On Wed, Mar 17, 2021 at 01:12:01PM -0500, Connor Kuehl wrote:
> Hi,
>
> I've been familiarizing myself with the virtiofs guest kernel module and I'm
> trying to better understand how virtiofs maps a FUSE request into
> scattergather lists.
>
> sg_count_fuse_req() starts knowing that there will be at least one in
> header, as shown here (which makes sense):
>
> unsigned int size, total_sgs = 1 /* fuse_in_header */;
>
> However, I'm confused about this snippet right beneath it:
>
> if (args->in_numargs - args->in_pages)
> total_sgs += 1;
>
> What is the significance of the sg that is needed in the cases where this
> branch is taken? I'm not sure what its relationship is with args->in_numargs
> since it will increment total_sgs regardless args->in_numargs is 3, 2, or
> even 1 if args->in_pages is false.
Hi Conor,
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;
Not sure when we will have a case where args->in_numargs = 1 and
args->in_pages=true. Do we ever hit that.
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
>
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