Re: [PATCH v2 11/16] iomap: add caller-provided callbacks for read and readahead

Gao Xiang
2025-09-09T00:14:49Z

Hi Joanne,

On Mon, Sep 08, 2025 at 11:51:17AM -0700, Joanne Koong wrote:
> Add caller-provided callbacks for read and readahead so that it can be
> used generically, especially by filesystems that are not block-based.
>
> In particular, this:
> * Modifies the read and readahead interface to take in a
> struct iomap_read_folio_ctx that is publicly defined as:
>
> struct iomap_read_folio_ctx {
> const struct iomap_read_ops *ops;
> struct folio *cur_folio;
> struct readahead_control *rac;
> void *private;
> };
>
> where struct iomap_read_ops is defined as:
>
> struct iomap_read_ops {
> int (*read_folio_range)(const struct iomap_iter *iter,
> struct iomap_read_folio_ctx *ctx,
> loff_t pos, size_t len);
> int (*read_submit)(struct iomap_read_folio_ctx *ctx);
> };
>

No, I don't think `struct iomap_read_folio_ctx` has another
`.private` makes any sense, because:

- `struct iomap_iter *iter` already has `.private` and I think
it's mainly used for per-request usage; and your new
`.read_folio_range` already passes
`const struct iomap_iter *iter` which has `.private`
I don't think some read-specific `.private` is useful in any
case, also below.

- `struct iomap_read_folio_ctx` cannot be accessed by previous
.iomap_{begin,end} helpers, which means `struct iomap_read_ops`
is only useful for FUSE read iter/submit logic.

Also after my change, the prototype will be:

int iomap_read_folio(const struct iomap_ops *ops,
struct iomap_read_folio_ctx *ctx, void *private2);
void iomap_readahead(const struct iomap_ops *ops,
struct iomap_read_folio_ctx *ctx, void *private2);

Is it pretty weird due to `.iomap_{begin,end}` in principle can
only use `struct iomap_iter *` but have no way to access
` struct iomap_read_folio_ctx` to get more enough content for
read requests.

Thanks,
Gao Xiang