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

[Joseph Qi <joseph.qi@linux.alibaba.com>]

Gentle ping...

On 7/9/26 8:44 PM, Joseph Qi wrote:
> 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 <hch@lst.de>
> Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> ---
>  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;