[PATCH RFC 05/10] device-dax: Do not enforce MADV_DONTFORK on mmap()

[Joao Martins]

Currently check_vma() checks for VM_DONTCOPY for a device-dax
when the dax region is not backed by devmap (i.e. PFN_MAP is not set).
VM_DONTCOPY is set through madvise(MADV_DONTFORK) and it only sets it
at an address returned from mmap(). check_vma() is called at devdax
mmap hence checking VM_DONTCOPY prevents a process from mmap-ing the
device.

Let's not enforce MADV_DONTFORK at mmap(), but rather when it actually
gets used (on fault). The assumptions don't change, as it is
expected to still retain/madvise MADV_DONTFORK after mmap.

Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
 drivers/dax/device.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/drivers/dax/device.c b/drivers/dax/device.c
index 1af823b2fe6b..c6a7f5e12c54 100644
--- a/drivers/dax/device.c
+++ b/drivers/dax/device.c
@@ -14,7 +14,7 @@
 #include "dax-private.h"
 #include "bus.h"
 
-static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
+static int check_vma_mmap(struct dev_dax *dev_dax, struct vm_area_struct *vma,
 		const char *func)
 {
 	struct dax_region *dax_region = dev_dax->region;
@@ -41,17 +41,29 @@ static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
 		return -EINVAL;
 	}
 
-	if ((dax_region->pfn_flags & (PFN_DEV|PFN_MAP)) == PFN_DEV
-			&& (vma->vm_flags & VM_DONTCOPY) == 0) {
+	if (!vma_is_dax(vma)) {
 		dev_info_ratelimited(dev,
-				"%s: %s: fail, dax range requires MADV_DONTFORK\n",
+				"%s: %s: fail, vma is not DAX capable\n",
 				current->comm, func);
 		return -EINVAL;
 	}
 
-	if (!vma_is_dax(vma)) {
-		dev_info_ratelimited(dev,
-				"%s: %s: fail, vma is not DAX capable\n",
+	return 0;
+}
+
+static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
+		     const char *func)
+{
+	int rc;
+
+	rc = check_vma_mmap(dev_dax, vma, func);
+	if (rc < 0)
+		return rc;
+
+	if ((dev_dax->region->pfn_flags & (PFN_DEV|PFN_MAP)) == PFN_DEV
+			&& (vma->vm_flags & VM_DONTCOPY) == 0) {
+		dev_info_ratelimited(&dev_dax->dev,
+				"%s: %s: fail, dax range requires MADV_DONTFORK\n",
 				current->comm, func);
 		return -EINVAL;
 	}
@@ -315,7 +327,7 @@ static int dax_mmap(struct file *filp, struct vm_area_struct *vma)
 	 * fault time.
 	 */
 	id = dax_read_lock();
-	rc = check_vma(dev_dax, vma, __func__);
+	rc = check_vma_mmap(dev_dax, vma, __func__);
 	dax_read_unlock(id);
 	if (rc)
 		return rc;
-- 
2.17.1