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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Thu, 11 Apr 2019 07:51:48 -0700
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Subject: Re: [PATCH v5 1/6] libnvdimm: nd_region flush callback support
To:     Pankaj Gupta <pagupta@redhat.com>
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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(...);