linux-kernel - Re: [PATCH RESEND] drm/via: use get_user_pages

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Message-ID: <20170228193539.GT29622@ZenIV.linux.org.uk>
Date:   Tue, 28 Feb 2017 19:35:39 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Lorenzo Stoakes <lstoakes@...il.com>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org
Subject: Re: [PATCH RESEND] drm/via: use get_user_pages_unlocked()

On Tue, Feb 28, 2017 at 10:01:10AM +0100, Daniel Vetter wrote:

> > +	ret = get_user_pages_unlocked((unsigned long)xfer->mem_addr,
> > +			vsg->num_pages, vsg->pages,
> > +			(vsg->direction == DMA_FROM_DEVICE) ? FOLL_WRITE : 0);

Umm...  Why not
	ret = get_user_pages_fast((unsigned long)xfer->mem_addr,
			vsg->num_pages,
			vsg->direction == DMA_FROM_DEVICE,
			vsg->pages);

IOW, do you really need a warranty that ->mmap_sem will be grabbed and
released?