[RFC PATCH v2 6/8] fs: add FMODE_CAN_DMABUF_IO and require it for dma-buf I/O contexts

[Mengmeng Zhao <mengmeng.zhao@linux.dev>]

From: Mengmeng Zhao <zhaomengmeng@kylinos.cn>

Add a file mode bit for files that can be the target of dma-buf I/O and
require it when a buffer is registered.  Block device files set it when
the backing disk supports dma-buf I/O, and filesystems can set it for
inodes whose data can be reached without the page cache.  The bit is
coarse: a driver that installs ->init_dma_buf_io_ctx for every
namespace, like nvme, still decides per device at registration.

dma_buf_io_ctx_create() is the only caller of ->init_dma_buf_io_ctx, so
the capability and the O_DIRECT requirement are checked there for every
file type rather than in each helper; O_DIRECT belongs at registration
because F_SETFL can change it afterwards.  Add bdev_can_dmabuf_io() so
the open-time and registration checks share one predicate.

Signed-off-by: Mengmeng Zhao <zhaomengmeng@kylinos.cn>
Signed-off-by: Li Wang <liwang@kylinos.cn>
---
 block/bdev.c                 |  8 ++++----
 block/fops.c                 |  2 +-
 drivers/dma-buf/dma-buf-io.c | 13 +++++++++----
 include/linux/blkdev.h       |  7 ++++++-
 include/linux/fs.h           |  8 +++++++-
 5 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/block/bdev.c b/block/bdev.c
index 88e3f552f5d4..4e7a7f7e1e8b 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -61,14 +61,12 @@ struct block_device *file_bdev(struct file *bdev_file)
 }
 EXPORT_SYMBOL(file_bdev);
 
-int bdev_init_dma_buf_io_ctx(struct file *file, struct block_device *bdev,
+int bdev_init_dma_buf_io_ctx(struct block_device *bdev,
 			     struct dma_buf_io_ctx *ctx)
 {
-	if (!(file->f_flags & O_DIRECT))
-		return -EINVAL;
 	if (ctx->dmabuf->size > UINT_MAX)
 		return -EINVAL;
-	if (!bdev->bd_disk->fops->init_dma_buf_io_ctx)
+	if (!bdev_can_dmabuf_io(bdev))
 		return -EOPNOTSUPP;
 	return bdev->bd_disk->fops->init_dma_buf_io_ctx(bdev, ctx);
 }
@@ -1041,6 +1039,8 @@ int bdev_open(struct block_device *bdev, blk_mode_t mode, void *holder,
 
 	bdev_file->f_flags |= O_LARGEFILE;
 	bdev_file->f_mode |= FMODE_CAN_ODIRECT;
+	if (bdev_can_dmabuf_io(bdev))
+		bdev_file->f_mode |= FMODE_CAN_DMABUF_IO;
 	if (bdev_nowait(bdev))
 		bdev_file->f_mode |= FMODE_NOWAIT;
 	if (mode & BLK_OPEN_RESTRICT_WRITES)
diff --git a/block/fops.c b/block/fops.c
index 86bf1f298b79..7e5702e047a2 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -1008,7 +1008,7 @@ static int blkdev_mmap_prepare(struct vm_area_desc *desc)
 static int blkdev_init_dma_buf_io_ctx(struct file *file,
 				      struct dma_buf_io_ctx *ctx)
 {
-	return bdev_init_dma_buf_io_ctx(file, file_bdev(file), ctx);
+	return bdev_init_dma_buf_io_ctx(file_bdev(file), ctx);
 }
 
 const struct file_operations def_blk_fops = {
diff --git a/drivers/dma-buf/dma-buf-io.c b/drivers/dma-buf/dma-buf-io.c
index 8440151de860..9575b7d65ec5 100644
--- a/drivers/dma-buf/dma-buf-io.c
+++ b/drivers/dma-buf/dma-buf-io.c
@@ -152,9 +152,11 @@ static void dma_buf_io_ctx_shutdown(struct dma_buf_io_ctx *ctx)
  * @out_ctx:	[out]	resulting ctx
  *
  * Create a new I/O context from the given DMA buffer for the given file.
- * It only works with files implementing ->init_dma_buf_io_ctx and fails
- * with -EOPNOTSUPP otherwise. Once created, the user can create a map
- * and use it to issue I/O, such as read/write requests, to @file.
+ * It only works with files implementing ->init_dma_buf_io_ctx,
+ * advertising FMODE_CAN_DMABUF_IO and currently opened for direct I/O,
+ * and fails with -EOPNOTSUPP or -EINVAL otherwise. Once created, the
+ * user can create a map and use it to issue I/O, such as read/write
+ * requests, to @file.
  */
 int dma_buf_io_ctx_create(struct file *file,
 			   struct dma_buf *dmabuf,
@@ -164,8 +166,11 @@ int dma_buf_io_ctx_create(struct file *file,
 	struct dma_buf_io_ctx *ctx;
 	int ret;
 
-	if (!file->f_op->init_dma_buf_io_ctx)
+	if (!file->f_op->init_dma_buf_io_ctx ||
+	    !(file->f_mode & FMODE_CAN_DMABUF_IO))
 		return -EOPNOTSUPP;
+	if (!(file->f_flags & O_DIRECT))
+		return -EINVAL;
 
 	ctx = kzalloc_obj(*ctx);
 	if (!ctx)
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index b98f43478cc0..0cfb6fb6b4e2 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1610,9 +1610,14 @@ struct block_device_operations {
 	int (*alternative_gpt_sector)(struct gendisk *disk, sector_t *sector);
 };
 
-int bdev_init_dma_buf_io_ctx(struct file *file, struct block_device *bdev,
+int bdev_init_dma_buf_io_ctx(struct block_device *bdev,
 			     struct dma_buf_io_ctx *ctx);
 
+static inline bool bdev_can_dmabuf_io(struct block_device *bdev)
+{
+	return !!bdev->bd_disk->fops->init_dma_buf_io_ctx;
+}
+
 #ifdef CONFIG_COMPAT
 extern int blkdev_compat_ptr_ioctl(struct block_device *, blk_mode_t,
 				      unsigned int, unsigned long);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 99caef43ff3c..f5524fc770d4 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -121,7 +121,13 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 /* File supports atomic writes */
 #define FMODE_CAN_ATOMIC_WRITE	((__force fmode_t)(1 << 7))
 
-/* FMODE_* bit 8 */
+/*
+ * File can be the target of dma-buf I/O.  Set it for files whose data can be
+ * reached with direct I/O without the page cache: a dma-buf iterator has no
+ * pages to copy, so a buffered path cannot serve it.  The bit is coarse; both
+ * the device behind it and O_DIRECT are checked when a buffer is registered.
+ */
+#define FMODE_CAN_DMABUF_IO	((__force fmode_t)(1 << 8))
 
 /* 32bit hashes as llseek() offset (for directories) */
 #define FMODE_32BITHASH         ((__force fmode_t)(1 << 9))
-- 
2.43.0