linux-kernel - Re: [RFC 8/9] dm thin: add llseek(SEEK_HOLE/SEEK

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Date: Wed, 3 Apr 2024 11:03:46 -0400
From: Stefan Hajnoczi <stefanha@...hat.com>
To: Eric Blake <eblake@...hat.com>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alasdair Kergon <agk@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>, dm-devel@...ts.linux.dev,
	David Teigland <teigland@...hat.com>,
	Mike Snitzer <snitzer@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@....de>, Joe Thornber <ejt@...hat.com>
Subject: Re: [RFC 8/9] dm thin: add llseek(SEEK_HOLE/SEEK_DATA) support

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 <stefanha@...hat.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.

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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