Re: [RFC v2 PATCH 05/10] dm thin: add methods to set and get reserved space

Brian Foster
2016-04-13T18:34:03Z

On Wed, Apr 13, 2016 at 10:44:42AM -0700, Darrick J. Wong wrote:
> On Tue, Apr 12, 2016 at 12:42:48PM -0400, Brian Foster wrote:
> > From: Joe Thornber <ejt@redhat.com>
> >
> > Experimental reserve interface for XFS guys to play with.
> >
> > I have big reservations (no pun intended) about this patch.
> >
> > [BF:
> > - Support for reservation reduction.
> > - Support for space provisioning.
> > - Condensed to a single function.]
> >
> > Not-Signed-off-by: Joe Thornber <ejt@redhat.com>
> > Not-Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> > ---
> > drivers/md/dm-thin.c | 181 ++++++++++++++++++++++++++++++++++++++++++++++++---
> > 1 file changed, 171 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
> > index 92237b6..32bc5bd 100644
> > --- a/drivers/md/dm-thin.c
> > +++ b/drivers/md/dm-thin.c
...
> > @@ -4271,6 +4343,94 @@ static void thin_io_hints(struct dm_target *ti, struct queue_limits *limits)
> > limits->max_discard_sectors = 2048 * 1024 * 16; /* 16G */
> > }
> >
> > +static int thin_provision_space(struct dm_target *ti, sector_t offset,
> > + sector_t len, sector_t *res)
> > +{
> > + struct thin_c *tc = ti->private;
> > + struct pool *pool = tc->pool;
> > + sector_t end;
> > + dm_block_t pblock;
> > + dm_block_t vblock;
> > + int error;
> > + struct dm_thin_lookup_result lookup;
> > +
> > + if (!is_factor(offset, pool->sectors_per_block))
> > + return -EINVAL;
> > +
> > + if (!len || !is_factor(len, pool->sectors_per_block))
> > + return -EINVAL;
> > +
> > + if (res && !is_factor(*res, pool->sectors_per_block))
> > + return -EINVAL;
> > +
> > + end = offset + len;
> > +
> > + while (offset < end) {
> > + vblock = offset;
> > + do_div(vblock, pool->sectors_per_block);
> > +
> > + error = dm_thin_find_block(tc->td, vblock, true, &lookup);
> > + if (error == 0)
> > + goto next;
> > + if (error != -ENODATA)
> > + return error;
> > +
> > + error = alloc_data_block(tc, &pblock);
>
> So this means that if fallocate wants to BDEV_RES_PROVISION N blocks, it must
> first increase the reservation (BDEV_RES_MOD) by N blocks to avoid using up
> space that was previously reserved by some other caller. I think?
>

Yes, assuming this is being called from a filesystem using the
reservation mechanism.

> > + if (error)
> > + return error;
> > +
> > + error = dm_thin_insert_block(tc->td, vblock, pblock);
>
> Having reserved and mapped blocks, what happens when we try to read them?
> Do we actually get zeroes, or does the read go straight through to whatever
> happens to be in the disk blocks? I don't think it's correct that we could
> BDEV_RES_PROVISION and end up with stale credit card numbers from some other
> thin device.
>

Agree, but I'm not really sure how this works in thinp tbh. fallocate
wasn't really on my mind when doing this. I was simply trying to cobble
together what I could to facilitate making progress on the fs parts
(e.g., I just needed a call that allocated blocks and consumed
reservation in the process).

Skimming through the dm-thin code, it looks like a (configurable) block
zeroing mechanism can be triggered from somewhere around
provision_block()->schedule_zero(), depending on whether the incoming
write overwrites the newly allocated block. If that's the case, then I
suspect that means reads would just fall through to the block and return
whatever was on disk. This code would probably need to tie into that
zeroing mechanism one way or another to deal with that issue. (Though
somebody who actually knows something about dm-thin should verify that.
:)

Brian

> (PS: I don't know enough about thinp to know if this has already been taken
> care of. I didn't see anything, but who knows what I missed. :))
>
> --D
>
> > + if (error)
> > + return error;
> > +
> > + if (res && *res)
> > + *res -= pool->sectors_per_block;
> > +next:
> > + offset += pool->sectors_per_block;
> > + }
> > +
> > + return 0;
> > +}
> > +
> > +static int thin_reserve_space(struct dm_target *ti, int mode, sector_t offset,
> > + sector_t len, sector_t *res)
> > +{
> > + struct thin_c *tc = ti->private;
> > + struct pool *pool = tc->pool;
> > + sector_t blocks;
> > + unsigned long flags;
> > + int error;
> > +
> > + if (mode == BDEV_RES_PROVISION)
> > + return thin_provision_space(ti, offset, len, res);
> > +
> > + /* res required for get/set */
> > + error = -EINVAL;
> > + if (!res)
> > + return error;
> > +
> > + if (mode == BDEV_RES_GET) {
> > + spin_lock_irqsave(&tc->pool->lock, flags);
> > + *res = tc->reserve_count * pool->sectors_per_block;
> > + spin_unlock_irqrestore(&tc->pool->lock, flags);
> > + error = 0;
> > + } else if (mode == BDEV_RES_MOD) {
> > + /*
> > + * @res must always be a factor of the pool's blocksize; upper
> > + * layers can rely on the bdev's minimum_io_size for this.
> > + */
> > + if (!is_factor(*res, pool->sectors_per_block))
> > + return error;
> > +
> > + blocks = *res;
> > + (void) sector_div(blocks, pool->sectors_per_block);
> > +
> > + error = set_reserve_count(tc, blocks);
> > + }
> > +
> > + return error;
> > +}
> > +
> > static struct target_type thin_target = {
> > .name = "thin",
> > .version = {1, 18, 0},
> > @@ -4285,6 +4445,7 @@ static struct target_type thin_target = {
> > .status = thin_status,
> > .iterate_devices = thin_iterate_devices,
> > .io_hints = thin_io_hints,
> > + .reserve_space = thin_reserve_space,
> > };
> >
> > /*----------------------------------------------------------------*/
> > --
> > 2.4.11
> >
> > _______________________________________________
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> > xfs@oss.sgi.com
> > http://oss.sgi.com/mailman/listinfo/xfs
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