Re: [PATCH 5/6] block: use iomap for writes to block devices

[Xu Yang]

Hi Christoph,

On Tue, Aug 01, 2023 at 07:22:00PM +0200, Christoph Hellwig wrote:
> Use iomap in buffer_head compat mode to write to block devices.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> Reviewed-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> Reviewed-by: Hannes Reinecke <hare@xxxxxxx>
> ---
>  block/Kconfig |  1 +
>  block/fops.c  | 31 +++++++++++++++++++++++++++++--
>  2 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/block/Kconfig b/block/Kconfig
> index 86122e459fe046..1a13ef0b1ca10c 100644
> --- a/block/Kconfig
> +++ b/block/Kconfig
> @@ -5,6 +5,7 @@
>  menuconfig BLOCK
>         bool "Enable the block layer" if EXPERT
>         default y
> +       select FS_IOMAP
>         select SBITMAP
>         help
>  	 Provide block layer support for the kernel.
> diff --git a/block/fops.c b/block/fops.c
> index f0b822c28ddfe2..063ece37d44e44 100644
> --- a/block/fops.c
> +++ b/block/fops.c
> @@ -15,6 +15,7 @@
>  #include <linux/falloc.h>
>  #include <linux/suspend.h>
>  #include <linux/fs.h>
> +#include <linux/iomap.h>
>  #include <linux/module.h>
>  #include "blk.h"
>  
> @@ -386,6 +387,27 @@ static ssize_t blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>  	return __blkdev_direct_IO(iocb, iter, bio_max_segs(nr_pages));
>  }
>  
> +static int blkdev_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
> +		unsigned int flags, struct iomap *iomap, struct iomap *srcmap)
> +{
> +	struct block_device *bdev = I_BDEV(inode);
> +	loff_t isize = i_size_read(inode);
> +
> +	iomap->bdev = bdev;
> +	iomap->offset = ALIGN_DOWN(offset, bdev_logical_block_size(bdev));
> +	if (iomap->offset >= isize)
> +		return -EIO;
> +	iomap->type = IOMAP_MAPPED;
> +	iomap->addr = iomap->offset;
> +	iomap->length = isize - iomap->offset;
> +	iomap->flags |= IOMAP_F_BUFFER_HEAD;
> +	return 0;
> +}
> +
> +static const struct iomap_ops blkdev_iomap_ops = {
> +	.iomap_begin		= blkdev_iomap_begin,
> +};
> +
>  static int blkdev_writepage(struct page *page, struct writeback_control *wbc)
>  {
>  	return block_write_full_page(page, blkdev_get_block, wbc);
> @@ -556,6 +578,11 @@ blkdev_direct_write(struct kiocb *iocb, struct iov_iter *from)
>  	return written;
>  }
>  
> +static ssize_t blkdev_buffered_write(struct kiocb *iocb, struct iov_iter *from)
> +{
> +	return iomap_file_buffered_write(iocb, from, &blkdev_iomap_ops);
> +}
> +
>  /*
>   * Write data to the block device.  Only intended for the block device itself
>   * and the raw driver which basically is a fake block device.
> @@ -605,9 +632,9 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  		ret = blkdev_direct_write(iocb, from);
>  		if (ret >= 0 && iov_iter_count(from))
>  			ret = direct_write_fallback(iocb, from, ret,
> -					generic_perform_write(iocb, from));
> +					blkdev_buffered_write(iocb, from));
>  	} else {
> -		ret = generic_perform_write(iocb, from);
> +		ret = blkdev_buffered_write(iocb, from);
>  	}
>  
>  	if (ret > 0)

I'm testing SSD block device write performance recently. I found the write
speed descrased greatly on my board (330MB/s -> 130MB/s). Then I spent some
time to find cause, finally find that it's caused by this patch and if I
revert this patch, write speed can recover to 330MB/s.

I'm using below command to test write performance:
dd if=/dev/zero of=/dev/sda bs=4M count=1024

And I also do more tests to get more findings. In short, I found write
speed changes with the "bs=" parameter.

I totally write 4GB data to sda for each test, the results as below:

 - dd if=/dev/zero of=/dev/sda bs=400K  count=10485  (334 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=800K  count=5242   (278 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=1600K count=2621   (204 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=2200K count=1906   (170 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=3000K count=1398   (150 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=4500K count=932    (139 MB/s)

When this patch reverted, I got below results:

 - dd if=/dev/zero of=/dev/sda bs=400K  count=10485  (339 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=800K  count=5242   (330 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=1600K count=2621   (332 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=2200K count=1906   (333 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=3000K count=1398   (333 MB/s)
 - dd if=/dev/zero of=/dev/sda bs=4500K count=932    (333 MB/s)

I just want to know if this results is expected when uses iomap, or it's
a real issue?

Many thanks in advance!

Best Regards,
Xu Yang

> -- 
> 2.39.2
>