linux-kernel - Re: [PATCH] mm: extend prefault helpers to fault in more than PAGE

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Date:	Thu, 1 Mar 2012 12:15:57 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Daniel Vetter <daniel.vetter@...ll.ch>
Cc:	Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: extend prefault helpers to fault in more than
 PAGE_SIZE

On Thu,  1 Mar 2012 20:22:59 +0100
Daniel Vetter <daniel.vetter@...ll.ch> wrote:

> drm/i915 wants to read/write more than one page in its fastpath
> and hence needs to prefault more than PAGE_SIZE bytes.
> 
> Add new functions in filemap.h to make that possible.
> 
> Also kill a copy&pasted spurious space in both functions while at it.
> 
>
> ...
>
> +/* Multipage variants of the above prefault helpers, useful if more than
> + * PAGE_SIZE of date needs to be prefaulted. These are separate from the above
> + * functions (which only handle up to PAGE_SIZE) to avoid clobbering the
> + * filemap.c hotpaths. */

Like this please:

/*
 * Multipage variants of the above prefault helpers, useful if more than
 * PAGE_SIZE of date needs to be prefaulted. These are separate from the above
 * functions (which only handle up to PAGE_SIZE) to avoid clobbering the
 * filemap.c hotpaths.
 */

and s/date/data/

> +static inline int fault_in_multipages_writeable(char __user *uaddr, int size)
> +{
> +	int ret;
> +	const char __user *end = uaddr + size - 1;
> +
> +	if (unlikely(size == 0))
> +		return 0;
> +
> +	/*
> +	 * Writing zeroes into userspace here is OK, because we know that if
> +	 * the zero gets there, we'll be overwriting it.
> +	 */

Yeah, like that.

> +	while (uaddr <= end) {
> +		ret = __put_user(0, uaddr);
> +		if (ret != 0)
> +			return ret;
> +		uaddr += PAGE_SIZE;
> +	}
> +
> +	/* Check whether the range spilled into the next page. */
> +	if (((unsigned long)uaddr & PAGE_MASK) ==
> +			((unsigned long)end & PAGE_MASK))
> +		ret = __put_user(0, end);
> +
> +	return ret;
> +}
> +
> +static inline int fault_in_multipages_readable(const char __user *uaddr,
> +					       int size)
> +{
> +	volatile char c;
> +	int ret;
> +	const char __user *end = uaddr + size - 1;
> +
> +	if (unlikely(size == 0))
> +		return 0;
> +
> +	while (uaddr <= end) {
> +		ret = __get_user(c, uaddr);
> +		if (ret != 0)
> +			return ret;
> +		uaddr += PAGE_SIZE;
> +	}
> +
> +	/* Check whether the range spilled into the next page. */
> +	if (((unsigned long)uaddr & PAGE_MASK) ==
> +			((unsigned long)end & PAGE_MASK)) {
> +		ret = __get_user(c, end);
> +		(void)c;
> +	}
> +
> +	return ret;
> +}

Please merge it via the DRI tree.
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