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From:   Jeff Moyer <jmoyer@redhat.com>
To:     adam.manzanares@wdc.com
Cc:     viro@zeniv.linux.org.uk, bcrl@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
        linux-api@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] fs: Add aio iopriority support for block_dev
References: <20180503182114.2797-1-adam.manzanares@wdc.com>
Date:   Thu, 03 May 2018 14:36:15 -0400
In-Reply-To: <20180503182114.2797-1-adam.manzanares@wdc.com> (adam
        manzanares's message of "Thu, 3 May 2018 11:21:14 -0700")
Message-ID: <x49po2cr3hc.fsf@segfault.boston.devel.redhat.com>
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Hi, Adam,

adam.manzanares@wdc.com writes:

> From: Adam Manzanares <adam.manzanares@wdc.com>
>
> This is the per-I/O equivalent of the ioprio_set system call.
>
> When IOCB_FLAG_IOPRIO is set on the iocb aio_flags field, then we set the
> newly added kiocb ki_ioprio field to the value in the iocb aio_reqprio field.
>
> When a bio is created for an aio request by the block dev we set the priority
> value of the bio to the user supplied value.
>
> See the following link for performance implications on a SATA HDD:
> https://lkml.org/lkml/2016/12/6/495
>
> Given that WRR support for NVME devices has patches floating around and it was
> discussed at LSFMM, we may soon have a lower latency storage device that can 
> make use of iopriorities. A per command iopriority interface seems timely 
> given these developments. 
>
> If we want to avoid bloating struct kiocb, I suggest we turn the private field 
> into a union of the private and ki_ioprio field. It seems like the users of 
> the private field all use it at a point where we can yank the priority from 
> the kiocb before the private field is used. Comments and suggestions welcome.

The ioprio_set system call requires CAP_SYS_ADMIN for setting
IOPRIO_CLASS_RT.  I think we need similar checks here.

-Jeff

>
> v2: merge patches
>     use IOCB_FLAG_IOPRIO
>     validate intended use with IOCB_IOPRIO
>     add linux-api and linux-block to cc
>
> Signed-off-by: Adam Manzanares <adam.manzanares@wdc.com>
> ---
>  fs/aio.c                     | 10 ++++++++++
>  fs/block_dev.c               |  2 ++
>  include/linux/fs.h           |  2 ++
>  include/uapi/linux/aio_abi.h |  1 +
>  4 files changed, 15 insertions(+)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 88d7927ffbc6..f36636d8ff2c 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1597,6 +1597,16 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
>  		req->common.ki_flags |= IOCB_EVENTFD;
>  	}
>  
> +	if (iocb->aio_flags & IOCB_FLAG_IOPRIO) {
> +		/*
> +		 * If the IOCB_FLAG_IOPRIO flag of aio_flags is set, then
> +		 * aio_reqprio is interpreted as an I/O scheduling
> +		 * class and priority.
> +		 */
> +		req->common.ki_ioprio = iocb->aio_reqprio;
> +		req->common.ki_flags |= IOCB_IOPRIO;
> +	}
> +
>  	ret = kiocb_set_rw_flags(&req->common, iocb->aio_rw_flags);
>  	if (unlikely(ret)) {
>  		pr_debug("EINVAL: aio_rw_flags\n");
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 7ec920e27065..970bef79caa6 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -355,6 +355,8 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>  		bio->bi_write_hint = iocb->ki_hint;
>  		bio->bi_private = dio;
>  		bio->bi_end_io = blkdev_bio_end_io;
> +		if (iocb->ki_flags & IOCB_IOPRIO)
> +			bio->bi_ioprio = iocb->ki_ioprio;
>  
>  		ret = bio_iov_iter_get_pages(bio, iter);
>  		if (unlikely(ret)) {
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 760d8da1b6c7..ab63ce720305 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -292,6 +292,7 @@ enum rw_hint {
>  #define IOCB_SYNC		(1 << 5)
>  #define IOCB_WRITE		(1 << 6)
>  #define IOCB_NOWAIT		(1 << 7)
> +#define IOCB_IOPRIO		(1 << 8)
>  
>  struct kiocb {
>  	struct file		*ki_filp;
> @@ -300,6 +301,7 @@ struct kiocb {
>  	void			*private;
>  	int			ki_flags;
>  	enum rw_hint		ki_hint;
> +	u16			ki_ioprio; /* See linux/ioprio.h */
>  } __randomize_layout;
>  
>  static inline bool is_sync_kiocb(struct kiocb *kiocb)
> diff --git a/include/uapi/linux/aio_abi.h b/include/uapi/linux/aio_abi.h
> index a04adbc70ddf..d4593a6062ef 100644
> --- a/include/uapi/linux/aio_abi.h
> +++ b/include/uapi/linux/aio_abi.h
> @@ -54,6 +54,7 @@ enum {
>   *                   is valid.
>   */
>  #define IOCB_FLAG_RESFD		(1 << 0)
> +#define IOCB_FLAG_IOPRIO	(1 << 1)
>  
>  /* read() from /dev/aio returns these structures. */
>  struct io_event {