linux-kernel - Re: [PATCH v5 1/3] fuse: add compound command to combine multiple requests

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Message-ID: <CAJfpegvt0HwHOmOTzkCoOqdmvU6pf-wM228QQSauDsbcL+mmUA@mail.gmail.com>
Date: Wed, 11 Feb 2026 17:13:21 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: Horst Birthelmer <horst@...thelmer.com>
Cc: Bernd Schubert <bschubert@....com>, Joanne Koong <joannelkoong@...il.com>, 
	Luis Henriques <luis@...lia.com>, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, Horst Birthelmer <hbirthelmer@....com>
Subject: Re: [PATCH v5 1/3] fuse: add compound command to combine multiple requests

On Tue, 10 Feb 2026 at 09:46, Horst Birthelmer <horst@...thelmer.com> wrote:

> +static char *fuse_compound_build_one_op(struct fuse_conn *fc,
> +                                        struct fuse_args *op_args,
> +                                        char *buffer_pos)
> +{
> +       struct fuse_in_header *hdr;
> +       size_t needed_size = sizeof(struct fuse_in_header);
> +       int j;
> +
> +       for (j = 0; j < op_args->in_numargs; j++)
> +               needed_size += op_args->in_args[j].size;
> +
> +       hdr = (struct fuse_in_header *)buffer_pos;
> +       memset(hdr, 0, sizeof(*hdr));
> +       hdr->len = needed_size;
> +       hdr->opcode = op_args->opcode;
> +       hdr->nodeid = op_args->nodeid;

hdr->unique is notably missing.

I don't know.  Maybe just fill it with the index?

> +       hdr->uid = from_kuid(fc->user_ns, current_fsuid());
> +       hdr->gid = from_kgid(fc->user_ns, current_fsgid());

uid/gid are not needed except for creation ops, and those need idmap
to calculate the correct values.  I don't think we want to keep legacy
behavior of always setting these.

> +       hdr->pid = pid_nr_ns(task_pid(current), fc->pid_ns);

This will be the same as the value in the compound header, so it's
redundant.  That might not be bad, but I feel that we're better off
setting this to zero and letting the userspace server fetch the pid
value from the compound header if that's needed.

> +#define FUSE_MAX_COMPOUND_OPS   16        /* Maximum operations per compound */

Don't see a good reason to declare this in the API.   More sensible
would be to negotiate a max_request_size during INIT.

> +
> +#define FUSE_COMPOUND_SEPARABLE (1<<0)
> +#define FUSE_COMPOUND_ATOMIC (1<<1)

What is the meaning of these flags?

> +
> +/*
> + * Compound request header
> + *
> + * This header is followed by the fuse requests
> + */
> +struct fuse_compound_in {
> +       uint32_t        count;                  /* Number of operations */

This is redundant, as the sum of the sub-request lengths is equal to
the compound request length, hence calculating the number of ops is
trivial.

> +       uint32_t        flags;                  /* Compound flags */
> +
> +       /* Total size of all results.
> +        * This is needed for preallocating the whole result for all
> +        * commands in this compound.
> +        */
> +       uint32_t        result_size;

I don't understand why this is needed.  Preallocation by the userspace
server?  Why is this different from a simple request?

> +       uint64_t        reserved;
> +};
> +
> +/*
> + * Compound response header
> + *
> + * This header is followed by complete fuse responses
> + */
> +struct fuse_compound_out {
> +       uint32_t        count;     /* Number of results */

Again, redundant.

Thanks,
Miklos