Re: [RFC 8/9] dm thin: add llseek(SEEK_HOLE/SEEK_DATA) support

Eric Blake
2024-03-29T01:31:34Z

On Thu, Mar 28, 2024 at 04:39:09PM -0400, Stefan Hajnoczi wrote:
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> Open issues:
> - Locking?
> - thin_seek_hole_data() does not run as a bio or request. This patch
> assumes dm_thin_find_mapped_range() synchronously performs I/O if
> metadata needs to be loaded from disk. Is that a valid assumption?
> ---
> drivers/md/dm-thin.c | 77 ++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 77 insertions(+)
>
> diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
> index 4793ad2aa1f7e..3c5dc4f0fe8a3 100644
> --- a/drivers/md/dm-thin.c
> +++ b/drivers/md/dm-thin.c
> @@ -4501,6 +4501,82 @@ static void thin_io_hints(struct dm_target *ti, struct queue_limits *limits)
> }
> }
>
> +static dm_block_t loff_to_block(struct pool *pool, loff_t offset)
> +{
> + sector_t offset_sectors = offset >> SECTOR_SHIFT;
> + dm_block_t ret;
> +
> + if (block_size_is_power_of_two(pool))
> + ret = offset_sectors >> pool->sectors_per_block_shift;
> + else {
> + ret = offset_sectors;
> + (void) sector_div(ret, pool->sectors_per_block);
> + }
> +
> + return ret;
> +}
> +
> +static loff_t block_to_loff(struct pool *pool, dm_block_t block)
> +{
> + return block_to_sectors(pool, block) << SECTOR_SHIFT;
> +}
> +
> +static loff_t thin_seek_hole_data(struct dm_target *ti, loff_t offset,
> + int whence)
> +{
> + struct thin_c *tc = ti->private;
> + struct dm_thin_device *td = tc->td;
> + struct pool *pool = tc->pool;
> + dm_block_t begin;
> + dm_block_t end;
> + dm_block_t mapped_begin;
> + dm_block_t mapped_end;
> + dm_block_t pool_begin;
> + bool maybe_shared;
> + int ret;
> +
> + /* TODO locking? */
> +
> + if (block_size_is_power_of_two(pool))
> + end = ti->len >> pool->sectors_per_block_shift;
> + else {
> + end = ti->len;
> + (void) sector_div(end, pool->sectors_per_block);
> + }
> +
> + offset -= ti->begin << SECTOR_SHIFT;
> +
> + while (true) {
> + begin = loff_to_block(pool, offset);
> + ret = dm_thin_find_mapped_range(td, begin, end,
> + &mapped_begin, &mapped_end,
> + &pool_begin, &maybe_shared);
> + if (ret == -ENODATA) {
> + if (whence == SEEK_DATA)
> + return -ENXIO;
> + break;
> + } else if (ret < 0) {
> + /* TODO handle EWOULDBLOCK? */
> + return -ENXIO;

This should probably be -EIO, not -ENXIO.

> + }
> +
> + /* SEEK_DATA finishes here... */
> + if (whence == SEEK_DATA) {
> + if (mapped_begin != begin)
> + offset = block_to_loff(pool, mapped_begin);
> + break;
> + }
> +
> + /* ...while SEEK_HOLE may need to look further */
> + if (mapped_begin != begin)
> + break; /* offset is in a hole */
> +
> + offset = block_to_loff(pool, mapped_end);
> + }
> +
> + return offset + (ti->begin << SECTOR_SHIFT);

It's hard to follow, but I'm fairly certain that if whence ==
SEEK_HOLE, you end up returning ti->begin + ti->len instead of -ENXIO
if the range from begin to end is fully mapped; which is inconsistent
with the semantics you have in 4/9 (although in 6/9 I argue that
having all of the dm callbacks return ti->begin + ti->len instead of
-ENXIO might make logic easier for iterating through consecutive ti,
and then convert to -ENXIO only in the caller).

> +}
> +
> static struct target_type thin_target = {
> .name = "thin",
> .version = {1, 23, 0},
> @@ -4515,6 +4591,7 @@ static struct target_type thin_target = {
> .status = thin_status,
> .iterate_devices = thin_iterate_devices,
> .io_hints = thin_io_hints,
> + .seek_hole_data = thin_seek_hole_data,
> };
>
> /*----------------------------------------------------------------*/
> --
> 2.44.0
>

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization: qemu.org | libguestfs.org