Re: Question about sg_count_fuse_req() in linux/fs/fuse/virtio_fs.c

[Connor Kuehl]

On 3/18/21 8:56 AM, Vivek Goyal wrote:
> I think all the in args are being mapped into a single scatter gather
> element and that's why it does not matter whether in_numargs is 3, 2 or 1.
> They will be mapped in a single element.
>
> sg_init_fuse_args()
> {
>          len = fuse_len_args(numargs - argpages, args);
>          if (len)
>                  sg_init_one(&sg[total_sgs++], argbuf, len);
> }
>
>          out_sgs += sg_init_fuse_args(&sg[out_sgs], req,
>                                       (struct fuse_arg *)args->in_args,
>                                       args->in_numargs, args->in_pages,
>                                       req->argbuf, &argbuf_used);
>
> When we are sending some data in some pages, then we set args->in_pages
> to true. And in that case, last element of args->in_args[] contains the
> total size of bytes in additional pages we are sending and is not part
> of in_args being mapped to scatter gather element. That's why this
> check.
>
> 	if (args->in_numargs - args->in_pages)
> 		total_sgs += 1;

Aha! Thank you, Vivek. That makes sense.

> Not sure when we will have a case where args->in_numargs = 1 and
> args->in_pages=true. Do we ever hit that.

Not in my experience. My previous mail was examining this routine mainly 
in a vacuum.

Connor

> Thanks
> Vivek
>
> > Especially since the block right below it counts pages if args->in_pages is
> > true:
> >
> >          if (args->in_pages) {
> >                  size = args->in_args[args->in_numargs - 1].size;
> >                  total_sgs += sg_count_fuse_pages(ap->descs, ap->num_pages,
> >                                                   size);
> >          }
> >
> > The rest of the routine goes on similarly but for the 'out' components.
> >
> > I doubt incrementing 'total_sgs' in the first if-statement I showed above is
> > vestigial, I just think my mental model of what is happening here is
> > incomplete.
> >
> > Any clarification is much appreciated!
>
> > Connor