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

Darrick J. Wong
2021-09-22T18:54:18Z

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