Re: [fuse-devel] Sector aligned address & offsets to all read/write requests from lib-fuse

[Goswin v. B. <gos...@we...>]

Miklos Szeredi <mi...@sz...> writes:

> Goswin von Brederlow <gos...@we...> writes:
>
>> I don't know how feasable this is, but: Could request be split up into a
>> command fd and data fd? The main thread would open the command fd and
>> each thread would request it own data fd over the command fd. Then when
>> a thread reads a request from the command fd (and it is a big one) the
>> kernel would dump the data to the data fd for the requesting thread.
>>
>> With this fuse wouldn't even have to splice the data into an extra pipe
>> but could just pass the data fd to the callback as is.
>
> The kernel should not need to care about threads and such.  It should
> not contain knowledge about the implementation of libfuse.
>
> So slightly modifying your idea: the kernel can attach a data fd
> (basically the read end of a pipe) to the request and dump the data into
> that pipe.
>
> The question is: how to attach the data fd to the request?  File
> descriptor passing over unix domain sockets (SCM_RIGHTS) comes to mind.
> But changing the device interface into a socket interface is not quite
> trivial.
>
> Passing the file descriptor in the fuse_write_in header would be
> possible, but that's really messy.
>
> An ioctl() to request that the data be dumped onto the given pipe is
> perhaps the cleanest way to do this.  E.g.
>
> struct fuse_data_request {
> 	u64 unique;
>         int fd;
> };
>
> static void do_write()
> {
> 	struct fuse_data_request fdr;
>         int pip[2];
> 	...
>         pipe(pip);
>         fdr.unique = req->unique;
>         fdr.fd = pip[1];
> 	ioctl(fuse_dev, FUSE_IOCTL_GETDATA, &fdr);
>         /* data available in pip[0] */
>         ...
> }
>
> Thanks,
> Miklos

One more thing I forgot.

Does the FD have to be a pipe? Think of an overlay filesystem. Why not
pass the FD of the underlying file:

struct fuse_data_request {
	u64 unique;
        u64 offset;
        int fd;
};

static void do_write(...)
{
	struct fuse_data_request fdr;
        fdr.unique = req->unique;
        fdt.offset = off;
        fdr.fd = fi->fh;
	ioctl(fuse_dev, FUSE_IOCTL_GETDATA, &fdr);
}

Plus some error handling.

MfG
        Goswin