linux-kernel - Re: [Patch v9 07/10] block: Add fops atomic write support

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Date: Fri, 21 Jun 2024 14:23:20 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: John Garry <john.g.garry@...cle.com>
Cc: Hannes Reinecke <hare@...e.de>, axboe@...nel.dk, kbusch@...nel.org,
	hch@....de, sagi@...mberg.me, jejb@...ux.ibm.com,
	martin.petersen@...cle.com, viro@...iv.linux.org.uk,
	brauner@...nel.org, dchinner@...hat.com, jack@...e.cz,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
	tytso@....edu, jbongio@...gle.com, linux-scsi@...r.kernel.org,
	ojaswin@...ux.ibm.com, linux-aio@...ck.org,
	linux-btrfs@...r.kernel.org, io-uring@...r.kernel.org,
	nilay@...ux.ibm.com, ritesh.list@...il.com, willy@...radead.org,
	agk@...hat.com, snitzer@...nel.org, mpatocka@...hat.com,
	dm-devel@...ts.linux.dev
Subject: Re: [Patch v9 07/10] block: Add fops atomic write support

On Fri, Jun 21, 2024 at 01:02:34PM +0100, John Garry wrote:
> On 21/06/2024 07:13, Hannes Reinecke wrote:
> > On 6/20/24 14:53, John Garry wrote:
> > > Support atomic writes by submitting a single BIO with the REQ_ATOMIC set.
> > > 
> > > It must be ensured that the atomic write adheres to its rules, like
> > > naturally aligned offset, so call blkdev_dio_invalid() ->
> > > blkdev_atomic_write_valid() [with renaming blkdev_dio_unaligned() to
> > > blkdev_dio_invalid()] for this purpose. The BIO submission path currently
> > > checks for atomic writes which are too large, so no need to check here.
> > > 
> > > In blkdev_direct_IO(), if the nr_pages exceeds BIO_MAX_VECS, then we
> > > cannot
> > > produce a single BIO, so error in this case.
> > > 
> > > Finally set FMODE_CAN_ATOMIC_WRITE when the bdev can support atomic
> > > writes
> > > and the associated file flag is for O_DIRECT.
> > > 
> > > Reviewed-by: Martin K. Petersen <martin.petersen@...cle.com>
> > > Signed-off-by: John Garry <john.g.garry@...cle.com>
> > > ---
> > >   block/fops.c | 20 +++++++++++++++++---
> > >   1 file changed, 17 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/block/fops.c b/block/fops.c
> > > index 376265935714..be36c9fbd500 100644
> > > --- a/block/fops.c
> > > +++ b/block/fops.c
> > > @@ -34,9 +34,12 @@ static blk_opf_t dio_bio_write_op(struct kiocb *iocb)
> > >       return opf;
> > >   }
> > > -static bool blkdev_dio_unaligned(struct block_device *bdev, loff_t pos,
> > > -                  struct iov_iter *iter)
> > > +static bool blkdev_dio_invalid(struct block_device *bdev, loff_t pos,
> > > +                struct iov_iter *iter, bool is_atomic)
> > >   {
> > > +    if (is_atomic && !generic_atomic_write_valid(iter, pos))
> > > +        return true;
> > > +
> > >       return pos & (bdev_logical_block_size(bdev) - 1) ||
> > >           !bdev_iter_is_aligned(bdev, iter);
> > >   }
> > > @@ -72,6 +75,8 @@ static ssize_t __blkdev_direct_IO_simple(struct
> > > kiocb *iocb,
> > >       bio.bi_iter.bi_sector = pos >> SECTOR_SHIFT;
> > >       bio.bi_write_hint = file_inode(iocb->ki_filp)->i_write_hint;
> > >       bio.bi_ioprio = iocb->ki_ioprio;
> > > +    if (iocb->ki_flags & IOCB_ATOMIC)
> > > +        bio.bi_opf |= REQ_ATOMIC;
> > >       ret = bio_iov_iter_get_pages(&bio, iter);
> > >       if (unlikely(ret))
> > > @@ -343,6 +348,9 @@ static ssize_t __blkdev_direct_IO_async(struct
> > > kiocb *iocb,
> > >           task_io_account_write(bio->bi_iter.bi_size);
> > >       }
> > > +    if (iocb->ki_flags & IOCB_ATOMIC)
> > > +        bio->bi_opf |= REQ_ATOMIC;
> > > +
> > >       if (iocb->ki_flags & IOCB_NOWAIT)
> > >           bio->bi_opf |= REQ_NOWAIT;
> > > @@ -359,12 +367,13 @@ static ssize_t __blkdev_direct_IO_async(struct
> > > kiocb *iocb,
> > >   static ssize_t blkdev_direct_IO(struct kiocb *iocb, struct
> > > iov_iter *iter)
> > >   {
> > >       struct block_device *bdev = I_BDEV(iocb->ki_filp->f_mapping->host);
> > > +    bool is_atomic = iocb->ki_flags & IOCB_ATOMIC;
> > >       unsigned int nr_pages;
> > >       if (!iov_iter_count(iter))
> > >           return 0;
> > > -    if (blkdev_dio_unaligned(bdev, iocb->ki_pos, iter))
> > > +    if (blkdev_dio_invalid(bdev, iocb->ki_pos, iter, is_atomic))
> > 
> > Why not passing in iocb->ki_flags here?
> > Or, indeed, the entire iocb?
> 
> We could (pass the iocb), but we only need to look up one thing - ki_pos. We
> already have is_atomic local. I am just trying to make things as efficient
> as possible. If you really think it's better (to pass iocb), then it can be
> changed.

I certainly do. ;)

https://lore.kernel.org/linux-xfs/20240620212401.GA3058325@frogsfrogsfrogs/

--D

> Thanks,
> John
> 
>