On Tue, Jun 3, 2025 at 11:53 AM wangtao <tao.wang...@honor.com> wrote:
>
> Memory files can optimize copy performance via copy_file_range callbacks:
> -Compared to mmap&read: reduces GUP (get_user_pages) overhead
> -Compared to sendfile/splice: eliminates one memory copy
> -Supports dma-buf direct I/O zero-copy implementation
>
> Suggested by: Christian König <christian.koe...@amd.com>
> Suggested by: Amir Goldstein <amir7...@gmail.com>
> Signed-off-by: wangtao <tao.wang...@honor.com>
> ---
> fs/read_write.c | 64 +++++++++++++++++++++++++++++++++++++---------
> include/linux/fs.h | 2 ++
> 2 files changed, 54 insertions(+), 12 deletions(-)
>
> diff --git a/fs/read_write.c b/fs/read_write.c
> index bb0ed26a0b3a..ecb4f753c632 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1469,6 +1469,31 @@ COMPAT_SYSCALL_DEFINE4(sendfile64, int, out_fd, int,
> in_fd,
> }
> #endif
>
> +static const struct file_operations *memory_copy_file_ops(
> + struct file *file_in, struct file *file_out)
> +{
> + if ((file_in->f_op->fop_flags & FOP_MEMORY_FILE) &&
> + (file_in->f_mode & FMODE_CAN_ODIRECT) &&
> + file_in->f_op->copy_file_range && file_out->f_op->write_iter)
> + return file_in->f_op;
> + else if ((file_out->f_op->fop_flags & FOP_MEMORY_FILE) &&
> + (file_out->f_mode & FMODE_CAN_ODIRECT) &&
> + file_in->f_op->read_iter && file_out->f_op->copy_file_range)
> + return file_out->f_op;
> + else
> + return NULL;
> +}
> +
> +static int essential_file_rw_checks(struct file *file_in, struct file
> *file_out)
> +{
> + if (!(file_in->f_mode & FMODE_READ) ||
> + !(file_out->f_mode & FMODE_WRITE) ||
> + (file_out->f_flags & O_APPEND))
> + return -EBADF;
> +
> + return 0;
> +}
> +
> /*
> * Performs necessary checks before doing a file copy
> *
> @@ -1484,9 +1509,16 @@ static int generic_copy_file_checks(struct file
> *file_in, loff_t pos_in,
> struct inode *inode_out = file_inode(file_out);
> uint64_t count = *req_count;
> loff_t size_in;
> + bool splice = flags & COPY_FILE_SPLICE;
> + const struct file_operations *mem_fops;
> int ret;
>
> - ret = generic_file_rw_checks(file_in, file_out);
> + /* The dma-buf file is not a regular file. */
> + mem_fops = memory_copy_file_ops(file_in, file_out);
> + if (splice || mem_fops == NULL)
nit: use !mem_fops please
Considering that the flag COPY_FILE_SPLICE is not allowed
from userspace and is only called by nfsd and ksmbd
I think we should assert and deny the combination of
mem_fops && splice because it is very much unexpected.
After asserting this, it would be nicer to write as:
if (mem_fops)
ret = essential_file_rw_checks(file_in, file_out);
else
ret = generic_file_rw_checks(file_in, file_out);
> + else
> + ret = essential_file_rw_checks(file_in, file_out);
> if (ret)
> return ret;
>
> @@ -1500,8 +1532,10 @@ static int generic_copy_file_checks(struct file
> *file_in, loff_t pos_in,
> * and several different sets of file_operations, but they all end up
> * using the same ->copy_file_range() function pointer.
> */
> - if (flags & COPY_FILE_SPLICE) {
> + if (splice) {
> /* cross sb splice is allowed */
> + } else if (mem_fops != NULL) {
With the assertion that splice && mem_fops is not allowed
if (splice || mem_fops) {
would go well together because they both allow cross-fs
copy not only cross sb.
> + /* cross-fs copy is allowed for memory file. */
> } else if (file_out->f_op->copy_file_range) {
> if (file_in->f_op->copy_file_range !=
> file_out->f_op->copy_file_range)
> @@ -1554,6 +1588,7 @@ ssize_t vfs_copy_file_range(struct file *file_in,
> loff_t pos_in,
> ssize_t ret;
> bool splice = flags & COPY_FILE_SPLICE;
> bool samesb = file_inode(file_in)->i_sb == file_inode(file_out)->i_sb;
> + const struct file_operations *mem_fops;
>
> if (flags & ~COPY_FILE_SPLICE)
> return -EINVAL;
> @@ -1574,18 +1609,27 @@ ssize_t vfs_copy_file_range(struct file *file_in,
> loff_t pos_in,
> if (len == 0)
> return 0;
>
> + if (splice)
> + goto do_splice;
> +
> file_start_write(file_out);
>
goto do_splice needs to be after file_start_write
Please wait for feedback from vfs maintainers before posting another
version addressing my review comments.
Thanks,
Amir.
