linux-kernel - Re: [PATCH] ata-sff: always map page before data transfer

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Date:   Thu, 4 May 2017 02:51:34 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Tycho Andersen <tycho@...ker.com>
Cc:     Tejun Heo <tj@...nel.org>,
        Juerg Haefliger <juerg.haefliger@....com>,
        linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] ata-sff: always map page before data transfer

> I don't understand all the factors at play here, so thoughts are definitely
> welcome.

I don't fully understand the old code either.  One thing that is weird
is the "use a bounce buffer comment" which doesn't make any sense.

The other is the local_irq_save, which isn't really needed for
kmap_atomic to start with, but maybe that's the reason why the original
author didn't want to do it unconditionally?

So based on that:

> +	/* FIXME: use a bounce buffer */

drop this comment..

> +	local_irq_save(flags);

.. remove the local_irq_save/local_irq_restore ..

> +	/* do the actual data transfer */
> +	ap->ops->sff_data_xfer(qc, buf + offset, qc->sect_size,
> +			       do_write);

.. and a nice alittle cleanup move the do_write onto the previous line.

> +	/* FIXME: use bounce buffer */
> +	local_irq_save(flags);
> +	buf = kmap_atomic(page);
>  
> +	/* do the actual data transfer */
> +	consumed = ap->ops->sff_data_xfer(qc, buf + offset,
> +							count, rw);

And same here.

And we should be fine.