Re: [PATCH v5 1/6] libnvdimm: nd_region flush callback support

Dan Williams
2019-04-11T14:52:07Z

On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta <pagupta@redhat.com> wrote:
>
> This patch adds functionality to perform flush from guest
> to host over VIRTIO. We are registering a callback based
> on 'nd_region' type. virtio_pmem driver requires this special
> flush function. For rest of the region types we are registering
> existing flush function. Report error returned by host fsync
> failure to userspace.
>
> This also handles asynchronous flush requests from the block layer
> by creating a child bio and chaining it with parent bio.
>
> Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
> ---
[..]
> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
> index b4ef7d9ff22e..fb1041ab32a6 100644
> --- a/drivers/nvdimm/region_devs.c
> +++ b/drivers/nvdimm/region_devs.c
> @@ -295,7 +295,9 @@ static ssize_t deep_flush_store(struct device *dev, struct device_attribute *att
> return rc;
> if (!flush)
> return -EINVAL;
> - nvdimm_flush(nd_region);
> + rc = nvdimm_flush(nd_region, NULL, false);
> + if (rc)
> + return rc;
>
> return len;
> }
> @@ -1085,6 +1087,11 @@ static struct nd_region *nd_region_create(struct nvdimm_bus *nvdimm_bus,
> dev->of_node = ndr_desc->of_node;
> nd_region->ndr_size = resource_size(ndr_desc->res);
> nd_region->ndr_start = ndr_desc->res->start;
> + if (ndr_desc->flush)
> + nd_region->flush = ndr_desc->flush;
> + else
> + nd_region->flush = generic_nvdimm_flush;
> +
> nd_device_register(dev);
>
> return nd_region;
> @@ -1125,11 +1132,36 @@ struct nd_region *nvdimm_volatile_region_create(struct nvdimm_bus *nvdimm_bus,
> }
> EXPORT_SYMBOL_GPL(nvdimm_volatile_region_create);
>
> +int nvdimm_flush(struct nd_region *nd_region, struct bio *bio, bool async)
> +{

I don't quite see the point of the 'async' argument. All the usages of
this routine are either

nvdimm_flush(nd_region, bio, true)
...or:
nvdimm_flush(nd_region, NULL, false)

...so why not gate async behavior on the presence of the 'bio' argument?

> + int rc = 0;
> +
> + /* Create child bio for asynchronous flush and chain with
> + * parent bio. Otherwise directly call nd_region flush.
> + */
> + if (async && bio->bi_iter.bi_sector != -1) {
> +
> + struct bio *child = bio_alloc(GFP_ATOMIC, 0);
> +
> + if (!child)
> + return -ENOMEM;
> + bio_copy_dev(child, bio);
> + child->bi_opf = REQ_PREFLUSH;
> + child->bi_iter.bi_sector = -1;
> + bio_chain(child, bio);
> + submit_bio(child);

I understand how this works, but it's a bit too "magical" for my
taste. I would prefer that all flush implementations take an optional
'bio' argument rather than rely on the make_request implementation to
stash the bio away on a driver specific list.

> + } else {
> + if (nd_region->flush(nd_region))
> + rc = -EIO;

Given the common case wants to be fast and synchronous I think we
should try to avoid retpoline overhead by default. So something like
this:

if (nd_region->flush == generic_nvdimm_flush)
rc = generic_nvdimm_flush(...);