Re: [PATCH RESEND V12 3/8] fuse: Definitions and ioctl for passthrough

[Peng Tao <bergwolf@gmail.com>]

On Wed, Feb 17, 2021 at 9:41 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Mon, Jan 25, 2021 at 4:31 PM Alessio Balsini <balsini@android.com> wrote:
> >
> > Expose the FUSE_PASSTHROUGH interface to user space and declare all the
> > basic data structures and functions as the skeleton on top of which the
> > FUSE passthrough functionality will be built.
> >
> > As part of this, introduce the new FUSE passthrough ioctl, which allows
> > the FUSE daemon to specify a direct connection between a FUSE file and a
> > lower file system file. Such ioctl requires user space to pass the file
> > descriptor of one of its opened files through the fuse_passthrough_out
> > data structure introduced in this patch. This structure includes extra
> > fields for possible future extensions.
> > Also, add the passthrough functions for the set-up and tear-down of the
> > data structures and locks that will be used both when fuse_conns and
> > fuse_files are created/deleted.
> >
> > Signed-off-by: Alessio Balsini <balsini@android.com>
>
> [...]
>
> > diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> > index 54442612c48b..9d7685ce0acd 100644
> > --- a/include/uapi/linux/fuse.h
> > +++ b/include/uapi/linux/fuse.h
> > @@ -360,6 +360,7 @@ struct fuse_file_lock {
> >  #define FUSE_MAP_ALIGNMENT     (1 << 26)
> >  #define FUSE_SUBMOUNTS         (1 << 27)
> >  #define FUSE_HANDLE_KILLPRIV_V2        (1 << 28)
> > +#define FUSE_PASSTHROUGH       (1 << 29)
>
> This header has a version and a changelog.  Please update those as well.
>
> >
> >  /**
> >   * CUSE INIT request/reply flags
> > @@ -625,7 +626,7 @@ struct fuse_create_in {
> >  struct fuse_open_out {
> >         uint64_t        fh;
> >         uint32_t        open_flags;
> > -       uint32_t        padding;
> > +       uint32_t        passthrough_fh;
>
> I think it would be cleaner to add a FOPEN_PASSTHROUGH flag to
> explicitly request passthrough instead of just passing a non-null
> value to passthrough_fh.
>
> >  };
> >
> >  struct fuse_release_in {
> > @@ -828,6 +829,13 @@ struct fuse_in_header {
> >         uint32_t        padding;
> >  };
> >
> > +struct fuse_passthrough_out {
> > +       uint32_t        fd;
> > +       /* For future implementation */
> > +       uint32_t        len;
> > +       void            *vec;
> > +};
>
> I don't see why we'd need these extensions.    The ioctl just needs to
> establish an ID to open file mapping that can be referenced on the
> regular protocol, i.e. it just needs to be passed an open file
> descriptor and return an unique ID.
>
> Mapping the fuse file's data to the underlying file's data is a
> different matter.  That can be an identity mapping established at open
> time (this is what this series does) or it can be an arbitrary extent
> mapping to one or more underlying open files, established at open time
> or on demand.  All of these can be done in band using the fuse
> protocol, no need to involve the ioctl mechanism.
>
> So I think we can just get rid of "struct fuse_passthrough_out"
> completely and use "uint32_t *" as the ioctl argument.
>
> What I think would be useful is to have an explicit
> FUSE_DEV_IOC_PASSTHROUGH_CLOSE ioctl, that would need to be called
> once the fuse server no longer needs this ID.   If this turns out to
> be a performance problem, we could still add the auto-close behavior
> with an explicit FOPEN_PASSTHROUGH_AUTOCLOSE flag later.
Hi Miklos,

W/o auto closing, what happens if user space daemon forgets to call
FUSE_DEV_IOC_PASSTHROUGH_CLOSE? Do we keep the ID alive somewhere?

Thanks,
Tao
-- 
Into Sth. Rich & Strange