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Date: Tue, 28 Feb 2017 19:35:39 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Lorenzo Stoakes <lstoakes@gmail.com>, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-mm@kvack.org
Subject: Re: [PATCH RESEND] drm/via: use get_user_pages_unlocked()
Message-ID: <20170228193539.GT29622@ZenIV.linux.org.uk>
References: <20170227215008.21457-1-lstoakes@gmail.com>
 <20170228090110.m4pxtjlbgaft7oet@phenom.ffwll.local>
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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?