* Question about sg_count_fuse_req() in linux/fs/fuse/virtio_fs.c
@ 2021-03-17 18:12 Connor Kuehl
2021-03-18 13:56 ` Vivek Goyal
0 siblings, 1 reply; 3+ messages in thread
From: Connor Kuehl @ 2021-03-17 18:12 UTC (permalink / raw)
To: virtio-fs
Cc: vgoyal, stefanha, miklos, virtualization, linux-fsdevel, linux-kernel
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
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: Question about sg_count_fuse_req() in linux/fs/fuse/virtio_fs.c 2021-03-17 18:12 Question about sg_count_fuse_req() in linux/fs/fuse/virtio_fs.c Connor Kuehl @ 2021-03-18 13:56 ` Vivek Goyal 2021-03-18 13:58 ` Connor Kuehl 0 siblings, 1 reply; 3+ messages in thread From: Vivek Goyal @ 2021-03-18 13:56 UTC (permalink / raw) To: Connor Kuehl Cc: virtio-fs, stefanha, miklos, virtualization, linux-fsdevel, linux-kernel 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 > ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Question about sg_count_fuse_req() in linux/fs/fuse/virtio_fs.c 2021-03-18 13:56 ` Vivek Goyal @ 2021-03-18 13:58 ` Connor Kuehl 0 siblings, 0 replies; 3+ messages in thread From: Connor Kuehl @ 2021-03-18 13:58 UTC (permalink / raw) To: Vivek Goyal Cc: virtio-fs, stefanha, miklos, virtualization, linux-fsdevel, linux-kernel 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 >> > ^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2021-03-18 13:59 UTC | newest] Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2021-03-17 18:12 Question about sg_count_fuse_req() in linux/fs/fuse/virtio_fs.c Connor Kuehl 2021-03-18 13:56 ` Vivek Goyal 2021-03-18 13:58 ` Connor Kuehl
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