Re: [PATCH v2 2/2] [SCSI] sg: fix EWOULDBLOCK errors with scsi-mq

[Douglas Gilbert <dgilbert@interlog.com>]

On 15-02-13 12:10 PM, Tony Battersby wrote:
> With scsi-mq enabled, userspace programs can get unexpected EWOULDBLOCK
> (a.k.a. EAGAIN) errors when submitting commands to the SCSI generic
> driver.  Fix by calling blk_get_request() with GFP_KERNEL instead of
> GFP_ATOMIC.
>
> Note: to avoid introducing a potential deadlock, this patch should be
> applied after the patch titled "sg: fix unkillable I/O wait deadlock
> with scsi-mq".
>
> Cc: Douglas Gilbert <dgilbert@interlog.com>
> Cc: <stable@vger.kernel.org> # 3.17+
> Signed-off-by: Tony Battersby <tonyb@cybernetics.com>

Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Tested-by: Douglas Gilbert <dgilbert@interlog.com>

> For inclusion in kernel 3.20.
>
> The difference in behavior is due to bt_get() in block/blk-mq-tag.c
> checking for __GFP_WAIT.
>
> The bsg driver already calls blk_get_request() with GFP_KERNEL, so there
> is no need for a change there.
>
> --- linux-3.19.0/drivers/scsi/sg.c.orig	2015-02-13 11:04:40.000000000 -0500
> +++ linux-3.19.0/drivers/scsi/sg.c	2015-02-13 11:05:14.000000000 -0500
> @@ -1695,7 +1695,22 @@ sg_start_req(Sg_request *srp, unsigned c
>   			return -ENOMEM;
>   	}
>
> +	/*
> +	 * NOTE
> +	 *
> +	 * With scsi-mq enabled, there are a fixed number of preallocated
> +	 * requests equal in number to shost->can_queue.  If all of the
> +	 * preallocated requests are already in use, then using GFP_ATOMIC with
> +	 * blk_get_request() will return -EWOULDBLOCK, whereas using GFP_KERNEL
> +	 * will cause blk_get_request() to sleep until an active command
> +	 * completes, freeing up a request.  Neither option is ideal, but
> +	 * GFP_KERNEL is the better choice to prevent userspace from getting an
> +	 * unexpected EWOULDBLOCK.
> +	 *
> +	 * With scsi-mq disabled, blk_get_request() with GFP_KERNEL usually
> +	 * does not sleep except under memory pressure.
> +	 */
> +	rq = blk_get_request(q, rw, GFP_KERNEL);
> -	rq = blk_get_request(q, rw, GFP_ATOMIC);
>   	if (IS_ERR(rq)) {
>   		kfree(long_cmdp);
>   		return PTR_ERR(rq);
>
> --