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

Stefan Hajnoczi
2024-03-28T20:40:38Z

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;
+ }
+
+ /* 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);
+}
+
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