Messages in this thread | | | Subject | Re: Question about sg_count_fuse_req() in linux/fs/fuse/virtio_fs.c | From | Connor Kuehl <> | Date | Thu, 18 Mar 2021 08:58:54 -0500 |
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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 >> >
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