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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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




[Index of Archives]     [Linux Ext4 Filesystem]     [Union Filesystem]     [Filesystem Testing]     [Ceph Users]     [Ecryptfs]     [AutoFS]     [Kernel Newbies]     [Share Photos]     [Security]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux Cachefs]     [Reiser Filesystem]     [Linux RAID]     [Samba]     [Device Mapper]     [CEPH Development]

  Powered by Linux