Message-Id: <20161026191810.12275-6-dh.herrmann@gmail.com>
Date:   Wed, 26 Oct 2016 21:18:01 +0200
From:   David Herrmann <dh.herrmann@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, Greg KH <greg@...ah.com>,
        Hannes Reinecke <hare@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Arnd Bergmann <arnd@...db.de>, Tom Gundersen <teg@...m.no>,
        David Herrmann <dh.herrmann@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [RFC v1 05/14] bus1: util - pool utility library

From: Tom Gundersen <teg@...m.no>

A bus1-pool is a shmem-backed memory pool shared between userspace and
the kernel. The pool is used to transfer memory from the kernel to
userspace without requiring userspace to pre-allocate space.

The pool is managed in slices, which are published to userspace when
they are ready to be read and must be released by userspace when
userspace is done with them.

Userspace has read-only access to its pools and the kernel has
read-write access, but published slices are not altered.

This pool implementation will be used by bus1 message transactions to
support single-copy data transfers, directly from the sender's address
space into the pool of the destination peer. The allocation algorithm
is based on the Android Binder code and has served their needs well for
many years now.

Signed-off-by: Tom Gundersen <teg@...m.no>
Signed-off-by: David Herrmann <dh.herrmann@...il.com>
---
 ipc/bus1/Makefile    |   3 +-
 ipc/bus1/util/pool.c | 572 +++++++++++++++++++++++++++++++++++++++++++++++++++
 ipc/bus1/util/pool.h | 164 +++++++++++++++
 3 files changed, 738 insertions(+), 1 deletion(-)
 create mode 100644 ipc/bus1/util/pool.c
 create mode 100644 ipc/bus1/util/pool.h

diff --git a/ipc/bus1/Makefile b/ipc/bus1/Makefile
index 6db6d13..ca8e19d 100644
--- a/ipc/bus1/Makefile
+++ b/ipc/bus1/Makefile
@@ -1,7 +1,8 @@
 bus1-y :=			\
 	main.o			\
 	util/active.o		\
-	util/flist.o
+	util/flist.o		\
+	util/pool.o
 
 obj-$(CONFIG_BUS1) += bus1.o
 
diff --git a/ipc/bus1/util/pool.c b/ipc/bus1/util/pool.c
new file mode 100644
index 0000000..2ddbffb
--- /dev/null
+++ b/ipc/bus1/util/pool.c
@@ -0,0 +1,572 @@
+/*
+ * Copyright (C) 2013-2016 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation; either version 2.1 of the License, or (at
+ * your option) any later version.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/aio.h>
+#include <linux/err.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/highmem.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/rbtree.h>
+#include <linux/sched.h>
+#include <linux/shmem_fs.h>
+#include <linux/sizes.h>
+#include <linux/slab.h>
+#include <linux/uaccess.h>
+#include <linux/uio.h>
+#include "pool.h"
+
+static struct bus1_pool_slice *bus1_pool_slice_new(size_t offset, size_t size)
+{
+	struct bus1_pool_slice *slice;
+
+	if (offset > U32_MAX || size == 0 || size > BUS1_POOL_SLICE_SIZE_MAX)
+		return ERR_PTR(-EMSGSIZE);
+
+	slice = kmalloc(sizeof(*slice), GFP_KERNEL);
+	if (!slice)
+		return ERR_PTR(-ENOMEM);
+
+	slice->offset = offset;
+	slice->size = size;
+
+	return slice;
+}
+
+static struct bus1_pool_slice *
+bus1_pool_slice_free(struct bus1_pool_slice *slice)
+{
+	if (!slice)
+		return NULL;
+
+	kfree(slice);
+
+	return NULL;
+}
+
+/* insert slice into the free tree */
+static void bus1_pool_slice_link_free(struct bus1_pool_slice *slice,
+				      struct bus1_pool *pool)
+{
+	struct rb_node **n, *prev = NULL;
+	struct bus1_pool_slice *ps;
+
+	n = &pool->slices_free.rb_node;
+	while (*n) {
+		prev = *n;
+		ps = container_of(prev, struct bus1_pool_slice, rb);
+		if (slice->size < ps->size)
+			n = &prev->rb_left;
+		else
+			n = &prev->rb_right;
+	}
+
+	rb_link_node(&slice->rb, prev, n);
+	rb_insert_color(&slice->rb, &pool->slices_free);
+}
+
+/* insert slice into the busy tree */
+static void bus1_pool_slice_link_busy(struct bus1_pool_slice *slice,
+				      struct bus1_pool *pool)
+{
+	struct rb_node **n, *prev = NULL;
+	struct bus1_pool_slice *ps;
+
+	n = &pool->slices_busy.rb_node;
+	while (*n) {
+		prev = *n;
+		ps = container_of(prev, struct bus1_pool_slice, rb);
+		if (WARN_ON(slice->offset == ps->offset))
+			n = &prev->rb_right; /* add anyway */
+		else if (slice->offset < ps->offset)
+			n = &prev->rb_left;
+		else /* if (slice->offset > ps->offset) */
+			n = &prev->rb_right;
+	}
+
+	rb_link_node(&slice->rb, prev, n);
+	rb_insert_color(&slice->rb, &pool->slices_busy);
+
+	pool->allocated_size += slice->size;
+}
+
+/* find free slice big enough to hold @size bytes */
+static struct bus1_pool_slice *
+bus1_pool_slice_find_by_size(struct bus1_pool *pool, size_t size)
+{
+	struct bus1_pool_slice *ps, *closest = NULL;
+	struct rb_node *n;
+
+	n = pool->slices_free.rb_node;
+	while (n) {
+		ps = container_of(n, struct bus1_pool_slice, rb);
+		if (size < ps->size) {
+			closest = ps;
+			n = n->rb_left;
+		} else if (size > ps->size) {
+			n = n->rb_right;
+		} else /* if (size == ps->size) */ {
+			return ps;
+		}
+	}
+
+	return closest;
+}
+
+/* find used slice with given offset */
+static struct bus1_pool_slice *
+bus1_pool_slice_find_by_offset(struct bus1_pool *pool, size_t offset)
+{
+	struct bus1_pool_slice *ps;
+	struct rb_node *n;
+
+	n = pool->slices_busy.rb_node;
+	while (n) {
+		ps = container_of(n, struct bus1_pool_slice, rb);
+		if (offset < ps->offset)
+			n = n->rb_left;
+		else if (offset > ps->offset)
+			n = n->rb_right;
+		else /* if (offset == ps->offset) */
+			return ps;
+	}
+
+	return NULL;
+}
+
+/**
+ * bus1_pool_init() - create memory pool
+ * @pool:	pool to operate on
+ * @filename:	name to use for the shmem-file (only visible via /proc)
+ *
+ * Initialize a new pool object.
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int bus1_pool_init(struct bus1_pool *pool, const char *filename)
+{
+	struct bus1_pool_slice *slice;
+	struct page *p;
+	struct file *f;
+	int r;
+
+	/* cannot calculate width of bitfields, so hardcode '4' as flag-size */
+	BUILD_BUG_ON(BUS1_POOL_SLICE_SIZE_BITS + 3 > 32);
+	BUILD_BUG_ON(BUS1_POOL_SLICE_SIZE_MAX > U32_MAX);
+
+	f = shmem_file_setup(filename, ALIGN(BUS1_POOL_SLICE_SIZE_MAX, 8),
+			     VM_NORESERVE);
+	if (IS_ERR(f))
+		return PTR_ERR(f);
+
+	r = get_write_access(file_inode(f));
+	if (r < 0) {
+		fput(f);
+		return r;
+	}
+
+	pool->f = f;
+	pool->allocated_size = 0;
+	INIT_LIST_HEAD(&pool->slices);
+	pool->slices_free = RB_ROOT;
+	pool->slices_busy = RB_ROOT;
+
+	slice = bus1_pool_slice_new(0, BUS1_POOL_SLICE_SIZE_MAX);
+	if (IS_ERR(slice)) {
+		bus1_pool_deinit(pool);
+		return PTR_ERR(slice);
+	}
+
+	slice->free = true;
+	slice->ref_kernel = false;
+	slice->ref_user = false;
+
+	list_add(&slice->entry, &pool->slices);
+	bus1_pool_slice_link_free(slice, pool);
+
+	/*
+	 * Touch first page of client pool so the initial allocation overhead
+	 * is done during peer setup rather than a message transaction. This is
+	 * really just an optimization to avoid some random peaks in common
+	 * paths. It is not meant as ultimate protection.
+	 */
+	p = shmem_read_mapping_page(file_inode(f)->i_mapping, 0);
+	if (!IS_ERR(p))
+		put_page(p);
+
+	return 0;
+}
+
+/**
+ * bus1_pool_deinit() - destroy pool
+ * @pool:	pool to destroy, or NULL
+ *
+ * This destroys a pool that was previously create via bus1_pool_init(). If
+ * NULL is passed, or if @pool->f is NULL (i.e., the pool was initialized to 0
+ * but not created via bus1_pool_init(), yet), then this is a no-op.
+ *
+ * The caller must make sure that no kernel reference to any slice exists. Any
+ * pending user-space reference to any slice is dropped by this function.
+ */
+void bus1_pool_deinit(struct bus1_pool *pool)
+{
+	struct bus1_pool_slice *slice;
+
+	if (!pool || !pool->f)
+		return;
+
+	while ((slice = list_first_entry_or_null(&pool->slices,
+						 struct bus1_pool_slice,
+						 entry))) {
+		WARN_ON(slice->ref_kernel);
+		list_del(&slice->entry);
+		bus1_pool_slice_free(slice);
+	}
+
+	put_write_access(file_inode(pool->f));
+	fput(pool->f);
+	pool->f = NULL;
+}
+
+/**
+ * bus1_pool_alloc() - allocate memory
+ * @pool:	pool to allocate memory from
+ * @size:	number of bytes to allocate
+ *
+ * This allocates a new slice of @size bytes from the memory pool at @pool. The
+ * slice must be released via bus1_pool_release_kernel() by the caller. All
+ * slices are aligned to 8 bytes (both offset and size).
+ *
+ * If no suitable slice can be allocated, an error is returned.
+ *
+ * Each pool slice can have two different references, a kernel reference and a
+ * user-space reference. Initially, it only has a kernel-reference, which must
+ * be dropped via bus1_pool_release_kernel(). However, if you previously
+ * publish the slice via bus1_pool_publish(), it will also have a user-space
+ * reference, which user-space must (indirectly) release via a call to
+ * bus1_pool_release_user().
+ * A slice is only actually freed if neither reference exists, anymore. Hence,
+ * pool-slice can be held by both, the kernel and user-space, and both can rely
+ * on it staying around as long as they wish.
+ *
+ * Return: Pointer to new slice, or ERR_PTR on failure.
+ */
+struct bus1_pool_slice *bus1_pool_alloc(struct bus1_pool *pool, size_t size)
+{
+	struct bus1_pool_slice *slice, *ps;
+	size_t slice_size;
+
+	slice_size = ALIGN(size, 8);
+	if (slice_size == 0 || slice_size > BUS1_POOL_SLICE_SIZE_MAX)
+		return ERR_PTR(-EMSGSIZE);
+
+	/* find smallest suitable, free slice */
+	slice = bus1_pool_slice_find_by_size(pool, slice_size);
+	if (!slice)
+		return ERR_PTR(-EXFULL);
+
+	/* split slice if it doesn't match exactly */
+	if (slice_size < slice->size) {
+		ps = bus1_pool_slice_new(slice->offset + slice_size,
+					 slice->size - slice_size);
+		if (IS_ERR(ps))
+			return ERR_CAST(ps);
+
+		ps->free = true;
+		ps->ref_kernel = false;
+		ps->ref_user = false;
+
+		list_add(&ps->entry, &slice->entry); /* add after @slice */
+		bus1_pool_slice_link_free(ps, pool);
+
+		slice->size = slice_size;
+	}
+
+	/* move from free-tree to busy-tree */
+	rb_erase(&slice->rb, &pool->slices_free);
+	bus1_pool_slice_link_busy(slice, pool);
+
+	slice->ref_kernel = true;
+	slice->ref_user = false;
+	slice->free = false;
+
+	return slice;
+}
+
+static void bus1_pool_free(struct bus1_pool *pool,
+			   struct bus1_pool_slice *slice)
+{
+	struct bus1_pool_slice *ps;
+
+	/* don't free the slice if either has a reference */
+	if (slice->ref_kernel || slice->ref_user || WARN_ON(slice->free))
+		return;
+
+	/*
+	 * To release a pool-slice, we first drop it from the busy-tree, then
+	 * merge it with possible previous/following free slices and re-add it
+	 * to the free-tree.
+	 */
+
+	rb_erase(&slice->rb, &pool->slices_busy);
+
+	if (!WARN_ON(slice->size > pool->allocated_size))
+		pool->allocated_size -= slice->size;
+
+	if (pool->slices.next != &slice->entry) {
+		ps = container_of(slice->entry.prev, struct bus1_pool_slice,
+				  entry);
+		if (ps->free) {
+			rb_erase(&ps->rb, &pool->slices_free);
+			list_del(&slice->entry);
+			ps->size += slice->size;
+			bus1_pool_slice_free(slice);
+			slice = ps; /* switch to previous slice */
+		}
+	}
+
+	if (pool->slices.prev != &slice->entry) {
+		ps = container_of(slice->entry.next, struct bus1_pool_slice,
+				  entry);
+		if (ps->free) {
+			rb_erase(&ps->rb, &pool->slices_free);
+			list_del(&ps->entry);
+			slice->size += ps->size;
+			bus1_pool_slice_free(ps);
+		}
+	}
+
+	slice->free = true;
+	bus1_pool_slice_link_free(slice, pool);
+}
+
+/**
+ * bus1_pool_release_kernel() - release kernel-owned slice reference
+ * @pool:	pool to free memory on
+ * @slice:	slice to release
+ *
+ * This releases the kernel-reference to a slice that was previously allocated
+ * via bus1_pool_alloc(). This only releases the kernel reference to the slice.
+ * If the slice was already published to user-space, then their reference is
+ * left untouched. Once both references are gone, the memory is actually freed.
+ *
+ * Return: NULL is returned.
+ */
+struct bus1_pool_slice *
+bus1_pool_release_kernel(struct bus1_pool *pool, struct bus1_pool_slice *slice)
+{
+	if (!slice || WARN_ON(!slice->ref_kernel))
+		return NULL;
+
+	/* kernel must own a ref to @slice */
+	slice->ref_kernel = false;
+
+	bus1_pool_free(pool, slice);
+
+	return NULL;
+}
+
+/**
+ * bus1_pool_publish() - publish a slice
+ * @pool:		pool to operate on
+ * @slice:		slice to publish
+ *
+ * Publish a pool slice to user-space, so user-space can get access to it via
+ * the mapped pool memory. If the slice was already published, this is a no-op.
+ * Otherwise, the slice is marked as public and will only get freed once both
+ * the user-space reference *and* kernel-space reference are released.
+ */
+void bus1_pool_publish(struct bus1_pool *pool, struct bus1_pool_slice *slice)
+{
+	/* kernel must own a ref to @slice to publish it */
+	WARN_ON(!slice->ref_kernel);
+	slice->ref_user = true;
+}
+
+/**
+ * bus1_pool_release_user() - release a public slice
+ * @pool:	pool to operate on
+ * @offset:	offset of slice to release
+ * @n_slicesp:	output variable to store number of released slices, or NULL
+ *
+ * Release the user-space reference to a pool-slice, specified via the offset
+ * of the slice. If both, the user-space reference *and* the kernel-space
+ * reference to the slice are gone, the slice will be actually freed.
+ *
+ * If no slice exists with the given offset, or if there is no user-space
+ * reference to the specified slice, an error is returned.
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int bus1_pool_release_user(struct bus1_pool *pool,
+			   size_t offset,
+			   size_t *n_slicesp)
+{
+	struct bus1_pool_slice *slice;
+
+	slice = bus1_pool_slice_find_by_offset(pool, offset);
+	if (!slice || !slice->ref_user)
+		return -ENXIO;
+
+	if (n_slicesp)
+		*n_slicesp = !slice->ref_kernel;
+
+	slice->ref_user = false;
+	bus1_pool_free(pool, slice);
+
+	return 0;
+}
+
+/**
+ * bus1_pool_flush() - flush all user references
+ * @pool:	pool to flush
+ * @n_slicesp:	output variable to store number of released slices, or NULL
+ *
+ * This flushes all user-references to any slice in @pool. Kernel references
+ * are left untouched.
+ */
+void bus1_pool_flush(struct bus1_pool *pool, size_t *n_slicesp)
+{
+	struct bus1_pool_slice *slice;
+	struct rb_node *node, *t;
+	size_t n_slices = 0;
+
+	for (node = rb_first(&pool->slices_busy);
+	     node && ((t = rb_next(node)), true);
+	     node = t) {
+		slice = container_of(node, struct bus1_pool_slice, rb);
+		if (!slice->ref_user)
+			continue;
+
+		if (!slice->ref_kernel)
+			++n_slices;
+
+		/*
+		 * @slice (or the logically previous/next slice) might be freed
+		 * by bus1_pool_free(). However, this only ever affects 'free'
+		 * slices, never busy slices. Hence, @t is protected from
+		 * removal.
+		 */
+		slice->ref_user = false;
+		bus1_pool_free(pool, slice);
+	}
+
+	if (n_slicesp)
+		*n_slicesp = n_slices;
+}
+
+/**
+ * bus1_pool_mmap() - mmap the pool
+ * @pool:		pool to operate on
+ * @vma:		VMA to map to
+ *
+ * This maps the pools shmem file to the provided VMA. Only read-only mappings
+ * are allowed.
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int bus1_pool_mmap(struct bus1_pool *pool, struct vm_area_struct *vma)
+{
+	if (unlikely(vma->vm_flags & VM_WRITE))
+		return -EPERM; /* deny write-access to the pool */
+
+	/* replace the connection file with our shmem file */
+	if (vma->vm_file)
+		fput(vma->vm_file);
+	vma->vm_file = get_file(pool->f);
+	vma->vm_flags &= ~VM_MAYWRITE;
+
+	/* calls into shmem_mmap(), which simply sets vm_ops */
+	return pool->f->f_op->mmap(pool->f, vma);
+}
+
+/**
+ * bus1_pool_write_iovec() - copy user memory to a slice
+ * @pool:		pool to operate on
+ * @slice:		slice to write to
+ * @offset:		relative offset into slice memory
+ * @iov:		iovec array, pointing to data to copy
+ * @n_iov:		number of elements in @iov
+ * @total_len:		total number of bytes to copy
+ *
+ * This copies the memory pointed to by @iov into the memory slice @slice at
+ * relative offset @offset (relative to begin of slice).
+ *
+ * Return: Numbers of bytes copied, negative error code on failure.
+ */
+ssize_t bus1_pool_write_iovec(struct bus1_pool *pool,
+			      struct bus1_pool_slice *slice,
+			      loff_t offset,
+			      struct iovec *iov,
+			      size_t n_iov,
+			      size_t total_len)
+{
+	struct iov_iter iter;
+	ssize_t len;
+
+	if (WARN_ON(offset + total_len < offset) ||
+	    WARN_ON(offset + total_len > slice->size) ||
+	    WARN_ON(slice->ref_user))
+		return -EFAULT;
+	if (total_len < 1)
+		return 0;
+
+	offset += slice->offset;
+	iov_iter_init(&iter, WRITE, iov, n_iov, total_len);
+
+	len = vfs_iter_write(pool->f, &iter, &offset);
+
+	return (len >= 0 && len != total_len) ? -EFAULT : len;
+}
+
+/**
+ * bus1_pool_write_kvec() - copy kernel memory to a slice
+ * @pool:		pool to operate on
+ * @slice:		slice to write to
+ * @offset:		relative offset into slice memory
+ * @iov:		kvec array, pointing to data to copy
+ * @n_iov:		number of elements in @iov
+ * @total_len:		total number of bytes to copy
+ *
+ * This copies the memory pointed to by @iov into the memory slice @slice at
+ * relative offset @offset (relative to begin of slice).
+ *
+ * Return: Numbers of bytes copied, negative error code on failure.
+ */
+ssize_t bus1_pool_write_kvec(struct bus1_pool *pool,
+			     struct bus1_pool_slice *slice,
+			     loff_t offset,
+			     struct kvec *iov,
+			     size_t n_iov,
+			     size_t total_len)
+{
+	struct iov_iter iter;
+	mm_segment_t old_fs;
+	ssize_t len;
+
+	if (WARN_ON(offset + total_len < offset) ||
+	    WARN_ON(offset + total_len > slice->size) ||
+	    WARN_ON(slice->ref_user))
+		return -EFAULT;
+	if (total_len < 1)
+		return 0;
+
+	offset += slice->offset;
+	iov_iter_kvec(&iter, WRITE | ITER_KVEC, iov, n_iov, total_len);
+
+	old_fs = get_fs();
+	set_fs(get_ds());
+	len = vfs_iter_write(pool->f, &iter, &offset);
+	set_fs(old_fs);
+
+	return (len >= 0 && len != total_len) ? -EFAULT : len;
+}
diff --git a/ipc/bus1/util/pool.h b/ipc/bus1/util/pool.h
new file mode 100644
index 0000000..f0e369b
--- /dev/null
+++ b/ipc/bus1/util/pool.h
@@ -0,0 +1,164 @@
+#ifndef __BUS1_POOL_H
+#define __BUS1_POOL_H
+
+/*
+ * Copyright (C) 2013-2016 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation; either version 2.1 of the License, or (at
+ * your option) any later version.
+ */
+
+/**
+ * DOC: Pools
+ *
+ * A pool is a shmem-backed memory pool shared between userspace and the kernel.
+ * The pool is used to transfer memory from the kernel to userspace without
+ * requiring userspace to allocate the memory.
+ *
+ * The pool is managed in slices, which are published to userspace when they are
+ * ready to be read and must be released by userspace when userspace is done
+ * with them.
+ *
+ * Userspace has read-only access to its pools and the kernel has read-write
+ * access, but published slices are not altered.
+ */
+
+#include <linux/kernel.h>
+#include <linux/rbtree.h>
+#include <linux/types.h>
+
+struct file;
+struct iovec;
+struct kvec;
+
+/* internal: number of bits available to slice size */
+#define BUS1_POOL_SLICE_SIZE_BITS (29)
+#define BUS1_POOL_SLICE_SIZE_MAX ((1 << BUS1_POOL_SLICE_SIZE_BITS) - 1)
+
+/**
+ * struct bus1_pool_slice - pool slice
+ * @offset:		relative offset in parent pool
+ * @size:		slice size
+ * @free:		whether this slice is in-use or not
+ * @ref_kernel:		whether a kernel reference exists
+ * @ref_user:		whether a user reference exists
+ * @entry:		link into linear list of slices
+ * @rb:			link to busy/free rb-tree
+ *
+ * Each chunk of memory in the pool is managed as a slice. A slice can be
+ * accessible by both the kernel and user-space, and their access rights are
+ * managed independently. As long as the kernel has a reference to a slice, its
+ * offset and size can be accessed freely and will not change. Once the kernel
+ * drops its reference, it must not access the slice, anymore.
+ *
+ * To allow user-space access, the slice must be published. This marks the slice
+ * as referenced by user-space. Note that all slices are always readable by
+ * user-space, since the entire pool can be mapped. Publishing a slice only
+ * marks the slice as referenced by user-space, so it will not be modified or
+ * removed. Once user-space releases its reference, it should no longer access
+ * the slice as it might be modified and/or overwritten by other data.
+ *
+ * Only if neither kernel nor user-space have a reference to a slice, the slice
+ * is released. The kernel reference can only be acquired/released once, but
+ * user-space references can be published/released several times. In particular,
+ * if the kernel retains a reference when a slice is published and later
+ * released by userspace, the same slice can be published again in the future.
+ *
+ * Note that both kernel-space and user-space must be aware that slice
+ * references are not ref-counted. They are simple booleans. For the kernel-side
+ * this is obvious, as no ref/unref functions are provided. But user-space must
+ * be aware that the same slice being published several times does not increase
+ * the reference count.
+ */
+struct bus1_pool_slice {
+	u32 offset;
+
+	/* merge @size with flags to save 8 bytes per existing slice */
+	u32 size : BUS1_POOL_SLICE_SIZE_BITS;
+	u32 free : 1;
+	u32 ref_kernel : 1;
+	u32 ref_user : 1;
+
+	struct list_head entry;
+	struct rb_node rb;
+};
+
+/**
+ * struct bus1_pool - client pool
+ * @f:			backing shmem file
+ * @allocated_size:	currently allocated memory in bytes
+ * @slices:		all slices sorted by address
+ * @slices_busy:	tree of allocated slices
+ * @slices_free:	tree of free slices
+ *
+ * A pool is used to allocate memory slices that can be shared between
+ * kernel-space and user-space. A pool is always backed by a shmem-file and puts
+ * a simple slice-allocator on top. User-space gets read-only access to the
+ * entire pool, kernel-space gets read/write access via accessor-functions.
+ *
+ * Pools are used to transfer large sets of data to user-space, without
+ * requiring a round-trip to ask user-space for a suitable memory chunk.
+ * Instead, the kernel simply allocates slices in the pool and tells user-space
+ * where it put the data.
+ *
+ * All pool operations must be serialized by the caller. No internal lock is
+ * provided. Slices can be queried/modified unlocked. But any pool operation
+ * (allocation, release, flush, ...) must be serialized.
+ */
+struct bus1_pool {
+	struct file *f;
+	size_t allocated_size;
+	struct list_head slices;
+	struct rb_root slices_busy;
+	struct rb_root slices_free;
+};
+
+#define BUS1_POOL_NULL ((struct bus1_pool){})
+
+int bus1_pool_init(struct bus1_pool *pool, const char *filename);
+void bus1_pool_deinit(struct bus1_pool *pool);
+
+struct bus1_pool_slice *bus1_pool_alloc(struct bus1_pool *pool, size_t size);
+struct bus1_pool_slice *bus1_pool_release_kernel(struct bus1_pool *pool,
+						 struct bus1_pool_slice *slice);
+void bus1_pool_publish(struct bus1_pool *pool, struct bus1_pool_slice *slice);
+int bus1_pool_release_user(struct bus1_pool *pool,
+			   size_t offset,
+			   size_t *n_slicesp);
+void bus1_pool_flush(struct bus1_pool *pool, size_t *n_slicesp);
+int bus1_pool_mmap(struct bus1_pool *pool, struct vm_area_struct *vma);
+
+ssize_t bus1_pool_write_iovec(struct bus1_pool *pool,
+			      struct bus1_pool_slice *slice,
+			      loff_t offset,
+			      struct iovec *iov,
+			      size_t n_iov,
+			      size_t total_len);
+ssize_t bus1_pool_write_kvec(struct bus1_pool *pool,
+			     struct bus1_pool_slice *slice,
+			     loff_t offset,
+			     struct kvec *iov,
+			     size_t n_iov,
+			     size_t total_len);
+
+/**
+ * bus1_pool_slice_is_public() - check whether a slice is public
+ * @slice:		slice to check
+ *
+ * This checks whether @slice is public. That is, bus1_pool_publish() has been
+ * called and the user has not released their reference, yet.
+ *
+ * Note that if you need reliable results, you better make sure this cannot
+ * race calls to bus1_pool_publish() or bus1_pool_release_user().
+ *
+ * Return: True if public, false if not.
+ */
+static inline bool bus1_pool_slice_is_public(struct bus1_pool_slice *slice)
+{
+	WARN_ON(!slice->ref_kernel);
+	return slice->ref_user;
+}
+
+#endif /* __BUS1_POOL_H */
-- 
2.10.1

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