Re: [PATCH V8 2/3] fuse: Introduce synchronous read and write for passthrough

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Sep 11, 2020 at 7:34 PM Alessio Balsini <balsini@android.com> wrote:
>
> All the read and write operations performed on fuse_files which have the
> passthrough feature enabled are forwarded to the associated lower file
> system file.
>
> Sending the request directly to the lower file system avoids the userspace
> round-trip that, because of possible context switches and additional
> operations might reduce the overall performance, especially in those cases
> where caching doesn't help, for example in reads at random offsets.
>
> If a fuse_file has a lower file system file associated for passthrough can
> be verified by checking the validity of its passthrough_filp pointer, which
> is not null only passthrough has been successfully enabled via the
> appropriate ioctl(). When a read/write operation is requested for a FUSE
> file with passthrough enabled, the request is directly forwarded to the
> corresponding file_operations of the lower file system file. After the
> read/write operation is completed, the file stats change is notified (and
> propagated) to the lower file system.
>
> This change only implements synchronous requests in passthrough, returning
> an error in the case of ansynchronous operations, yet covering the majority
> of the use cases.
>
> Signed-off-by: Alessio Balsini <balsini@android.com>
> ---
>  fs/fuse/file.c        |  8 +++--
>  fs/fuse/fuse_i.h      |  2 ++
>  fs/fuse/passthrough.c | 81 +++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 89 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 6c0ec742ce74..c3289ff0cd33 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1552,7 +1552,9 @@ static ssize_t fuse_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>         if (is_bad_inode(file_inode(file)))
>                 return -EIO;
>
> -       if (!(ff->open_flags & FOPEN_DIRECT_IO))
> +       if (ff->passthrough_filp)
> +               return fuse_passthrough_read_iter(iocb, to);
> +       else if (!(ff->open_flags & FOPEN_DIRECT_IO))
>                 return fuse_cache_read_iter(iocb, to);
>         else
>                 return fuse_direct_read_iter(iocb, to);
> @@ -1566,7 +1568,9 @@ static ssize_t fuse_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>         if (is_bad_inode(file_inode(file)))
>                 return -EIO;
>
> -       if (!(ff->open_flags & FOPEN_DIRECT_IO))
> +       if (ff->passthrough_filp)
> +               return fuse_passthrough_write_iter(iocb, from);
> +       else if (!(ff->open_flags & FOPEN_DIRECT_IO))
>                 return fuse_cache_write_iter(iocb, from);
>         else
>                 return fuse_direct_write_iter(iocb, from);
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 6c5166447905..21ba30a6a661 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -1106,5 +1106,7 @@ void fuse_free_conn(struct fuse_conn *fc);
>
>  int fuse_passthrough_setup(struct fuse_req *req, unsigned int fd);
>  void fuse_passthrough_release(struct fuse_file *ff);
> +ssize_t fuse_passthrough_read_iter(struct kiocb *iocb, struct iov_iter *to);
> +ssize_t fuse_passthrough_write_iter(struct kiocb *iocb, struct iov_iter *from);
>
>  #endif /* _FS_FUSE_I_H */
> diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
> index 86ab4eafa7bf..44a78e02f45d 100644
> --- a/fs/fuse/passthrough.c
> +++ b/fs/fuse/passthrough.c
> @@ -2,6 +2,87 @@
>
>  #include "fuse_i.h"
>
> +#include <linux/fs_stack.h>
> +#include <linux/fsnotify.h>
> +#include <linux/uio.h>
> +
> +static void fuse_copyattr(struct file *dst_file, struct file *src_file,
> +                         bool write)
> +{
> +       if (write) {
> +               struct inode *dst = file_inode(dst_file);
> +               struct inode *src = file_inode(src_file);
> +
> +               fsnotify_modify(src_file);
> +               fsstack_copy_inode_size(dst, src);
> +       } else {
> +               fsnotify_access(src_file);
> +       }
> +}
> +
> +
> +ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
> +                                  struct iov_iter *iter)
> +{
> +       ssize_t ret;
> +       struct file *fuse_filp = iocb_fuse->ki_filp;
> +       struct fuse_file *ff = fuse_filp->private_data;
> +       struct file *passthrough_filp = ff->passthrough_filp;
> +
> +       if (!iov_iter_count(iter))
> +               return 0;
> +
> +       if (is_sync_kiocb(iocb_fuse)) {
> +               struct kiocb iocb;
> +
> +               kiocb_clone(&iocb, iocb_fuse, passthrough_filp);
> +               ret = call_read_iter(passthrough_filp, &iocb, iter);
> +               iocb_fuse->ki_pos = iocb.ki_pos;
> +               if (ret >= 0)
> +                       fuse_copyattr(fuse_filp, passthrough_filp, false);
> +
> +       } else {
> +               ret = -EIO;
> +       }
> +
> +       return ret;
> +}
> +
> +ssize_t fuse_passthrough_write_iter(struct kiocb *iocb_fuse,
> +                                   struct iov_iter *iter)
> +{
> +       ssize_t ret;
> +       struct file *fuse_filp = iocb_fuse->ki_filp;
> +       struct fuse_file *ff = fuse_filp->private_data;
> +       struct inode *fuse_inode = file_inode(fuse_filp);
> +       struct file *passthrough_filp = ff->passthrough_filp;
> +
> +       if (!iov_iter_count(iter))
> +               return 0;
> +
> +       inode_lock(fuse_inode);
> +
> +       if (is_sync_kiocb(iocb_fuse)) {
> +               struct kiocb iocb;
> +
> +               kiocb_clone(&iocb, iocb_fuse, passthrough_filp);
> +
> +               file_start_write(passthrough_filp);
> +               ret = call_write_iter(passthrough_filp, &iocb, iter);

Why not vfs_iter_write()/vfs_iter_read()?

You are bypassing many internal VFS checks that seem pretty important.

Thanks,
Amir.