Re: [RFC PATCH] block: wire blkdev_fallocate() to block_device_operations' reserve_space

Darrick J. Wong
2016-04-13T00:12:53Z

On Tue, Apr 12, 2016 at 05:04:27PM -0400, Mike Snitzer wrote:
> On Tue, Apr 12 2016 at 4:39pm -0400,
> Darrick J. Wong <darrick.wong@oracle.com> wrote:
>
> > On Tue, Apr 12, 2016 at 04:04:59PM -0400, Mike Snitzer wrote:
> > > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > > index 5a2c3ab..b34c07b 100644
> > > --- a/fs/block_dev.c
> > > +++ b/fs/block_dev.c
> > > @@ -1801,17 +1801,13 @@ long blkdev_fallocate(struct file *file, int mode, loff_t start, loff_t len)
> > > struct request_queue *q = bdev_get_queue(bdev);
> > > struct address_space *mapping;
> > > loff_t end = start + len - 1;
> > > - loff_t bs_mask, isize;
> > > + loff_t isize;
> > > int error;
> > >
> > > /* We only support zero range and punch hole. */
> > > if (mode & ~BLKDEV_FALLOC_FL_SUPPORTED)
> > > return -EOPNOTSUPP;
> > >
> > > - /* We haven't a primitive for "ensure space exists" right now. */
> > > - if (!(mode & ~FALLOC_FL_KEEP_SIZE))
> > > - return -EOPNOTSUPP;
> > > -
> > > /* Only punch if the device can do zeroing discard. */
> > > if ((mode & FALLOC_FL_PUNCH_HOLE) &&
> > > (!blk_queue_discard(q) || !q->limits.discard_zeroes_data))
> > > @@ -1829,9 +1825,12 @@ long blkdev_fallocate(struct file *file, int mode, loff_t start, loff_t len)
> > > return -EINVAL;
> > > }
> > >
> > > - /* Don't allow IO that isn't aligned to logical block size */
> > > - bs_mask = bdev_logical_block_size(bdev) - 1;
> > > - if ((start | len) & bs_mask)
> > > + /*
> > > + * Don't allow IO that isn't aligned to minimum IO size (io_min)
> > > + * - for normal device's io_min is usually logical block size
> > > + * - but for more exotic devices (e.g. DM thinp) it may be larger
> > > + */
> > > + if ((start | len) % bdev_io_min(bdev))

I started by noticing the 64-bit division. However, in researching alignment
requirements for fallocate, I noticed that nothing says that we can return
-EINVAL for unaligned offset/len for allocate or punch. For file allocations
ext4 and xfs simply enlarge the range so that the ends are aligned to the
logical block size; for punch they both shrink the range to deallocate until
the ends are aligned, and write zeroes to the partial blocks.

At least for user-visible fallocate we should do likewise, but for the internal
blkdev_ helpers I think it makes more sense to check lbs alignment and let the
lower level driver reject the IO if min_io alignment is a hard requirement.
Documentation/block/queue-sysfs.txt says that the min_io is the smallest
/preferred/ size.

But, before that, I'll push out some new fallocate patches for -rc3.

> > > return -EINVAL;
> >
> > Noted. Will update the original patch.
>
> BTW, I just noticed your "block: require write_same and discard requests
> align to logical block size" -- doesn't look right.

What happens if we pass a request to thinp that isn't aligned to
minimum_io_size? Does it reject the command?

> But maybe I'm just too hyper-focused on DM thinp's needs (which would
> much prefer these checks be done in terms of minimum_io_size, rather
> than logical_block_size, and _not_ assuming power-of-2 math will work).
>
> But at least for discard: your lbs-based check is fine; since we have
> discard_granularity to cover thinp's more specific requirements.

--D