[Intel-gfx] [PATCH 2/2] drm/i915: properly prefault for pread/pwrite

[Daniel Vetter]

The helper functions used are designed for pagecache io and splice,
i.e. they prefault at most PAGE_SIZE bytes spanning at most 2 pages.

pread/pwrite want to write/read much more to avoid dropping the
struct_mutex lock in between. So write our helper function to prefault.
We're the only user of these pagemap.h helpers that want this behaviour,
so keep these new helpers private.

Based on a patch by Chris Wilson. In addition to his approach this
alos tries to prefault the last page in case the user address range
crosses a page boundary at the end (and hence might sit on n+1 pages
for at most n*PAGE_SIZE of date).

As a nice side-effect this rather reliably papers over the current
code's inability to handle non-struct page-backed user memory in the
slow paths. Because the real fix is grossly invasive, I think this
patch is the right thing for backporting.

Cc: stable at kernel.org
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38115
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/i915_gem.c |   59 ++++++++++++++++++++++++++++++++++++--
 1 files changed, 55 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 5f0f46e..42dc922 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -503,6 +503,32 @@ out:
 	return ret;
 }
 
+static inline int __prefault_writeable(char __user *uaddr, int size)
+{
+	int ret;
+	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.
+	 */
+	while (uaddr <= end) {
+		ret = __put_user(0, uaddr);
+		if (ret != 0)
+			return ret;
+		uaddr += PAGE_SIZE;
+	}
+	if (ret == 0) {
+		if (((unsigned long)uaddr & PAGE_MASK) !=
+				((unsigned long)end & PAGE_MASK))
+		 	ret = __put_user(0, end);
+	}
+	return ret;
+}
+
 /**
  * Reads data from the object referenced by handle.
  *
@@ -524,8 +550,8 @@ i915_gem_pread_ioctl(struct drm_device *dev, void *data,
 		       args->size))
 		return -EFAULT;
 
-	ret = fault_in_pages_writeable((char __user *)(uintptr_t)args->data_ptr,
-				       args->size);
+	ret = __prefault_writeable((char __user *)(uintptr_t)args->data_ptr,
+				   args->size);
 	if (ret)
 		return -EFAULT;
 
@@ -943,6 +969,31 @@ out:
 	return ret;
 }
 
+static inline int __prefault_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;
+	}
+	if (ret == 0) {
+		if (((unsigned long)uaddr & PAGE_MASK) !=
+				((unsigned long)end & PAGE_MASK)) {
+		 	ret = __get_user(c, end);
+			(void)c;
+		}
+	}
+	return ret;
+}
+
 /**
  * Writes data to the object referenced by handle.
  *
@@ -964,8 +1015,8 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
 		       args->size))
 		return -EFAULT;
 
-	ret = fault_in_pages_readable((char __user *)(uintptr_t)args->data_ptr,
-				      args->size);
+	ret = __prefault_readable((char __user *)(uintptr_t)args->data_ptr,
+				  args->size);
 	if (ret)
 		return -EFAULT;
 
-- 
1.7.6.2