Message-Id: <20260210-fuse-compounds-upstream-v5-1-ea0585f62daa@ddn.com>
Date: Tue, 10 Feb 2026 09:46:16 +0100
From: Horst Birthelmer <horst@...thelmer.com>
To: Miklos Szeredi <miklos@...redi.hu>, Bernd Schubert <bschubert@....com>, 
 Joanne Koong <joannelkoong@...il.com>, Luis Henriques <luis@...lia.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
 Horst Birthelmer <hbirthelmer@....com>
Subject: [PATCH v5 1/3] fuse: add compound command to combine multiple
 requests

From: Horst Birthelmer <hbirthelmer@....com>

For a FUSE_COMPOUND we add a header that contains information
about how many commands there are in the compound and about the
size of the expected result. This will make the interpretation
in libfuse easier, since we can preallocate the whole result.
Then we append the requests that belong to this compound.

The API for the compound command has:
  fuse_compound_alloc()
  fuse_compound_add()
  fuse_compound_send()

Signed-off-by: Horst Birthelmer <hbirthelmer@....com>
---
 fs/fuse/Makefile          |   2 +-
 fs/fuse/compound.c        | 224 ++++++++++++++++++++++++++++++++++++++++++++++
 fs/fuse/fuse_i.h          |  11 +++
 include/uapi/linux/fuse.h |  40 +++++++++
 4 files changed, 276 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/Makefile b/fs/fuse/Makefile
index 22ad9538dfc4b80c6d9b52235bdfead6a6567ae4..4c09038ef995d1b9133c2b6871b97b280a4693b0 100644
--- a/fs/fuse/Makefile
+++ b/fs/fuse/Makefile
@@ -11,7 +11,7 @@ obj-$(CONFIG_CUSE) += cuse.o
 obj-$(CONFIG_VIRTIO_FS) += virtiofs.o
 
 fuse-y := trace.o	# put trace.o first so we see ftrace errors sooner
-fuse-y += dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o
+fuse-y += dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o compound.o
 fuse-y += iomode.o
 fuse-$(CONFIG_FUSE_DAX) += dax.o
 fuse-$(CONFIG_FUSE_PASSTHROUGH) += passthrough.o backing.o
diff --git a/fs/fuse/compound.c b/fs/fuse/compound.c
new file mode 100644
index 0000000000000000000000000000000000000000..1a85209f4e99a0e5e54955bcd951eaf395176c12
--- /dev/null
+++ b/fs/fuse/compound.c
@@ -0,0 +1,224 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * FUSE: Filesystem in Userspace
+ * Copyright (C) 2025
+ *
+ * Compound operations for FUSE - batch multiple operations into a single
+ * request to reduce round trips between kernel and userspace.
+ */
+
+#include "fuse_i.h"
+
+/*
+ * Compound request builder, state tracker, and args pointer storage
+ */
+struct fuse_compound_req {
+	struct fuse_mount *fm;
+	struct fuse_compound_in compound_header;
+	struct fuse_compound_out result_header;
+	int op_errors[FUSE_MAX_COMPOUND_OPS];
+	struct fuse_args *op_args[FUSE_MAX_COMPOUND_OPS];
+};
+
+struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm, u32 flags)
+{
+	struct fuse_compound_req *compound;
+
+	compound = kzalloc(sizeof(*compound), GFP_KERNEL);
+	if (!compound)
+		return NULL;
+
+	compound->fm = fm;
+	compound->compound_header.flags = flags;
+
+	return compound;
+}
+
+int fuse_compound_add(struct fuse_compound_req *compound, struct fuse_args *args)
+{
+	if (!compound ||
+	    compound->compound_header.count >= FUSE_MAX_COMPOUND_OPS)
+		return -EINVAL;
+
+	if (args->in_pages)
+		return -EINVAL;
+
+	compound->op_args[compound->compound_header.count] = args;
+	compound->compound_header.count++;
+	return 0;
+}
+
+static void *fuse_copy_resp_data_per_req(const struct fuse_args *args,
+					  char *resp)
+{
+	const struct fuse_arg *arg;
+	int i;
+
+	for (i = 0; i < args->out_numargs; i++) {
+		arg = &args->out_args[i];
+		memcpy(arg->value, resp, arg->size);
+		resp += arg->size;
+	}
+
+	return resp;
+}
+
+int fuse_compound_get_error(struct fuse_compound_req *compound, int op_idx)
+{
+	return compound->op_errors[op_idx];
+}
+
+static void *fuse_compound_parse_one_op(struct fuse_compound_req *compound,
+					 int op_index, char *response,
+					 char *response_end)
+{
+	struct fuse_out_header *op_hdr = (struct fuse_out_header *)response;
+	struct fuse_args *args = compound->op_args[op_index];
+
+	if (op_hdr->len < sizeof(struct fuse_out_header))
+		return NULL;
+
+	if (response + op_hdr->len > response_end)
+		return NULL;
+
+	if (op_hdr->error)
+		compound->op_errors[op_index] = op_hdr->error;
+	else
+		fuse_copy_resp_data_per_req(args, response +
+					    sizeof(struct fuse_out_header));
+	/* in case of error, we still need to advance to the next op */
+	return response + op_hdr->len;
+}
+
+static int fuse_compound_parse_resp(struct fuse_compound_req *compound,
+				     char *response, size_t response_size)
+{
+	char *response_end = response + response_size;
+	int req_count;
+	int i;
+
+	req_count = min(compound->compound_header.count,
+			compound->result_header.count);
+
+	for (i = 0; i < req_count; i++) {
+		response = fuse_compound_parse_one_op(compound, i, response,
+						      response_end);
+		if (!response)
+			return -EIO;
+	}
+
+	return 0;
+}
+
+/*
+ * Build a single operation request in the buffer
+ *
+ * Returns the new buffer position after writing the operation.
+ */
+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->uid = from_kuid(fc->user_ns, current_fsuid());
+	hdr->gid = from_kgid(fc->user_ns, current_fsgid());
+	hdr->pid = pid_nr_ns(task_pid(current), fc->pid_ns);
+	buffer_pos += sizeof(*hdr);
+
+	for (j = 0; j < op_args->in_numargs; j++) {
+		memcpy(buffer_pos, op_args->in_args[j].value,
+		       op_args->in_args[j].size);
+		buffer_pos += op_args->in_args[j].size;
+	}
+
+	return buffer_pos;
+}
+
+ssize_t fuse_compound_send(struct fuse_compound_req *compound)
+{
+	struct fuse_conn *fc = compound->fm->fc;
+	struct fuse_args args = {
+		.opcode = FUSE_COMPOUND,
+		.in_numargs = 2,
+		.out_numargs = 2,
+		.out_argvar = true,
+	};
+	unsigned int req_count = compound->compound_header.count;
+	size_t total_expected_out_size = 0;
+	size_t actual_response_size;
+	size_t buffer_size = 0;
+	void *resp_payload_buffer;
+	char *buffer_pos;
+	void *buffer = NULL;
+	ssize_t ret;
+	int i, j;
+
+	for (i = 0; i < req_count; i++) {
+		struct fuse_args *op_args = compound->op_args[i];
+		size_t needed_size = sizeof(struct fuse_in_header);
+
+		for (j = 0; j < op_args->in_numargs; j++)
+			needed_size += op_args->in_args[j].size;
+
+		buffer_size += needed_size;
+
+		for (j = 0; j < op_args->out_numargs; j++)
+			total_expected_out_size += op_args->out_args[j].size;
+	}
+
+	buffer = kmalloc(buffer_size, GFP_KERNEL | __GFP_ZERO);
+	if (!buffer)
+		return -ENOMEM;
+
+	buffer_pos = buffer;
+	for (i = 0; i < req_count; i++)
+		buffer_pos = fuse_compound_build_one_op(fc,
+							compound->op_args[i],
+							buffer_pos);
+
+	compound->compound_header.result_size = total_expected_out_size;
+
+	args.in_args[0].size = sizeof(compound->compound_header);
+	args.in_args[0].value = &compound->compound_header;
+	args.in_args[1].size = buffer_size;
+	args.in_args[1].value = buffer;
+
+	buffer_size = total_expected_out_size +
+		      (req_count * sizeof(struct fuse_out_header));
+
+	resp_payload_buffer = kmalloc(buffer_size, GFP_KERNEL | __GFP_ZERO);
+	if (!resp_payload_buffer) {
+		ret = -ENOMEM;
+		goto out_free_buffer;
+	}
+
+	args.out_args[0].size = sizeof(compound->result_header);
+	args.out_args[0].value = &compound->result_header;
+	args.out_args[1].size = buffer_size;
+	args.out_args[1].value = resp_payload_buffer;
+
+	ret = fuse_simple_request(compound->fm, &args);
+	if (ret < 0)
+		goto out;
+
+	actual_response_size = args.out_args[1].size;
+
+	ret = fuse_compound_parse_resp(compound, (char *)resp_payload_buffer,
+				       actual_response_size);
+out:
+	kfree(resp_payload_buffer);
+out_free_buffer:
+	kfree(buffer);
+	return ret;
+}
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 7f16049387d15e869db4be23a93605098588eda9..9ebcd96b6b309d75c86a9c716cbd88aaa55c57ef 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1273,6 +1273,17 @@ static inline ssize_t fuse_simple_idmap_request(struct mnt_idmap *idmap,
 int fuse_simple_background(struct fuse_mount *fm, struct fuse_args *args,
 			   gfp_t gfp_flags);
 
+/**
+ * Compound request API
+ */
+struct fuse_compound_req;
+ssize_t fuse_compound_send(struct fuse_compound_req *compound);
+
+struct fuse_compound_req *fuse_compound_alloc(struct fuse_mount *fm, u32 flags);
+int fuse_compound_add(struct fuse_compound_req *compound,
+		      struct fuse_args *args);
+int fuse_compound_get_error(struct fuse_compound_req *compound, int op_idx);
+
 /**
  * Assign a unique id to a fuse request
  */
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index c13e1f9a2f12bd39f535188cb5466688eba42263..113583c4efb67268174dbe4f68e9ea1c21b45eb6 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -664,6 +664,13 @@ enum fuse_opcode {
 	FUSE_STATX		= 52,
 	FUSE_COPY_FILE_RANGE_64	= 53,
 
+	/* A compound request is handled like a single request,
+	 * but contains multiple requests as input.
+	 * This can be used to signal to the fuse server that
+	 * the requests can be combined atomically.
+	 */
+	FUSE_COMPOUND		= 54,
+
 	/* CUSE specific operations */
 	CUSE_INIT		= 4096,
 
@@ -1245,6 +1252,39 @@ struct fuse_supp_groups {
 	uint32_t	groups[];
 };
 
+#define FUSE_MAX_COMPOUND_OPS   16        /* Maximum operations per compound */
+
+#define FUSE_COMPOUND_SEPARABLE (1<<0)
+#define FUSE_COMPOUND_ATOMIC (1<<1)
+
+/*
+ * Compound request header
+ *
+ * This header is followed by the fuse requests
+ */
+struct fuse_compound_in {
+	uint32_t	count;			/* Number of operations */
+	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;
+	uint64_t	reserved;
+};
+
+/*
+ * Compound response header
+ *
+ * This header is followed by complete fuse responses
+ */
+struct fuse_compound_out {
+	uint32_t	count;     /* Number of results */
+	uint32_t	flags;     /* Result flags */
+	uint64_t	reserved;
+};
+
 /**
  * Size of the ring buffer header
  */

-- 
2.53.0

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