linux-kernel - Re: [RFC] weird semantics of SG_DXFER_TO_FROM_DEV in BLK_DEV

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Date:	Thu, 7 Apr 2016 08:53:29 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Christoph Hellwig <hch@...radead.org>, Jens Axboe <axboe@...com>,
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [RFC] weird semantics of SG_DXFER_TO_FROM_DEV in BLK_DEV_SKD
 (drivers/block/skd*)

On Mon, Apr 04, 2016 at 06:16:12PM +0100, Al Viro wrote:
> Another fun question: should the normal sg_io() copy the buffer in on
> SG_DXFER_TO_FROM_DEV?  Right now it doesn't; in !copy case (when it goes
> through bio_map_user_iov()) the effect is achieved simply by doing the
> read into the pages user has mapped in that area, but bio_copy_user_iov()
> doesn't do it:
>         /*
>          * success
>          */
>         if (((iter->type & WRITE) && (!map_data || !map_data->null_mapped)) ||
>             (map_data && map_data->from_user)) {
>                 ret = bio_copy_from_iter(bio, *iter);
>                 if (ret)
>                         goto cleanup;
>         }
> will see NULL map_data; the ->from_user case is sg_start_req() stuff.  IOW,
> SG_IO behaviour for /dev/sg* is different from the generic one...

meh.  I really wish /dev/sg was just using the generic page pool :(