[PATCH 3/5] fs: sys_ringbuffer

Kent Overstreet
2024-06-03T00:33:21Z

Add new syscalls for generic ringbuffers that can be attached to
arbitrary (supporting) file descriptors.

A ringbuffer consists of:
- a single page for head/tail pointers, size/mask, and other ancilliary
metadata, described by 'struct ringbuffer_ptrs'
- a data buffer, consisting of one or more pages mapped at
'ringbuffer_ptrs.data_offset' above the address of 'ringbuffer_ptrs'

The data buffer is always a power of two size. Head and tail pointers
are u32 byte offsets, and they are stored unmasked (i.e., they use the
full 32 bit range) - they must be masked for reading.

- ringbuffer(int fd, int rw, u32 size, ulong *addr)

Create or get address of an existing ringbuffer for either reads or
writes, of at least size bytes, and attach it to the given file
descriptor; the address of the ringbuffer is returned via addr.

Since files can be shared between processes in different address spaces
a ringbuffer may be mapped into multiple address spaces via this
syscall.

- ringbuffer_wait(int fd, int rw)

Wait for space to be availaable (on a ringbuffer for writing), or data
to be available (on a ringbuffer for writing).

todo: add parameters for timeout, minimum amount of data/space to wait for

- ringbuffer_wakeup(int fd, int rw)

Required after writing to a previously empty ringbuffer, or reading from
a previously full ringbuffer to notify waiters on the other end

todo - investigate integrating with futexes?
todo - add extra fields to ringbuffer_ptrs for waiting on a minimum
amount of data/space, i.e. to signal when a wakeup is required

Kernel interfaces:
- To indicate that ringbuffers are supported on a file, set
FOP_RINGBUFFER_READ and/or FOP_RINGBUFFER_WRITE in your
file_operations.
- To read or write to a file's associated ringbuffers
(file->f_ringbuffer), use ringbuffer_read() or ringbuffer_write().

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
arch/x86/entry/syscalls/syscall_32.tbl | 3 +
arch/x86/entry/syscalls/syscall_64.tbl | 3 +
fs/Makefile | 1 +
fs/ringbuffer.c | 474 +++++++++++++++++++++++++
include/linux/fs.h | 2 +
include/linux/mm_types.h | 4 +
include/linux/ringbuffer_sys.h | 18 +
include/uapi/linux/futex.h | 1 +
include/uapi/linux/ringbuffer_sys.h | 40 +++
init/Kconfig | 9 +
kernel/fork.c | 2 +
11 files changed, 557 insertions(+)
create mode 100644 fs/ringbuffer.c
create mode 100644 include/linux/ringbuffer_sys.h
create mode 100644 include/uapi/linux/ringbuffer_sys.h

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 7fd1f57ad3d3..2385359eaf75 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -467,3 +467,6 @@
460 i386 lsm_set_self_attr sys_lsm_set_self_attr
461 i386 lsm_list_modules sys_lsm_list_modules
462 i386 mseal sys_mseal
+463 i386 ringbuffer sys_ringbuffer
+464 i386 ringbuffer_wait sys_ringbuffer_wait
+465 i386 ringbuffer_wakeup sys_ringbuffer_wakeup
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index a396f6e6ab5b..942602ece075 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -384,6 +384,9 @@
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
462 common mseal sys_mseal
+463 common ringbuffer sys_ringbuffer
+464 common ringbuffer_wait sys_ringbuffer_wait
+465 common ringbuffer_wakeup sys_ringbuffer_wakeup

#
# Due to a historical design error, certain syscalls are numbered differently
diff --git a/fs/Makefile b/fs/Makefile
index 6ecc9b0a53f2..48e54ac01fb1 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -28,6 +28,7 @@ obj-$(CONFIG_TIMERFD) += timerfd.o
obj-$(CONFIG_EVENTFD) += eventfd.o
obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
obj-$(CONFIG_AIO) += aio.o
+obj-$(CONFIG_RINGBUFFER) += ringbuffer.o
obj-$(CONFIG_FS_DAX) += dax.o
obj-$(CONFIG_FS_ENCRYPTION) += crypto/
obj-$(CONFIG_FS_VERITY) += verity/
diff --git a/fs/ringbuffer.c b/fs/ringbuffer.c
new file mode 100644
index 000000000000..82e042c1c89b
--- /dev/null
+++ b/fs/ringbuffer.c
@@ -0,0 +1,474 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define pr_fmt(fmt) "%s() " fmt "\n", __func__
+
+#include <linux/darray.h>
+#include <linux/errname.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/init.h>
+#include <linux/mman.h>
+#include <linux/mount.h>
+#include <linux/mutex.h>
+#include <linux/pagemap.h>
+#include <linux/pseudo_fs.h>
+#include <linux/ringbuffer_sys.h>
+#include <linux/syscalls.h>
+#include <linux/uio.h>
+
+#define RINGBUFFER_FS_MAGIC 0xa10a10a2
+
+static DEFINE_MUTEX(ringbuffer_lock);
+
+static struct vfsmount *ringbuffer_mnt;
+
+struct ringbuffer_mapping {
+ ulong addr;
+ struct mm_struct *mm;
+};
+
+struct ringbuffer {
+ u32 size; /* always a power of two */
+ u32 mask; /* size - 1 */
+ unsigned order;
+ wait_queue_head_t wait[2];
+ struct ringbuffer_ptrs *ptrs;
+ void *data;
+ /* hidden internal file for the mmap */
+ struct file *rb_file;
+ DARRAY(struct ringbuffer_mapping) mms;
+};
+
+static const struct address_space_operations ringbuffer_aops = {
+ .dirty_folio = noop_dirty_folio,
+#if 0
+ .migrate_folio = ringbuffer_migrate_folio,
+#endif
+};
+
+#if 0
+static int ringbuffer_mremap(struct vm_area_struct *vma)
+{
+ struct file *file = vma->vm_file;
+ struct mm_struct *mm = vma->vm_mm;
+ struct kioctx_table *table;
+ int i, res = -EINVAL;
+
+ spin_lock(&mm->ioctx_lock);
+ rcu_read_lock();
+ table = rcu_dereference(mm->ioctx_table);
+ if (!table)
+ goto out_unlock;
+
+ for (i = 0; i < table->nr; i++) {
+ struct kioctx *ctx;
+
+ ctx = rcu_dereference(table->table[i]);
+ if (ctx && ctx->ringbuffer_file == file) {
+ if (!atomic_read(&ctx->dead)) {
+ ctx->user_id = ctx->mmap_base = vma->vm_start;
+ res = 0;
+ }
+ break;
+ }
+ }
+
+out_unlock:
+ rcu_read_unlock();
+ spin_unlock(&mm->ioctx_lock);
+ return res;
+}
+#endif
+
+static const struct vm_operations_struct ringbuffer_vm_ops = {
+#if 0
+ .mremap = ringbuffer_mremap,
+#endif
+#if IS_ENABLED(CONFIG_MMU)
+ .fault = filemap_fault,
+ .map_pages = filemap_map_pages,
+ .page_mkwrite = filemap_page_mkwrite,
+#endif
+};
+
+static int ringbuffer_mmap(struct file *file, struct vm_area_struct *vma)
+{
+ vm_flags_set(vma, VM_DONTEXPAND);
+ vma->vm_ops = &ringbuffer_vm_ops;
+ return 0;
+}
+
+static const struct file_operations ringbuffer_fops = {
+ .mmap = ringbuffer_mmap,
+};
+
+void ringbuffer_free(struct ringbuffer *rb)
+{
+ pr_debug("%px", rb);
+
+ lockdep_assert_held(&ringbuffer_lock);
+
+ darray_for_each(rb->mms, map)
+ darray_for_each_reverse(map->mm->ringbuffers, rb2)
+ if (rb == *rb2)
+ darray_remove_item(&map->mm->ringbuffers, rb2);
+
+ if (rb->rb_file) {
+ /* Kills mapping: */
+ truncate_setsize(file_inode(rb->rb_file), 0);
+
+ struct address_space *mapping = rb->rb_file->f_mapping;
+ spin_lock(&mapping->i_private_lock);
+ mapping->i_private_data = NULL;
+ spin_unlock(&mapping->i_private_lock);
+
+ fput(rb->rb_file);
+ }
+
+ free_pages((ulong) rb->data, get_order(rb->size));
+ free_page((ulong) rb->ptrs);
+ kfree(rb);
+}
+
+static int ringbuffer_alloc_inode(struct ringbuffer *rb)
+{
+ struct inode *inode = alloc_anon_inode(ringbuffer_mnt->mnt_sb);
+ int ret = PTR_ERR_OR_ZERO(inode);
+ if (ret)
+ goto err;
+
+ inode->i_mapping->a_ops = &ringbuffer_aops;
+ inode->i_mapping->i_private_data = rb;
+ inode->i_size = rb->size * 2;
+ mapping_set_large_folios(inode->i_mapping);
+
+ rb->rb_file = alloc_file_pseudo(inode, ringbuffer_mnt, "[ringbuffer]",
+ O_RDWR, &ringbuffer_fops);
+ ret = PTR_ERR_OR_ZERO(rb->rb_file);
+ if (ret)
+ goto err_iput;
+
+ struct folio *f_ptrs = page_folio(virt_to_page(rb->ptrs));
+ struct folio *f_data = page_folio(virt_to_page(rb->data));
+
+ __folio_set_locked(f_ptrs);
+ __folio_mark_uptodate(f_ptrs);
+
+ void *shadow = NULL;
+ ret = __filemap_add_folio(rb->rb_file->f_mapping, f_ptrs,
+ (1U << rb->order) - 1, GFP_KERNEL, &shadow);
+ if (ret)
+ goto err;
+ folio_unlock(f_ptrs);
+
+ __folio_set_locked(f_data);
+ __folio_mark_uptodate(f_data);
+ shadow = NULL;
+ ret = __filemap_add_folio(rb->rb_file->f_mapping, f_data,
+ 1U << rb->order, GFP_KERNEL, &shadow);
+ if (ret)
+ goto err;
+ folio_unlock(f_data);
+ return 0;
+err_iput:
+ iput(inode);
+ return ret;
+err:
+ truncate_setsize(file_inode(rb->rb_file), 0);
+ fput(rb->rb_file);
+ return ret;
+}
+
+static int ringbuffer_map(struct ringbuffer *rb, ulong *addr)
+{
+ struct mm_struct *mm = current->mm;
+ int ret = 0;
+
+ lockdep_assert_held(&ringbuffer_lock);
+
+ if (!rb->rb_file) {
+ ret = ringbuffer_alloc_inode(rb);
+ if (ret)
+ return ret;
+ }
+
+ ret = darray_make_room(&rb->mms, 1) ?:
+ darray_make_room(&mm->ringbuffers, 1);
+ if (ret)
+ return ret;
+
+ ret = mmap_write_lock_killable(mm);
+ if (ret)
+ return ret;
+
+ ulong unused;
+ struct ringbuffer_mapping map = {
+ .addr = do_mmap(rb->rb_file, 0, rb->size + PAGE_SIZE,
+ PROT_READ|PROT_WRITE,
+ MAP_SHARED, 0,
+ (1U << rb->order) - 1,
+ &unused, NULL),
+ .mm = mm,
+ };
+ mmap_write_unlock(mm);
+
+ ret = PTR_ERR_OR_ZERO((void *) map.addr);
+ if (ret)
+ return ret;
+
+ ret = darray_push(&mm->ringbuffers, rb) ?:
+ darray_push(&rb->mms, map);
+ BUG_ON(ret); /* we preallocated */
+
+ *addr = map.addr;
+ return 0;
+}
+
+static int ringbuffer_get_addr_or_map(struct ringbuffer *rb, ulong *addr)
+{
+ lockdep_assert_held(&ringbuffer_lock);
+
+ struct mm_struct *mm = current->mm;
+
+ darray_for_each(rb->mms, map)
+ if (map->mm == mm) {
+ *addr = map->addr;
+ return 0;
+ }
+
+ return ringbuffer_map(rb, addr);
+}
+
+struct ringbuffer *ringbuffer_alloc(u32 size)
+{
+ unsigned order = get_order(size);
+ size = PAGE_SIZE << order;
+
+ struct ringbuffer *rb = kzalloc(sizeof(*rb), GFP_KERNEL);
+ if (!rb)
+ return ERR_PTR(-ENOMEM);
+
+ rb->size = size;
+ rb->mask = size - 1;
+ rb->order = order;
+ init_waitqueue_head(&rb->wait[READ]);
+ init_waitqueue_head(&rb->wait[WRITE]);
+
+ rb->ptrs = (void *) __get_free_page(GFP_KERNEL|__GFP_ZERO);
+ rb->data = (void *) __get_free_pages(GFP_KERNEL|__GFP_ZERO|__GFP_COMP, order);
+ if (!rb->ptrs || !rb->data) {
+ ringbuffer_free(rb);
+ return ERR_PTR(-ENOMEM);
+ }
+
+ /* todo - implement a fallback when high order allocation fails */
+
+ rb->ptrs->size = size;
+ rb->ptrs->mask = size - 1;
+ rb->ptrs->data_offset = PAGE_SIZE;
+ return rb;
+}
+
+/*
+ * XXX: we require synchronization when killing a ringbuffer (because no longer
+ * mapped anywhere) to a file that is still open (and in use)
+ */
+static void ringbuffer_mm_drop(struct mm_struct *mm, struct ringbuffer *rb)
+{
+ darray_for_each_reverse(rb->mms, map)
+ if (mm == map->mm) {
+ pr_debug("removing %px from %px", rb, mm);
+ darray_remove_item(&rb->mms, map);
+ }
+}
+
+void ringbuffer_mm_exit(struct mm_struct *mm)
+{
+ mutex_lock(&ringbuffer_lock);
+ darray_for_each_reverse(mm->ringbuffers, rb)
+ ringbuffer_mm_drop(mm, *rb);
+ mutex_unlock(&ringbuffer_lock);
+
+ darray_exit(&mm->ringbuffers);
+}
+
+SYSCALL_DEFINE4(ringbuffer, unsigned, fd, int, rw, u32, size, ulong __user *, ringbufferp)
+{
+ ulong rb_addr;
+
+ int ret = get_user(rb_addr, ringbufferp);
+ if (unlikely(ret))
+ return ret;
+
+ if (unlikely(rb_addr || !size || rw > WRITE))
+ return -EINVAL;
+
+ struct fd f = fdget(fd);
+ if (!f.file)
+ return -EBADF;
+
+ struct ringbuffer *rb = f.file->f_op->ringbuffer(f.file, rw);
+ if (!rb) {
+ ret = -EOPNOTSUPP;
+ goto err;
+ }
+
+ mutex_lock(&ringbuffer_lock);
+ ret = ringbuffer_get_addr_or_map(rb, &rb_addr);
+ if (ret)
+ goto err_unlock;
+
+ ret = put_user(rb_addr, ringbufferp);
+err_unlock:
+ mutex_unlock(&ringbuffer_lock);
+err:
+ fdput(f);
+ return ret;
+}
+
+ssize_t ringbuffer_read_iter(struct ringbuffer *rb, struct iov_iter *iter, bool nonblocking)
+{
+ u32 tail = rb->ptrs->tail, orig_tail = tail;
+ u32 head = smp_load_acquire(&rb->ptrs->head);
+
+ if (unlikely(head == tail)) {
+ if (nonblocking)
+ return -EAGAIN;
+ int ret = wait_event_interruptible(rb->wait[READ],
+ (head = smp_load_acquire(&rb->ptrs->head)) != rb->ptrs->tail);
+ if (ret)
+ return ret;
+ }
+
+ while (iov_iter_count(iter)) {
+ u32 tail_masked = tail & rb->mask;
+ u32 len = min(iov_iter_count(iter),
+ min(head - tail,
+ rb->size - tail_masked));
+ if (!len)
+ break;
+
+ len = copy_to_iter(rb->data + tail_masked, len, iter);
+
+ tail += len;
+ }
+
+ smp_store_release(&rb->ptrs->tail, tail);
+
+ smp_mb();
+
+ if (rb->ptrs->head - orig_tail >= rb->size)
+ wake_up(&rb->wait[WRITE]);
+
+ return tail - orig_tail;
+}
+EXPORT_SYMBOL_GPL(ringbuffer_read_iter);
+
+ssize_t ringbuffer_write_iter(struct ringbuffer *rb, struct iov_iter *iter, bool nonblocking)
+{
+ u32 head = rb->ptrs->head, orig_head = head;
+ u32 tail = smp_load_acquire(&rb->ptrs->tail);
+
+ if (unlikely(head - tail >= rb->size)) {
+ if (nonblocking)
+ return -EAGAIN;
+ int ret = wait_event_interruptible(rb->wait[WRITE],
+ head - (tail = smp_load_acquire(&rb->ptrs->tail)) < rb->size);
+ if (ret)
+ return ret;
+ }
+
+ while (iov_iter_count(iter)) {
+ u32 head_masked = head & rb->mask;
+ u32 len = min(iov_iter_count(iter),
+ min(tail + rb->size - head,
+ rb->size - head_masked));
+ if (!len)
+ break;
+
+ len = copy_from_iter(rb->data + head_masked, len, iter);
+
+ head += len;
+ }
+
+ smp_store_release(&rb->ptrs->head, head);
+
+ smp_mb();
+
+ if ((s32) (rb->ptrs->tail - orig_head) >= 0)
+ wake_up(&rb->wait[READ]);
+
+ return head - orig_head;
+}
+EXPORT_SYMBOL_GPL(ringbuffer_write_iter);
+
+SYSCALL_DEFINE2(ringbuffer_wait, unsigned, fd, int, rw)
+{
+ int ret = 0;
+
+ if (rw > WRITE)
+ return -EINVAL;
+
+ struct fd f = fdget(fd);
+ if (!f.file)
+ return -EBADF;
+
+ struct ringbuffer *rb = f.file->f_op->ringbuffer(f.file, rw);
+ if (!rb) {
+ ret = -EINVAL;
+ goto err;
+ }
+
+ struct ringbuffer_ptrs *rp = rb->ptrs;
+ wait_event(rb->wait[rw], rw == READ
+ ? rp->head != rp->tail
+ : rp->head - rp->tail < rb->size);
+err:
+ fdput(f);
+ return ret;
+}
+
+SYSCALL_DEFINE2(ringbuffer_wakeup, unsigned, fd, int, rw)
+{
+ int ret = 0;
+
+ if (rw > WRITE)
+ return -EINVAL;
+
+ struct fd f = fdget(fd);
+ if (!f.file)
+ return -EBADF;
+
+ struct ringbuffer *rb = f.file->f_op->ringbuffer(f.file, rw);
+ if (!rb) {
+ ret = -EINVAL;
+ goto err;
+ }
+
+ wake_up(&rb->wait[!rw]);
+err:
+ fdput(f);
+ return ret;
+}
+
+static int ringbuffer_init_fs_context(struct fs_context *fc)
+{
+ if (!init_pseudo(fc, RINGBUFFER_FS_MAGIC))
+ return -ENOMEM;
+ fc->s_iflags |= SB_I_NOEXEC;
+ return 0;
+}
+
+static int __init ringbuffer_init(void)
+{
+ static struct file_system_type ringbuffer_fs = {
+ .name = "ringbuffer",
+ .init_fs_context = ringbuffer_init_fs_context,
+ .kill_sb = kill_anon_super,
+ };
+ ringbuffer_mnt = kern_mount(&ringbuffer_fs);
+ if (IS_ERR(ringbuffer_mnt))
+ panic("Failed to create ringbuffer fs mount.");
+ return 0;
+}
+__initcall(ringbuffer_init);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 0283cf366c2a..3026f8f92d6f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1996,6 +1996,7 @@ struct offset_ctx;

typedef unsigned int __bitwise fop_flags_t;

+struct ringbuffer;
struct file_operations {
struct module *owner;
fop_flags_t fop_flags;
@@ -2004,6 +2005,7 @@ struct file_operations {
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
+ struct ringbuffer *(*ringbuffer)(struct file *, int);
int (*iopoll)(struct kiocb *kiocb, struct io_comp_batch *,
unsigned int flags);
int (*iterate_shared) (struct file *, struct dir_context *);
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 24323c7d0bd4..6e412718ce7e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -5,6 +5,7 @@
#include <linux/mm_types_task.h>

#include <linux/auxvec.h>
+#include <linux/darray_types.h>
#include <linux/kref.h>
#include <linux/list.h>
#include <linux/spinlock.h>
@@ -911,6 +912,9 @@ struct mm_struct {
spinlock_t ioctx_lock;
struct kioctx_table __rcu *ioctx_table;
#endif
+#ifdef CONFIG_RINGBUFFER
+ DARRAY(struct ringbuffer *) ringbuffers;
+#endif
#ifdef CONFIG_MEMCG
/*
* "owner" points to a task that is regarded as the canonical
diff --git a/include/linux/ringbuffer_sys.h b/include/linux/ringbuffer_sys.h
new file mode 100644
index 000000000000..843509f72514
--- /dev/null
+++ b/include/linux/ringbuffer_sys.h
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_RINGBUFFER_SYS_H
+#define _LINUX_RINGBUFFER_SYS_H
+
+#include <linux/darray_types.h>
+#include <linux/spinlock_types.h>
+#include <uapi/linux/ringbuffer_sys.h>
+
+struct mm_struct;
+void ringbuffer_mm_exit(struct mm_struct *mm);
+
+void ringbuffer_free(struct ringbuffer *rb);
+struct ringbuffer *ringbuffer_alloc(u32 size);
+
+ssize_t ringbuffer_read_iter(struct ringbuffer *rb, struct iov_iter *iter, bool nonblock);
+ssize_t ringbuffer_write_iter(struct ringbuffer *rb, struct iov_iter *iter, bool nonblock);
+
+#endif /* _LINUX_RINGBUFFER_SYS_H */
diff --git a/include/uapi/linux/futex.h b/include/uapi/linux/futex.h
index d2ee625ea189..09d94a5cb849 100644
--- a/include/uapi/linux/futex.h
+++ b/include/uapi/linux/futex.h
@@ -22,6 +22,7 @@
#define FUTEX_WAIT_REQUEUE_PI 11
#define FUTEX_CMP_REQUEUE_PI 12
#define FUTEX_LOCK_PI2 13
+#define FUTEX_WAIT_GE 14

#define FUTEX_PRIVATE_FLAG 128
#define FUTEX_CLOCK_REALTIME 256
diff --git a/include/uapi/linux/ringbuffer_sys.h b/include/uapi/linux/ringbuffer_sys.h
new file mode 100644
index 000000000000..a7afe8647cc1
--- /dev/null
+++ b/include/uapi/linux/ringbuffer_sys.h
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_RINGBUFFER_SYS_H
+#define _UAPI_LINUX_RINGBUFFER_SYS_H
+
+#include <uapi/linux/types.h>
+
+/*
+ * ringbuffer_ptrs - head and tail pointers for a ringbuffer, mappped to
+ * userspace:
+ */
+struct ringbuffer_ptrs {
+ /*
+ * We use u32s because this type is shared between the kernel and
+ * userspace - ulong/size_t won't work here, we might be 32bit userland
+ * and 64 bit kernel, and u64 would be preferable (reduced probability
+ * of ABA) but not all architectures can atomically read/write to a u64;
+ * we need to avoid torn reads/writes.
+ *
+ * head and tail pointers are incremented and stored without masking;
+ * this is to avoid ABA and differentiate between a full and empty
+ * buffer - they must be masked with @mask to get an actual offset into
+ * the data buffer.
+ *
+ * All units are in bytes.
+ *
+ * Data is emitted at head, consumed from tail.
+ */
+ __u32 head;
+ __u32 tail;
+ __u32 size; /* always a power of two */
+ __u32 mask; /* size - 1 */
+
+ /*
+ * Starting offset of data buffer, from the start of this struct - will
+ * always be PAGE_SIZE.
+ */
+ __u32 data_offset;
+};
+
+#endif /* _UAPI_LINUX_RINGBUFFER_SYS_H */
diff --git a/init/Kconfig b/init/Kconfig
index 72404c1f2157..c43d536d4898 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1673,6 +1673,15 @@ config IO_URING
applications to submit and complete IO through submission and
completion rings that are shared between the kernel and application.

+config RINGBUFFER
+ bool "Enable ringbuffer() syscall" if EXPERT
+ select XARRAY_MULTI
+ default y
+ help
+ This option adds support for generic ringbuffers, which can be
+ attached to any (supported) file descriptor, allowing for reading and
+ writing without syscall overhead.
+
config ADVISE_SYSCALLS
bool "Enable madvise/fadvise syscalls" if EXPERT
default y
diff --git a/kernel/fork.c b/kernel/fork.c
index 99076dbe27d8..9190a06a6365 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -103,6 +103,7 @@
#include <linux/rseq.h>
#include <uapi/linux/pidfd.h>
#include <linux/pidfs.h>
+#include <linux/ringbuffer_sys.h>

#include <asm/pgalloc.h>
#include <linux/uaccess.h>
@@ -1340,6 +1341,7 @@ static inline void __mmput(struct mm_struct *mm)
VM_BUG_ON(atomic_read(&mm->mm_users));

uprobe_clear_state(mm);
+ ringbuffer_mm_exit(mm);
exit_aio(mm);
ksm_exit(mm);
khugepaged_exit(mm); /* must run before exit_mmap */
--
2.45.1