Re: [PATCH RESEND V12 5/8] fuse: Introduce synchronous read and write for passthrough

[Miklos Szeredi]

On Mon, Jan 25, 2021 at 4:31 PM Alessio Balsini <balsini@xxxxxxxxxxx> 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 via VFS.
>
> 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.
>
> Verifying if a fuse_file has a lower file system file associated with
> can be done by checking the validity of its passthrough_filp pointer.
> This pointer is not NULL only if passthrough has been successfully
> enabled via the appropriate ioctl().
> When a read/write operation is requested for a FUSE file with
> passthrough enabled, a new equivalent VFS request is generated, which
> instead targets the lower file system file.
> The VFS layer performs additional checks that allow for safer operations
> but may cause the operation to fail if the process accessing the FUSE
> file system does not have access to the lower file system.
>
> This change only implements synchronous requests in passthrough,
> returning an error in the case of asynchronous operations, yet covering
> the majority of the use cases.
>
> Signed-off-by: Alessio Balsini <balsini@xxxxxxxxxxx>
> ---
>  fs/fuse/file.c        |  8 ++++--
>  fs/fuse/fuse_i.h      |  2 ++
>  fs/fuse/passthrough.c | 57 +++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 65 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 953f3034c375..cddada1e8bd9 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1581,7 +1581,9 @@ static ssize_t fuse_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>         if (FUSE_IS_DAX(inode))
>                 return fuse_dax_read_iter(iocb, to);
>
> -       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);
> @@ -1599,7 +1601,9 @@ static ssize_t fuse_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>         if (FUSE_IS_DAX(inode))
>                 return fuse_dax_write_iter(iocb, from);
>
> -       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 8d39f5304a11..c4730d893324 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -1239,5 +1239,7 @@ int fuse_passthrough_open(struct fuse_dev *fud,
>  int fuse_passthrough_setup(struct fuse_conn *fc, struct fuse_file *ff,
>                            struct fuse_open_out *openarg);
>  void fuse_passthrough_release(struct fuse_passthrough *passthrough);
> +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 cf993e83803e..d949ca07a83b 100644
> --- a/fs/fuse/passthrough.c
> +++ b/fs/fuse/passthrough.c
> @@ -4,6 +4,63 @@
>
>  #include <linux/fuse.h>
>  #include <linux/idr.h>
> +#include <linux/uio.h>
> +
> +#define PASSTHROUGH_IOCB_MASK                                                  \
> +       (IOCB_APPEND | IOCB_DSYNC | IOCB_HIPRI | IOCB_NOWAIT | IOCB_SYNC)
> +
> +static void fuse_copyattr(struct file *dst_file, struct file *src_file)
> +{
> +       struct inode *dst = file_inode(dst_file);
> +       struct inode *src = file_inode(src_file);
> +
> +       i_size_write(dst, i_size_read(src));
> +}

Hmm, I see why this is done, yet it's contrary to what's been set out
at the beginning: "All the requests other than reads or writes are
still handled by the userspace FUSE daemon."

Maybe just use fuse_write_update_size() instead of copying the size
from the underlying inode.

> +
> +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;
> +
> +       ret = vfs_iter_read(passthrough_filp, iter, &iocb_fuse->ki_pos,
> +                           iocb_to_rw_flags(iocb_fuse->ki_flags,
> +                                            PASSTHROUGH_IOCB_MASK));

Please split this line up into:

rwf = ioctb_to_rw_flags(...);
ret = vfs_iter_read(..., rwf);

> +
> +       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);
> +
> +       file_start_write(passthrough_filp);
> +       ret = vfs_iter_write(passthrough_filp, iter, &iocb_fuse->ki_pos,
> +                            iocb_to_rw_flags(iocb_fuse->ki_flags,
> +                                             PASSTHROUGH_IOCB_MASK));

Same here.

Thanks,
Miklos