linux-kernel - Re: [PATCH v5 3/9] iov_iter: Use IOCB/IOMAP_WRITE if available rather than iterator direction

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Date:   Thu, 12 Jan 2023 21:32:36 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Bart Van Assche <bart.vanassche@...il.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Douglas Gilbert <dgilbert@...erlog.com>
Subject: Re: [PATCH v5 3/9] iov_iter: Use IOCB/IOMAP_WRITE if available
 rather than iterator direction

On Thu, Jan 12, 2023 at 01:49:14PM -0800, Bart Van Assche wrote:
> I'm not sure that we still need the double copy in the sg driver. It seems
> obscure to me that there is user space software that relies on finding
> "0xec" in bytes not originating from a SCSI device. Additionally, SCSI
> drivers that do not support residuals should be something from the past.
> 
> Others may be better qualified to comment on this topic.

Yeah.  And that weird (ab)use of blk_rq_map_user_iov into preallocated
pages in sg has been a constant source of pain.  I'd be very happy to
make it match the generic SG_IO implementation and either do
get_user_pages or the normal bounce buffering implemented in the
common code.