linux-kernel - Question about sg_count_fuse_req() in linux/fs/fuse/virtio

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Message-ID: <810089e0-3a09-0d8f-9f8e-be5b3ac70587@redhat.com>
Date:   Wed, 17 Mar 2021 13:12:01 -0500
From:   Connor Kuehl <ckuehl@...hat.com>
To:     virtio-fs@...hat.com
Cc:     vgoyal@...hat.com, stefanha@...hat.com, miklos@...redi.hu,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Question about sg_count_fuse_req() in linux/fs/fuse/virtio_fs.c

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.

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