Re: [PATCH v5 6/7] dax: add recovery_write to dax_iomap_iter in failure path

[Christoph Hellwig <hch@infradead.org>]

On Fri, Jan 28, 2022 at 02:31:49PM -0700, Jane Chu wrote:
> +typedef size_t (*iter_func_t)(struct dax_device *dax_dev, pgoff_t pgoff,
> +		void *addr, size_t bytes, struct iov_iter *i);
>  static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
>  		struct iov_iter *iter)
>  {
> @@ -1210,6 +1212,7 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
>  	ssize_t ret = 0;
>  	size_t xfer;
>  	int id;
> +	iter_func_t write_func = dax_copy_from_iter;

This use of a function pointer causes indirect call overhead.  A simple
"bool in_recovery" or do_recovery does the trick in a way that is
both more readable and generates faster code.

> +		if ((map_len == -EIO) && (iov_iter_rw(iter) == WRITE)) {

No need for the braces.

>  		if (iov_iter_rw(iter) == WRITE)
> -			xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr,
> -					map_len, iter);
> +			xfer = write_func(dax_dev, pgoff, kaddr, map_len, iter);
>  		else
>  			xfer = dax_copy_to_iter(dax_dev, pgoff, kaddr,
>  					map_len, iter);

i.e.

		if (iov_iter_rw(iter) == READ)
			xfer = dax_copy_to_iter(dax_dev, pgoff, kaddr,
					map_len, iter);
		else if (unlikely(do_recovery))
			xfer = dax_recovery_write(dax_dev, pgoff, kaddr,
					map_len, iter);
		else
			xfer = dax_copy_from_iter(dax_dev, pgoff, kaddr,
					map_len, iter);