V4L2 memory registrations are incompatible with filesystem-dax that
needs the ability to revoke dma access to a mapping at will, or
otherwise allow the kernel to wait for completion of DMA. The
filesystem-dax implementation breaks the traditional solution of
truncate of active file backed mappings since there is no page-cache
page we can orphan to sustain ongoing DMA.
If v4l2 wants to support long lived DMA mappings it needs to arrange to
hold a file lease or use some other mechanism so that the kernel can
coordinate revoking DMA access when the filesystem needs to truncate
mappings.
Reported-by: Jan Kara <jack@xxxxxxx>
Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
Cc: linux-media@xxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
drivers/media/v4l2-core/videobuf-dma-sg.c | 39 ++++++++++++++++++++++++++++-
1 file changed, 37 insertions(+), 2 deletions(-)
diff --git a/drivers/media/v4l2-core/videobuf-dma-sg.c b/drivers/media/v4l2-core/videobuf-dma-sg.c
index 0b5c43f7e020..37a4ae61b2c0 100644
--- a/drivers/media/v4l2-core/videobuf-dma-sg.c
+++ b/drivers/media/v4l2-core/videobuf-dma-sg.c
@@ -155,8 +155,9 @@ static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
int direction, unsigned long data, unsigned long size)
{
unsigned long first, last;
- int err, rw = 0;
+ int err, rw = 0, i, nr_pages;
unsigned int flags = FOLL_FORCE;
+ struct vm_area_struct **vmas = NULL;
dma->direction = direction;
switch (dma->direction) {
@@ -179,6 +180,16 @@ static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
if (NULL == dma->pages)
return -ENOMEM;
+ if (IS_ENABLED(CONFIG_FS_DAX)) {
+ vmas = kmalloc(dma->nr_pages * sizeof(struct vm_area_struct *),
+ GFP_KERNEL);
+ if (NULL == vmas) {
+ kfree(dma->pages);
+ dma->pages = NULL;
+ return -ENOMEM;
+ }
+ }
+
if (rw == READ)
flags |= FOLL_WRITE;
@@ -186,7 +197,31 @@ static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
data, size, dma->nr_pages);
err = get_user_pages(data & PAGE_MASK, dma->nr_pages,
- flags, dma->pages, NULL);
+ flags, dma->pages, vmas);
+ nr_pages = err;
+
+ for (i = 0; vmas && i < nr_pages; i++) {
+ struct vm_area_struct *vma = vmas[i];
+ struct inode *inode;
+
+ if (!vma_is_dax(vma))
+ continue;
+
+ /* device-dax is safe for long-lived v4l2 mappings... */
+ inode = file_inode(vma->vm_file);
+ if (inode->i_mode == S_IFCHR)
+ continue;
+
+ /* ...filesystem-dax is not. */
+ err = -EOPNOTSUPP;
+ break;
+
+ /*
+ * FIXME: add a 'with lease' mechanism for v4l2 to
+ * obtain time bounded access to filesytem-dax mappings
+ */
+ }
+ kfree(vmas);
if (err != dma->nr_pages) {
dma->nr_pages = (err >= 0) ? err : 0;