Re: [RFCv1][WIP] ext2: Move direct-io to use iomap

[Jan Kara <jack@suse.cz>]

On Thu 16-03-23 20:10:29, Ritesh Harjani (IBM) wrote:
> [DO NOT MERGE] [WORK-IN-PROGRESS]
> 
> Hello Jan,
> 
> This is an initial version of the patch set which I wanted to share
> before today's call. This is still work in progress but atleast passes
> the set of test cases which I had kept for dio testing (except 1 from my
> list).
> 
> Looks like there won't be much/any changes required from iomap side to
> support ext2 moving to iomap apis.
> 
> I will be doing some more testing specifically test generic/083 which is
> occassionally failing in my testing.
> Also once this is stabilized, I can do some performance testing too if you
> feel so. Last I remembered we saw some performance regressions when ext4
> moved to iomap for dio.
> 
> PS: Please ignore if there are some silly mistakes. As I said, I wanted
> to get this out before today's discussion. :)
> 
> Thanks for your help!!
> 
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
>  fs/ext2/ext2.h  |   1 +
>  fs/ext2/file.c  | 114 ++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/ext2/inode.c |  20 +--------
>  3 files changed, 117 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/ext2/ext2.h b/fs/ext2/ext2.h
> index cb78d7dcfb95..cb5e309fe040 100644
> --- a/fs/ext2/ext2.h
> +++ b/fs/ext2/ext2.h
> @@ -753,6 +753,7 @@ extern unsigned long ext2_count_free (struct buffer_head *, unsigned);
>  extern struct inode *ext2_iget (struct super_block *, unsigned long);
>  extern int ext2_write_inode (struct inode *, struct writeback_control *);
>  extern void ext2_evict_inode(struct inode *);
> +extern void ext2_write_failed(struct address_space *mapping, loff_t to);
>  extern int ext2_get_block(struct inode *, sector_t, struct buffer_head *, int);
>  extern int ext2_setattr (struct mnt_idmap *, struct dentry *, struct iattr *);
>  extern int ext2_getattr (struct mnt_idmap *, const struct path *,
> diff --git a/fs/ext2/file.c b/fs/ext2/file.c
> index 6b4bebe982ca..7a8561304559 100644
> --- a/fs/ext2/file.c
> +++ b/fs/ext2/file.c
> @@ -161,12 +161,123 @@ int ext2_fsync(struct file *file, loff_t start, loff_t end, int datasync)
>  	return ret;
>  }
> 
> +static ssize_t ext2_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
> +{
> +	struct file *file = iocb->ki_filp;
> +	struct inode *inode = file->f_mapping->host;
> +	ssize_t ret;
> +
> +	inode_lock_shared(inode);
> +	ret = iomap_dio_rw(iocb, to, &ext2_iomap_ops, NULL, 0, NULL, 0);
> +	inode_unlock_shared(inode);
> +
> +	return ret;
> +}
> +
> +static int ext2_dio_write_end_io(struct kiocb *iocb, ssize_t size,
> +				 int error, unsigned int flags)
> +{
> +	loff_t pos = iocb->ki_pos;
> +	struct inode *inode = file_inode(iocb->ki_filp);
> +
> +	if (error)
> +		return error;
> +

I guess you should carry over here relevant bits of the comment from
ext4_dio_write_end_io() explaining that doing i_size update here is
necessary and actually safe.

> +	pos += size;
> +	if (pos > i_size_read(inode))
> +		i_size_write(inode, pos);
> +
> +	return 0;
> +}
> +
> +static const struct iomap_dio_ops ext2_dio_write_ops = {
> +	.end_io = ext2_dio_write_end_io,
> +};
> +
> +static ssize_t ext2_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
> +{
> +	struct file *file = iocb->ki_filp;
> +	struct inode *inode = file->f_mapping->host;
> +	ssize_t ret;
> +	unsigned int flags;
> +	unsigned long blocksize = inode->i_sb->s_blocksize;
> +	loff_t offset = iocb->ki_pos;
> +	loff_t count = iov_iter_count(from);
> +
> +
> +	inode_lock(inode);
> +	ret = generic_write_checks(iocb, from);
> +	if (ret <= 0)
> +		goto out_unlock;
> +	ret = file_remove_privs(file);
> +	if (ret)
> +		goto out_unlock;
> +	ret = file_update_time(file);
> +	if (ret)
> +		goto out_unlock;
> +
> +	/*
> +	 * We pass IOMAP_DIO_NOSYNC because otherwise iomap_dio_rw()
> +	 * calls for generic_write_sync in iomap_dio_complete().
> +	 * Since ext2_fsync nmust be called w/o inode lock,
> +	 * hence we pass IOMAP_DIO_NOSYNC and handle generic_write_sync()
> +	 * ourselves.
> +	 */
> +	flags = IOMAP_DIO_NOSYNC;

Meh, this is kind of ugly and we should come up with something better for
simple filesystems so that they don't have to play these games. Frankly,
these days I doubt there's anybody really needing inode_lock in
__generic_file_fsync(). Neither sync_mapping_buffers() nor
sync_inode_metadata() need inode_lock for their self-consistency. So it is
only about flushing more consistent set of metadata to disk when fsync(2)
races with other write(2)s to the same file so after a crash we have higher
chances of seeing some real state of the file. But I'm not sure it's really
worth keeping for filesystems that are still using sync_mapping_buffers().
People that care about consistency after a crash have IMHO moved to other
filesystems long ago.

> +
> +	/* use IOMAP_DIO_FORCE_WAIT for unaligned of extending writes */
						  ^^ or

> +	if (iocb->ki_pos + iov_iter_count(from) > i_size_read(inode) ||
> +	   (!IS_ALIGNED(iocb->ki_pos | iov_iter_alignment(from), blocksize)))
> +		flags |= IOMAP_DIO_FORCE_WAIT;
> +
> +	ret = iomap_dio_rw(iocb, from, &ext2_iomap_ops, &ext2_dio_write_ops,
> +			   flags, NULL, 0);
> +
> +	if (ret == -ENOTBLK)
> +		ret = 0;

So iomap_dio_rw() doesn't have the DIO_SKIP_HOLES behavior of
blockdev_direct_IO(). Thus you have to implement that in your
ext2_iomap_ops, in particular in iomap_begin...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR