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