fuse: enable dynamic configuration of fuse max pages limit (FUSE_MAX_MAX_PAGES)

Introduce the capability to dynamically configure the max pages limit
(FUSE_MAX_MAX_PAGES) through a sysctl. This allows system administrators
to dynamically set the maximum number of pages that can be used for
servicing requests in fuse.

Previously, this is gated by FUSE_MAX_MAX_PAGES which is statically set
to 256 pages. One result of this is that the buffer size for a write
request is limited to 1 MiB on a 4k-page system.

The default value for this sysctl is the original limit (256 pages).

$ sysctl -a | grep max_pages_limit
fs.fuse.max_pages_limit = 256

$ sysctl -n fs.fuse.max_pages_limit
256

$ echo 1024 | sudo tee /proc/sys/fs/fuse/max_pages_limit
1024

$ sysctl -n fs.fuse.max_pages_limit
1024

$ echo 65536 | sudo tee /proc/sys/fs/fuse/max_pages_limit
tee: /proc/sys/fs/fuse/max_pages_limit: Invalid argument

$ echo 0 | sudo tee /proc/sys/fs/fuse/max_pages_limit
tee: /proc/sys/fs/fuse/max_pages_limit: Invalid argument

$ echo 65535 | sudo tee /proc/sys/fs/fuse/max_pages_limit
65535

$ sysctl -n fs.fuse.max_pages_limit
65535

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Documentation/admin-guide/sysctl/fs.rst

@@ -332,3 +332,13 @@ Each "watch" costs roughly 90 bytes on a 32-bit kernel, and roughly 160 bytes
 on a 64-bit one.
 The current default value for ``max_user_watches`` is 4% of the
 available low memory, divided by the "watch" cost in bytes.
+
+5. /proc/sys/fs/fuse - Configuration options for FUSE filesystems
+=====================================================================
+
+This directory contains the following configuration options for FUSE
+filesystems:
+
+``/proc/sys/fs/fuse/max_pages_limit`` is a read/write file for
+setting/getting the maximum number of pages that can be used for servicing
+requests in FUSE.

fs/fuse/Makefile

@@ -14,5 +14,6 @@ fuse-y := dev.o dir.o file.o inode.o control.o xattr.o acl.o readdir.o ioctl.o
 fuse-y += iomode.o
 fuse-$(CONFIG_FUSE_DAX) += dax.o
 fuse-$(CONFIG_FUSE_PASSTHROUGH) += passthrough.o
+fuse-$(CONFIG_SYSCTL) += sysctl.o
 
 virtiofs-y := virtio_fs.o

fs/fuse/fuse_i.h

@@ -35,9 +35,6 @@
 /** Default max number of pages that can be used in a single read request */
 #define FUSE_DEFAULT_MAX_PAGES_PER_REQ 32
 
-/** Maximum of max_pages received in init_out */
-#define FUSE_MAX_MAX_PAGES 256
-
 /** Bias for fi->writectr, meaning new writepages must not be sent */
 #define FUSE_NOWRITE INT_MIN
 
@@ -47,6 +44,9 @@
 /** Number of dentries for each connection in the control filesystem */
 #define FUSE_CTL_NUM_DENTRIES 5
 
+/** Maximum of max_pages received in init_out */
+extern unsigned int fuse_max_pages_limit;
+
 /** List of active connections */
 extern struct list_head fuse_conn_list;
 
@@ -1480,4 +1480,12 @@ ssize_t fuse_passthrough_splice_write(struct pipe_inode_info *pipe,
 				      size_t len, unsigned int flags);
 ssize_t fuse_passthrough_mmap(struct file *file, struct vm_area_struct *vma);
 
+#ifdef CONFIG_SYSCTL
+extern int fuse_sysctl_register(void);
+extern void fuse_sysctl_unregister(void);
+#else
+#define fuse_sysctl_register()		(0)
+#define fuse_sysctl_unregister()	do { } while (0)
+#endif /* CONFIG_SYSCTL */
+
 #endif /* _FS_FUSE_I_H */

fs/fuse/inode.c

@@ -35,6 +35,8 @@ DEFINE_MUTEX(fuse_mutex);
 
 static int set_global_limit(const char *val, const struct kernel_param *kp);
 
+unsigned int fuse_max_pages_limit = 256;
+
 unsigned max_user_bgreq;
 module_param_call(max_user_bgreq, set_global_limit, param_get_uint,
 		  &max_user_bgreq, 0644);
@@ -944,7 +946,7 @@ void fuse_conn_init(struct fuse_conn *fc, struct fuse_mount *fm,
 	fc->pid_ns = get_pid_ns(task_active_pid_ns(current));
 	fc->user_ns = get_user_ns(user_ns);
 	fc->max_pages = FUSE_DEFAULT_MAX_PAGES_PER_REQ;
-	fc->max_pages_limit = FUSE_MAX_MAX_PAGES;
+	fc->max_pages_limit = fuse_max_pages_limit;
 
 	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
 		fuse_backing_files_init(fc);
@@ -2063,8 +2065,14 @@ static int __init fuse_fs_init(void)
 	if (err)
 		goto out3;
 
+	err = fuse_sysctl_register();
+	if (err)
+		goto out4;
+
 	return 0;
 
+ out4:
+	unregister_filesystem(&fuse_fs_type);
  out3:
 	unregister_fuseblk();
  out2:
@@ -2075,6 +2083,7 @@ static int __init fuse_fs_init(void)
 
 static void fuse_fs_cleanup(void)
 {
+	fuse_sysctl_unregister();
 	unregister_filesystem(&fuse_fs_type);
 	unregister_fuseblk();
 

fs/fuse/ioctl.c

@@ -10,6 +10,8 @@
 #include <linux/fileattr.h>
 #include <linux/fsverity.h>
 
+#define FUSE_VERITY_ENABLE_ARG_MAX_PAGES 256
+
 static ssize_t fuse_send_ioctl(struct fuse_mount *fm, struct fuse_args *args,
 			       struct fuse_ioctl_out *outarg)
 {
@@ -140,7 +142,7 @@ static int fuse_setup_enable_verity(unsigned long arg, struct iovec *iov,
 {
 	struct fsverity_enable_arg enable;
 	struct fsverity_enable_arg __user *uarg = (void __user *)arg;
-	const __u32 max_buffer_len = FUSE_MAX_MAX_PAGES * PAGE_SIZE;
+	const __u32 max_buffer_len = FUSE_VERITY_ENABLE_ARG_MAX_PAGES * PAGE_SIZE;
 
 	if (copy_from_user(&enable, uarg, sizeof(enable)))
 		return -EFAULT;

fs/fuse/sysctl.c

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * linux/fs/fuse/fuse_sysctl.c
+ *
+ * Sysctl interface to fuse parameters
+ */
+#include <linux/sysctl.h>
+
+#include "fuse_i.h"
+
+static struct ctl_table_header *fuse_table_header;
+
+/* Bound by fuse_init_out max_pages, which is a u16 */
+static unsigned int sysctl_fuse_max_pages_limit = 65535;
+
+static struct ctl_table fuse_sysctl_table[] = {
+	{
+		.procname	= "max_pages_limit",
+		.data		= &fuse_max_pages_limit,
+		.maxlen		= sizeof(fuse_max_pages_limit),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec_minmax,
+		.extra1		= SYSCTL_ONE,
+		.extra2		= &sysctl_fuse_max_pages_limit,
+	},
+};
+
+int fuse_sysctl_register(void)
+{
+	fuse_table_header = register_sysctl("fs/fuse", fuse_sysctl_table);
+	if (!fuse_table_header)
+		return -ENOMEM;
+	return 0;
+}
+
+void fuse_sysctl_unregister(void)
+{
+	unregister_sysctl_table(fuse_table_header);
+	fuse_table_header = NULL;
+}