Re: [PATCH v6 2/6] dax, pmem: Add a dax operation zero_page_range

[Pankaj Gupta <pankaj.gupta.linux@gmail.com>]

>
> Add a dax operation zero_page_range, to zero a page. This will also clear any
> known poison in the page being zeroed.
>
> As of now, zeroing of one page is allowed in a single call. There
> are no callers which are trying to zero more than a page in a single call.
> Once we grow the callers which zero more than a page in single call, we
> can add that support. Primary reason for not doing that yet is that this
> will add little complexity in dm implementation where a range might be
> spanning multiple underlying targets and one will have to split the range
> into multiple sub ranges and call zero_page_range() on individual targets.
>
> Suggested-by: Christoph Hellwig <hch@infradead.org>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
>  drivers/dax/super.c   | 20 ++++++++++++++++++++
>  drivers/nvdimm/pmem.c | 11 +++++++++++
>  include/linux/dax.h   |  4 ++++
>  3 files changed, 35 insertions(+)
>
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index 0aa4b6bc5101..e498daf3c0d7 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -344,6 +344,26 @@ size_t dax_copy_to_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
>  }
>  EXPORT_SYMBOL_GPL(dax_copy_to_iter);
>
> +int dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
> +                       size_t nr_pages)
> +{
> +       if (!dax_alive(dax_dev))
> +               return -ENXIO;
> +
> +       if (!dax_dev->ops->zero_page_range)
> +               return -EOPNOTSUPP;
> +       /*
> +        * There are no callers that want to zero more than one page as of now.
> +        * Once users are there, this check can be removed after the
> +        * device mapper code has been updated to split ranges across targets.
> +        */
> +       if (nr_pages != 1)
> +               return -EIO;
> +
> +       return dax_dev->ops->zero_page_range(dax_dev, pgoff, nr_pages);
> +}
> +EXPORT_SYMBOL_GPL(dax_zero_page_range);
> +
>  #ifdef CONFIG_ARCH_HAS_PMEM_API
>  void arch_wb_cache_pmem(void *addr, size_t size);
>  void dax_flush(struct dax_device *dax_dev, void *addr, size_t size)
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index 075b11682192..5b774ddd0efb 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -282,6 +282,16 @@ static const struct block_device_operations pmem_fops = {
>         .revalidate_disk =      nvdimm_revalidate_disk,
>  };
>
> +static int pmem_dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
> +                                   size_t nr_pages)
> +{
> +       struct pmem_device *pmem = dax_get_private(dax_dev);
> +
> +       return blk_status_to_errno(pmem_do_write(pmem, ZERO_PAGE(0), 0,
> +                                  PFN_PHYS(pgoff) >> SECTOR_SHIFT,
> +                                  PAGE_SIZE));
> +}
> +
>  static long pmem_dax_direct_access(struct dax_device *dax_dev,
>                 pgoff_t pgoff, long nr_pages, void **kaddr, pfn_t *pfn)
>  {
> @@ -313,6 +323,7 @@ static const struct dax_operations pmem_dax_ops = {
>         .dax_supported = generic_fsdax_supported,
>         .copy_from_iter = pmem_copy_from_iter,
>         .copy_to_iter = pmem_copy_to_iter,
> +       .zero_page_range = pmem_dax_zero_page_range,
>  };
>
>  static const struct attribute_group *pmem_attribute_groups[] = {
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 328c2dbb4409..71735c430c05 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -34,6 +34,8 @@ struct dax_operations {
>         /* copy_to_iter: required operation for fs-dax direct-i/o */
>         size_t (*copy_to_iter)(struct dax_device *, pgoff_t, void *, size_t,
>                         struct iov_iter *);
> +       /* zero_page_range: required operation. Zero page range   */
> +       int (*zero_page_range)(struct dax_device *, pgoff_t, size_t);
>  };
>
>  extern struct attribute_group dax_attribute_group;
> @@ -199,6 +201,8 @@ size_t dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
>                 size_t bytes, struct iov_iter *i);
>  size_t dax_copy_to_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
>                 size_t bytes, struct iov_iter *i);
> +int dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
> +                       size_t nr_pages);
>  void dax_flush(struct dax_device *dax_dev, void *addr, size_t size);
>
>  ssize_t dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
> --
> 2.20.1

Zeroing single page seems right approach for now.
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
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