Re: [PATCH 2/5] iomap: use accelerated zeroing on a block device to zero a file range

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Sep 22, 2021 at 08:33:43AM +1000, Dave Chinner wrote:
> On Fri, Sep 17, 2021 at 06:30:55PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > Create a function that ensures that the storage backing part of a file
> > contains zeroes and will not trip over old media errors if the contents
> > are re-read.
> > 
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> >  fs/iomap/direct-io.c  |   75 +++++++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/iomap.h |    3 ++
> >  2 files changed, 78 insertions(+)
> > 
> > 
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index 4ecd255e0511..48826a49f976 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -652,3 +652,78 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> >  	return iomap_dio_complete(dio);
> >  }
> >  EXPORT_SYMBOL_GPL(iomap_dio_rw);
> > +
> > +static loff_t
> > +iomap_zeroinit_iter(struct iomap_iter *iter)
> > +{
> > +	struct iomap *iomap = &iter->iomap;
> > +	const struct iomap *srcmap = iomap_iter_srcmap(iter);
> > +	const u64 start = iomap->addr + iter->pos - iomap->offset;
> > +	const u64 nr_bytes = iomap_length(iter);
> > +	sector_t sector = start >> SECTOR_SHIFT;
> > +	sector_t nr_sectors = nr_bytes >> SECTOR_SHIFT;
> > +	int ret;
> > +
> > +	if (!iomap->bdev)
> > +		return -ECANCELED;
> > +
> > +	/* The physical extent must be sector-aligned for block layer APIs. */
> > +	if ((start | nr_bytes) & (SECTOR_SIZE - 1))
> > +		return -EINVAL;
> > +
> > +	/* Must be able to zero storage directly without fs intervention. */
> > +	if (iomap->flags & IOMAP_F_SHARED)
> > +		return -ECANCELED;
> > +	if (srcmap != iomap)
> > +		return -ECANCELED;
> > +
> > +	switch (iomap->type) {
> > +	case IOMAP_MAPPED:
> > +		ret = blkdev_issue_zeroout(iomap->bdev, sector, nr_sectors,
> > +				GFP_KERNEL, 0);
> 
> Pretty sure this needs to use BLKDEV_ZERO_NOUNMAP.

Oops, good catch!

--D

> The whole point of this is having zeroed space allocated ready for
> write on return, so having the hardware optimise away the physical
> storage zeroing by punching a hole in it's backing store and then
> potentially getting ENOSPC on the next write to this range would be
> .... suboptimal.
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com