On Thu, Mar 28, 2024 at 08:31:21PM -0500, Eric Blake wrote: > On Thu, Mar 28, 2024 at 04:39:09PM -0400, Stefan Hajnoczi wrote: > > Signed-off-by: Stefan Hajnoczi <[email protected]> > > --- > > 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.
Yes. XFS also returns -EIO, so I guess it's okay to do so.
I still need to get to the bottom of whether calling
dm_thin_find_mapped_range() is sane here and what to do when/if it
returns EWOULDBLOCK.
> > + }
> > +
> > + /* 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).
Returning (ti->begin + ti->len) << SECTOR_SHIFT for SEEK_HOLE when there
is data at the end of the target is intentional. This matches the
semantics of lseek().
I agree there is adjustment necessary in dm.c, but I want to seek the
semantics of all lseek() functions identical to avoid confusion.
Stefan
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