       struct virtio_device *vdev = nd_region->provider_data;
> +       struct virtio_pmem *vpmem = vdev->priv;
> +
>         /*
>          * Create child bio for asynchronous flush and chain with
>          * parent bio. Otherwise directly call nd_region flush.
>          */
>         if (bio && bio->bi_iter.bi_sector != -1) {
> -               struct bio *child = bio_alloc(bio->bi_bdev, 0,
> -                                             REQ_OP_WRITE | REQ_PREFLUSH,
> -                                             GFP_ATOMIC);
> +               struct bio *child = bio_alloc_bioset(bio->bi_bdev, 0,
> +                                       REQ_OP_WRITE | REQ_PREFLUSH, GFP_NOIO,
> +                                       &vpmem->flush_bio_set);
>
> -               if (!child)
> -                       return -ENOMEM;
>                 bio_clone_blkg_association(child, bio);
>                 child->bi_iter.bi_sector = -1;
>                 bio_chain(child, bio);
> diff --git a/drivers/nvdimm/virtio_pmem.c b/drivers/nvdimm/virtio_pmem.c
> index 77b1966619059..136179506b478 100644
> --- a/drivers/nvdimm/virtio_pmem.c
> +++ b/drivers/nvdimm/virtio_pmem.c
> @@ -65,12 +65,17 @@ static int virtio_pmem_probe(struct virtio_device *vdev)
>         }
>
>         mutex_init(&vpmem->flush_lock);
> +       err = bioset_init(&vpmem->flush_bio_set, BIO_POOL_SIZE, 0, 0);
> +       if (err) {
> +               dev_err(&vdev->dev, "failed to initialize flush bio_set\n");
> +               goto out_err;
> +       }
>         vpmem->vdev = vdev;
>         vdev->priv = vpmem;
>         err = init_vq(vpmem);
>         if (err) {
>                 dev_err(&vdev->dev, "failed to initialize virtio pmem vq's\n");
> -               goto out_err;
> +               goto out_bioset;
>         }
>
>         if (virtio_has_feature(vdev, VIRTIO_PMEM_F_SHMEM_REGION)) {
> @@ -131,6 +136,8 @@ static int virtio_pmem_probe(struct virtio_device *vdev)
>         nvdimm_bus_unregister(vpmem->nvdimm_bus);
>  out_vq:
>         vdev->config->del_vqs(vdev);
> +out_bioset:
> +       bioset_exit(&vpmem->flush_bio_set);
>  out_err:
>         return err;
>  }
> @@ -138,10 +145,12 @@ static int virtio_pmem_probe(struct virtio_device *vdev)
>  static void virtio_pmem_remove(struct virtio_device *vdev)
>  {
>         struct nvdimm_bus *nvdimm_bus = dev_get_drvdata(&vdev->dev);
> +       struct virtio_pmem *vpmem = vdev->priv;
>
>         nvdimm_bus_unregister(nvdimm_bus);
>         vdev->config->del_vqs(vdev);
>         virtio_reset_device(vdev);
> +       bioset_exit(&vpmem->flush_bio_set);
>  }
>
>  static int virtio_pmem_freeze(struct virtio_device *vdev)
> diff --git a/drivers/nvdimm/virtio_pmem.h b/drivers/nvdimm/virtio_pmem.h
> index f72cf17f9518f..4ff2076f75047 100644
> --- a/drivers/nvdimm/virtio_pmem.h
> +++ b/drivers/nvdimm/virtio_pmem.h
> @@ -15,6 +15,7 @@
>  #include <linux/libnvdimm.h>
>  #include <linux/mutex.h>
>  #include <linux/spinlock.h>
> +#include <linux/bio.h>
>
>  struct virtio_pmem_request {
>         struct virtio_pmem_req req;
> @@ -39,6 +40,9 @@ struct virtio_pmem {
>         /* Serialize flush requests to the device. */
>         struct mutex flush_lock;
>
> +       /* bio_set for allocating flush child bios */
> +       struct bio_set flush_bio_set;
> +
>         /* nvdimm bus registers virtio pmem device */
>         struct nvdimm_bus *nvdimm_bus;
>         struct nvdimm_bus_descriptor nd_desc;
> --
> 2.39.3
>