Re: [RFC PATCH v4 24/34] ext4: implement buffered write iomap path

[Dave Chinner <david@fromorbit.com>]

On Wed, May 01, 2024 at 06:11:13PM +1000, Dave Chinner wrote:
> On Wed, Apr 10, 2024 at 10:29:38PM +0800, Zhang Yi wrote:
> > From: Zhang Yi <yi.zhang@huawei.com>
> > 
> > Implement buffered write iomap path, use ext4_da_map_blocks() to map
> > delalloc extents and add ext4_iomap_get_blocks() to allocate blocks if
> > delalloc is disabled or free space is about to run out.
> > 
> > Note that we always allocate unwritten extents for new blocks in the
> > iomap write path, this means that the allocation type is no longer
> > controlled by the dioread_nolock mount option. After that, we could
> > postpone the i_disksize updating to the writeback path, and drop journal
> > handle in the buffered dealloc write path completely.
.....
> > +/*
> > + * Drop the staled delayed allocation range from the write failure,
> > + * including both start and end blocks. If not, we could leave a range
> > + * of delayed extents covered by a clean folio, it could lead to
> > + * inaccurate space reservation.
> > + */
> > +static int ext4_iomap_punch_delalloc(struct inode *inode, loff_t offset,
> > +				     loff_t length)
> > +{
> > +	ext4_es_remove_extent(inode, offset >> inode->i_blkbits,
> > +			DIV_ROUND_UP_ULL(length, EXT4_BLOCK_SIZE(inode->i_sb)));
> >  	return 0;
> >  }
> >  
> > +static int ext4_iomap_buffered_write_end(struct inode *inode, loff_t offset,
> > +					 loff_t length, ssize_t written,
> > +					 unsigned int flags,
> > +					 struct iomap *iomap)
> > +{
> > +	handle_t *handle;
> > +	loff_t end;
> > +	int ret = 0, ret2;
> > +
> > +	/* delalloc */
> > +	if (iomap->flags & IOMAP_F_EXT4_DELALLOC) {
> > +		ret = iomap_file_buffered_write_punch_delalloc(inode, iomap,
> > +			offset, length, written, ext4_iomap_punch_delalloc);
> > +		if (ret)
> > +			ext4_warning(inode->i_sb,
> > +			     "Failed to clean up delalloc for inode %lu, %d",
> > +			     inode->i_ino, ret);
> > +		return ret;
> > +	}
> 
> Why are you creating a delalloc extent for the write operation and
> then immediately deleting it from the extent tree once the write
> operation is done?

Ignore this, I mixed up the ext4_iomap_punch_delalloc() code
directly above with iomap_file_buffered_write_punch_delalloc().

In hindsight, iomap_file_buffered_write_punch_delalloc() is poorly
named, as it is handling a short write situation which requires
newly allocated delalloc blocks to be punched.
iomap_file_buffered_write_finish() would probably be a better name
for it....

> Also, why do you need IOMAP_F_EXT4_DELALLOC? Isn't a delalloc iomap
> set up with iomap->type = IOMAP_DELALLOC? Why can't that be used?

But this still stands - the first thing
iomap_file_buffered_write_punch_delalloc() is:

	if (iomap->type != IOMAP_DELALLOC)
                return 0;

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com