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 VivekEspecially 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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