[PATCH 11/13] mm: extend prefault helpers to fault in more than PAGE_SIZE

[PATCH 11/13] mm: extend prefault helpers to fault in more than PAGE_SIZE

[Daniel Vetter]

drm/i915 wants to read/write more than one page in its fastpath
and hence needs to prefault more than PAGE_SIZE bytes.

I've checked the callsites and they all already clamp size when
calling fault_in_pages_* to the same as for the subsequent
__copy_to|from_user and hence don't rely on the implicit clamping
to PAGE_SIZE.

Also kill a copy&pasted spurious space in both functions while at it.

Cc: linux-mm@kvack.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 include/linux/pagemap.h |   28 ++++++++++++++++++----------
 1 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index cfaaa69..689527d 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -408,6 +408,7 @@ extern void add_page_wait_queue(struct page *page, wait_queue_t *waiter);
 static inline int fault_in_pages_writeable(char __user *uaddr, int size)
 {
 	int ret;
+	char __user *end = uaddr + size - 1;
 
 	if (unlikely(size == 0))
 		return 0;
@@ -416,17 +417,20 @@ static inline int fault_in_pages_writeable(char __user *uaddr, int size)
 	 * Writing zeroes into userspace here is OK, because we know that if
 	 * the zero gets there, we'll be overwriting it.
 	 */
-	ret = __put_user(0, uaddr);
+	while (uaddr <= end) {
+		ret = __put_user(0, uaddr);
+		if (ret != 0)
+			return ret;
+		uaddr += PAGE_SIZE;
+	}
 	if (ret == 0) {
-		char __user *end = uaddr + size - 1;
-
 		/*
 		 * If the page was already mapped, this will get a cache miss
 		 * for sure, so try to avoid doing it.
 		 */
-		if (((unsigned long)uaddr & PAGE_MASK) !=
+		if (((unsigned long)uaddr & PAGE_MASK) ==
 				((unsigned long)end & PAGE_MASK))
-		 	ret = __put_user(0, end);
+			ret = __put_user(0, end);
 	}
 	return ret;
 }
@@ -435,17 +439,21 @@ static inline int fault_in_pages_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;
 
-	ret = __get_user(c, uaddr);
+	while (uaddr <= end) {
+		ret = __get_user(c, uaddr);
+		if (ret != 0)
+			return ret;
+		uaddr += PAGE_SIZE;
+	}
 	if (ret == 0) {
-		const char __user *end = uaddr + size - 1;
-
-		if (((unsigned long)uaddr & PAGE_MASK) !=
+		if (((unsigned long)uaddr & PAGE_MASK) ==
 				((unsigned long)end & PAGE_MASK)) {
-		 	ret = __get_user(c, end);
+			ret = __get_user(c, end);
 			(void)c;
 		}
 	}
-- 
1.7.6.4

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Re: [PATCH 11/13] mm: extend prefault helpers to fault in more than PAGE_SIZE

[Chris Wilson]

On Sun,  6 Nov 2011 20:13:58 +0100, Daniel Vetter <daniel.vetter@ffwll.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.
> 
> I've checked the callsites and they all already clamp size when
> calling fault_in_pages_* to the same as for the subsequent
> __copy_to|from_user and hence don't rely on the implicit clamping
> to PAGE_SIZE.
> 
> Also kill a copy&pasted spurious space in both functions while at it.
> 
> Cc: linux-mm@kvack.org
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  include/linux/pagemap.h |   28 ++++++++++++++++++----------
>  1 files changed, 18 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index cfaaa69..689527d 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -408,6 +408,7 @@ extern void add_page_wait_queue(struct page *page, wait_queue_t *waiter);
>  static inline int fault_in_pages_writeable(char __user *uaddr, int size)
>  {
>  	int ret;
> +	char __user *end = uaddr + size - 1;
>  
>  	if (unlikely(size == 0))
>  		return 0;
> @@ -416,17 +417,20 @@ static inline int fault_in_pages_writeable(char __user *uaddr, int size)
>  	 * Writing zeroes into userspace here is OK, because we know that if
>  	 * the zero gets there, we'll be overwriting it.
>  	 */
> -	ret = __put_user(0, uaddr);
> +	while (uaddr <= end) {
> +		ret = __put_user(0, uaddr);
> +		if (ret != 0)
> +			return ret;
> +		uaddr += PAGE_SIZE;
> +	}
>  	if (ret == 0) {
> -		char __user *end = uaddr + size - 1;
> -
>  		/*
>  		 * If the page was already mapped, this will get a cache miss
>  		 * for sure, so try to avoid doing it.
>  		 */
> -		if (((unsigned long)uaddr & PAGE_MASK) !=
> +		if (((unsigned long)uaddr & PAGE_MASK) ==
>  				((unsigned long)end & PAGE_MASK))
> -		 	ret = __put_user(0, end);
> +			ret = __put_user(0, end);
>  	}
>  	return ret;

You leave these functions in a worse mess by introducing a false
compiler warning about an uninitialized ret by the now redundant test
against zero, a do{}while loop would be clearer that the original
behaviour is merely extended upon. And please replace the open-coded
offset_in_page().
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

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