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

On Fri, 24 Feb 2012 14:34:31 +0100
Daniel Vetter [off-list ref] wrote:

quoted

quoted

--- 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;
+	}

The callsites in filemap.c are pretty hot paths, which is why this
thing remains explicitly inlined.  I think it would be worth adding a
bit of code here to avoid adding a pointless test-n-branch and larger
cache footprint to read() and write().

A way of doing that is to add another argument to these functions, say
"bool multipage".  Change the code to do

	if (multipage) {
		while (uaddr <= end) {
			...
		}
	}

and change the callsites to pass in constant "true" or "false".  Then
compile it up and manually check that the compiler completely removed
the offending code from the filemap.c callsites.

Wanna have a think about that?  If it all looks OK then please be sure
to add code comments explaining why we did this.

I wasn't really happy with the added branch either, but failed to come up
with a trick to avoid it. Imho adding new _multipage variants of these
functions instead of adding a constant argument is simpler because the
functions don't really share much thanks to the block below. I'll see what
it looks like (and obviously add a comment explaining what's going on).

well... that's just syntactic sugar:

static inline int __fault_in_pages_writeable(char __user *uaddr, int size, bool multipage)
{
	...
}

static inline int fault_in_pages_writeable(char __user *uaddr, int size)
{
	return __fault_in_pages_writeable(uaddr, size, false);
}

static inline int fault_in_multipages_writeable(char __user *uaddr, int size)
{
	return __fault_in_pages_writeable(uaddr, size, true);
}

which I don't think is worth bothering with given the very small number
of callsites.

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