Re: [PATCH 1/2] fs/block-dev.c:fix performance regression in O_DIRECT writes to md block devices.

[NeilBrown <neilb@suse.de>]

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On Mon, 16 Jul 2012 09:30:50 +0800 majianpeng <majianpeng@gmail.com> wrote:

> For regular file, write operaion used blk_plug function.But for block
> file,write operation did not use blk_plug.
> 
> Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
> ---
>  fs/block_dev.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index c2bbe1f..22cd436 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -215,9 +215,14 @@ blkdev_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
>  {
>  	struct file *file = iocb->ki_filp;
>  	struct inode *inode = file->f_mapping->host;
> +	struct blk_plug plug;
> +	ssize_t ret;
>  
> -	return __blockdev_direct_IO(rw, iocb, inode, I_BDEV(inode), iov, offset,
> +	blk_start_plug(&plug);
> +	ret =  __blockdev_direct_IO(rw, iocb, inode, I_BDEV(inode), iov, offset,
>  				    nr_segs, blkdev_get_blocks, NULL, NULL, 0);
> +	blk_finish_plug(&plug);
> +	return ret;
>  }
>  
>  int __sync_blockdev(struct block_device *bdev, int wait)

[cc:ing Jens Axboe]

I think we do need something like this, but I don't think this is the right
place for it.

For normal filesystem writes, the blk_{start,finish}_plug calls are in
generic_file_aio_write which is the "aio_write" function, or is called by it.
aio_write calls generic_file_aio_write, calls blk_start_plug, call
__generic_file_aio_write.

For block devices we bypass the generic_file_aio_write:

aio_write calls blkdev_aio_write calls __generic_file_aio_write - without
calling blk_start_plug.
So I think the calls to blk_start_plug and blk_finish_plug should go in
blkdev_aio_write.  i.e. blkdev_aio_write should be made to look more like
generic_file_aio_write (just without the mutex_lock/unlock).

If you redo the patch like that and test it I'll happily add my Reviewed-by.

I suspect it should be merged through Jens' tree.

Thanks,
NeilBrown


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