drivers/block/brd.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * Ram backed block device driver.
  *
  * Copyright (C) 2007 Nick Piggin
  * Copyright (C) 2007 Novell Inc.
  *
  * Parts derived from drivers/block/rd.c, and drivers/block/loop.c, copyright
  * of their respective owners.
  */

 #include <linux/init.h>
 #include <linux/initrd.h>
 #include <linux/module.h>
 #include <linux/moduleparam.h>
 #include <linux/major.h>
 #include <linux/blkdev.h>
 #include <linux/bio.h>
 #include <linux/highmem.h>
 #include <linux/mutex.h>
 #include <linux/radix-tree.h>
 #include <linux/fs.h>
 #include <linux/slab.h>
 #include <linux/backing-dev.h>

 #include <linux/uaccess.h>

 #define PAGE_SECTORS_SHIFT	(PAGE_SHIFT - SECTOR_SHIFT)
 #define PAGE_SECTORS		(1 << PAGE_SECTORS_SHIFT)

 /*
  * Each block ramdisk device has a radix_tree brd_pages of pages that stores
  * the pages containing the block device's contents. A brd page's ->index is
  * its offset in PAGE_SIZE units. This is similar to, but in no way connected
  * with, the kernel's pagecache or buffer cache (which sit above our block
  * device).
  */
 struct brd_device {
 	int		brd_number;

 	struct request_queue	*brd_queue;
 	struct gendisk		*brd_disk;
 	struct list_head	brd_list;

 	/*
 	 * Backing store of pages and lock to protect it. This is the contents
 	 * of the block device.
 	 */
 	spinlock_t		brd_lock;
 	struct radix_tree_root	brd_pages;
 };

 /*
  * Look up and return a brd's page for a given sector.
  */
 static struct page *brd_lookup_page(struct brd_device *brd, sector_t sector)
 {
 	pgoff_t idx;
 	struct page *page;

 	/*
 	 * The page lifetime is protected by the fact that we have opened the
 	 * device node -- brd pages will never be deleted under us, so we
 	 * don't need any further locking or refcounting.
 	 *
 	 * This is strictly true for the radix-tree nodes as well (ie. we
 	 * don't actually need the rcu_read_lock()), however that is not a
 	 * documented feature of the radix-tree API so it is better to be
 	 * safe here (we don't have total exclusion from radix tree updates
 	 * here, only deletes).
 	 */
 	rcu_read_lock();
 	idx = sector >> PAGE_SECTORS_SHIFT; /* sector to page index */
 	page = radix_tree_lookup(&brd->brd_pages, idx);
 	rcu_read_unlock();

 	BUG_ON(page && page->index != idx);

 	return page;
 }

 /*
  * Look up and return a brd's page for a given sector.
  * If one does not exist, allocate an empty page, and insert that. Then
  * return it.
  */
 static struct page *brd_insert_page(struct brd_device *brd, sector_t sector)
 {
 	pgoff_t idx;
 	struct page *page;
 	gfp_t gfp_flags;

 	page = brd_lookup_page(brd, sector);
 	if (page)
 		return page;

 	/*
 	 * Must use NOIO because we don't want to recurse back into the
 	 * block or filesystem layers from page reclaim.
 	 */
 	gfp_flags = GFP_NOIO | __GFP_ZERO | __GFP_HIGHMEM;
 	page = alloc_page(gfp_flags);
 	if (!page)
 		return NULL;

 	if (radix_tree_preload(GFP_NOIO)) {
 		__free_page(page);
 		return NULL;
 	}

 	spin_lock(&brd->brd_lock);
 	idx = sector >> PAGE_SECTORS_SHIFT;
 	page->index = idx;
 	if (radix_tree_insert(&brd->brd_pages, idx, page)) {
 		__free_page(page);
 		page = radix_tree_lookup(&brd->brd_pages, idx);
 		BUG_ON(!page);
 		BUG_ON(page->index != idx);
 	}
 	spin_unlock(&brd->brd_lock);

 	radix_tree_preload_end();

 	return page;
 }

 /*
  * Free all backing store pages and radix tree. This must only be called when
  * there are no other users of the device.
  */
 #define FREE_BATCH 16
 static void brd_free_pages(struct brd_device *brd)
 {
 	unsigned long pos = 0;
 	struct page *pages[FREE_BATCH];
 	int nr_pages;

 	do {
 		int i;

 		nr_pages = radix_tree_gang_lookup(&brd->brd_pages,
 				(void **)pages, pos, FREE_BATCH);

 		for (i = 0; i < nr_pages; i++) {
 			void *ret;

 			BUG_ON(pages[i]->index < pos);
 			pos = pages[i]->index;
 			ret = radix_tree_delete(&brd->brd_pages, pos);
 			BUG_ON(!ret || ret != pages[i]);
 			__free_page(pages[i]);
 		}

 		pos++;

 		/*
 		 * It takes 3.4 seconds to remove 80GiB ramdisk.
 		 * So, we need cond_resched to avoid stalling the CPU.
 		 */
 		cond_resched();

 		/*
 		 * This assumes radix_tree_gang_lookup always returns as
 		 * many pages as possible. If the radix-tree code changes,
 		 * so will this have to.
 		 */
 	} while (nr_pages == FREE_BATCH);
 }

 /*
  * copy_to_brd_setup must be called before copy_to_brd. It may sleep.
  */
 static int copy_to_brd_setup(struct brd_device *brd, sector_t sector, size_t n)
 {
 	unsigned int offset = (sector & (PAGE_SECTORS-1)) << SECTOR_SHIFT;
 	size_t copy;

 	copy = min_t(size_t, n, PAGE_SIZE - offset);
 	if (!brd_insert_page(brd, sector))
 		return -ENOSPC;
 	if (copy < n) {
 		sector += copy >> SECTOR_SHIFT;
 		if (!brd_insert_page(brd, sector))
 			return -ENOSPC;
 	}
 	return 0;
 }

 /*
  * Copy n bytes from src to the brd starting at sector. Does not sleep.
  */
 static void copy_to_brd(struct brd_device *brd, const void *src,
 			sector_t sector, size_t n)
 {
 	struct page *page;
 	void *dst;
 	unsigned int offset = (sector & (PAGE_SECTORS-1)) << SECTOR_SHIFT;
 	size_t copy;

 	copy = min_t(size_t, n, PAGE_SIZE - offset);
 	page = brd_lookup_page(brd, sector);
 	BUG_ON(!page);

 	dst = kmap_atomic(page);
 	memcpy(dst + offset, src, copy);
 	kunmap_atomic(dst);

 	if (copy < n) {
 		src += copy;
 		sector += copy >> SECTOR_SHIFT;
 		copy = n - copy;
 		page = brd_lookup_page(brd, sector);
 		BUG_ON(!page);

 		dst = kmap_atomic(page);
 		memcpy(dst, src, copy);
 		kunmap_atomic(dst);
 	}
 }

 /*
  * Copy n bytes to dst from the brd starting at sector. Does not sleep.
  */
 static void copy_from_brd(void *dst, struct brd_device *brd,
 			sector_t sector, size_t n)
 {
 	struct page *page;
 	void *src;
 	unsigned int offset = (sector & (PAGE_SECTORS-1)) << SECTOR_SHIFT;
 	size_t copy;

 	copy = min_t(size_t, n, PAGE_SIZE - offset);
 	page = brd_lookup_page(brd, sector);
 	if (page) {
 		src = kmap_atomic(page);
 		memcpy(dst, src + offset, copy);
 		kunmap_atomic(src);
 	} else
 		memset(dst, 0, copy);

 	if (copy < n) {
 		dst += copy;
 		sector += copy >> SECTOR_SHIFT;
 		copy = n - copy;
 		page = brd_lookup_page(brd, sector);
 		if (page) {
 			src = kmap_atomic(page);
 			memcpy(dst, src, copy);
 			kunmap_atomic(src);
 		} else
 			memset(dst, 0, copy);
 	}
 }

 /*
  * Process a single bvec of a bio.
  */
 static int brd_do_bvec(struct brd_device *brd, struct page *page,
 			unsigned int len, unsigned int off, unsigned int op,
 			sector_t sector)
 {
 	void *mem;
 	int err = 0;

 	if (op_is_write(op)) {
 		err = copy_to_brd_setup(brd, sector, len);
 		if (err)
 			goto out;
 	}

 	mem = kmap_atomic(page);
 	if (!op_is_write(op)) {
 		copy_from_brd(mem + off, brd, sector, len);
 		flush_dcache_page(page);
 	} else {
 		flush_dcache_page(page);
 		copy_to_brd(brd, mem + off, sector, len);
 	}
 	kunmap_atomic(mem);

 out:
 	return err;
 }

 static blk_qc_t brd_submit_bio(struct bio *bio)
 {
 	struct brd_device *brd = bio->bi_disk->private_data;
 	struct bio_vec bvec;
 	sector_t sector;
 	struct bvec_iter iter;

 	sector = bio->bi_iter.bi_sector;
 	if (bio_end_sector(bio) > get_capacity(bio->bi_disk))
 		goto io_error;

 	bio_for_each_segment(bvec, bio, iter) {
 		unsigned int len = bvec.bv_len;
 		int err;

 		/* Don't support un-aligned buffer */
 		WARN_ON_ONCE((bvec.bv_offset & (SECTOR_SIZE - 1)) ||
 				(len & (SECTOR_SIZE - 1)));

 		err = brd_do_bvec(brd, bvec.bv_page, len, bvec.bv_offset,
 				  bio_op(bio), sector);
 		if (err)
 			goto io_error;
 		sector += len >> SECTOR_SHIFT;
 	}

 	bio_endio(bio);
 	return BLK_QC_T_NONE;
 io_error:
 	bio_io_error(bio);
 	return BLK_QC_T_NONE;
 }

 static int brd_rw_page(struct block_device *bdev, sector_t sector,
 		       struct page *page, unsigned int op)
 {
 	struct brd_device *brd = bdev->bd_disk->private_data;
 	int err;

 	if (PageTransHuge(page))
 		return -ENOTSUPP;
 	err = brd_do_bvec(brd, page, PAGE_SIZE, 0, op, sector);
 	page_endio(page, op_is_write(op), err);
 	return err;
 }

 static const struct block_device_operations brd_fops = {
 	.owner =		THIS_MODULE,
 	.submit_bio =		brd_submit_bio,
 	.rw_page =		brd_rw_page,
 };

 /*
  * And now the modules code and kernel interface.
  */
 static int rd_nr = CONFIG_BLK_DEV_RAM_COUNT;
 module_param(rd_nr, int, 0444);
 MODULE_PARM_DESC(rd_nr, "Maximum number of brd devices");

 unsigned long rd_size = CONFIG_BLK_DEV_RAM_SIZE;
 module_param(rd_size, ulong, 0444);
 MODULE_PARM_DESC(rd_size, "Size of each RAM disk in kbytes.");

 static int max_part = 1;
 module_param(max_part, int, 0444);
 MODULE_PARM_DESC(max_part, "Num Minors to reserve between devices");

 MODULE_LICENSE("GPL");
 MODULE_ALIAS_BLOCKDEV_MAJOR(RAMDISK_MAJOR);
 MODULE_ALIAS("rd");

 #ifndef MODULE
 /* Legacy boot options - nonmodular */
 static int __init ramdisk_size(char *str)
 {
 	rd_size = simple_strtol(str, NULL, 0);
 	return 1;
 }
 __setup("ramdisk_size=", ramdisk_size);
 #endif

 /*
  * The device scheme is derived from loop.c. Keep them in synch where possible
  * (should share code eventually).
  */
 static LIST_HEAD(brd_devices);
 static DEFINE_MUTEX(brd_devices_mutex);

 static struct brd_device *brd_alloc(int i)
 {
 	struct brd_device *brd;
 	struct gendisk *disk;

 	brd = kzalloc(sizeof(*brd), GFP_KERNEL);
 	if (!brd)
 		goto out;
 	brd->brd_number		= i;
 	spin_lock_init(&brd->brd_lock);
 	INIT_RADIX_TREE(&brd->brd_pages, GFP_ATOMIC);

 	brd->brd_queue = blk_alloc_queue(NUMA_NO_NODE);
 	if (!brd->brd_queue)
 		goto out_free_dev;

 	/* This is so fdisk will align partitions on 4k, because of
 	 * direct_access API needing 4k alignment, returning a PFN
 	 * (This is only a problem on very small devices <= 4M,
 	 *  otherwise fdisk will align on 1M. Regardless this call
 	 *  is harmless)
 	 */
 	blk_queue_physical_block_size(brd->brd_queue, PAGE_SIZE);
 	disk = brd->brd_disk = alloc_disk(max_part);
 	if (!disk)
 		goto out_free_queue;
 	disk->major		= RAMDISK_MAJOR;
 	disk->first_minor	= i * max_part;
 	disk->fops		= &brd_fops;
 	disk->private_data	= brd;
 	disk->flags		= GENHD_FL_EXT_DEVT;
 	sprintf(disk->disk_name, "ram%d", i);
 	set_capacity(disk, rd_size * 2);
 	brd->brd_queue->backing_dev_info->capabilities |= BDI_CAP_SYNCHRONOUS_IO;

 	/* Tell the block layer that this is not a rotational device */
 	blk_queue_flag_set(QUEUE_FLAG_NONROT, brd->brd_queue);
 	blk_queue_flag_clear(QUEUE_FLAG_ADD_RANDOM, brd->brd_queue);

 	return brd;

 out_free_queue:
 	blk_cleanup_queue(brd->brd_queue);
 out_free_dev:
 	kfree(brd);
 out:
 	return NULL;
 }

 static void brd_free(struct brd_device *brd)
 {
 	put_disk(brd->brd_disk);
 	blk_cleanup_queue(brd->brd_queue);
 	brd_free_pages(brd);
 	kfree(brd);
 }

 static struct brd_device *brd_init_one(int i, bool *new)
 {
 	struct brd_device *brd;

 	*new = false;
 	list_for_each_entry(brd, &brd_devices, brd_list) {
 		if (brd->brd_number == i)
 			goto out;
 	}

 	brd = brd_alloc(i);
 	if (brd) {
 		brd->brd_disk->queue = brd->brd_queue;
 		add_disk(brd->brd_disk);
 		list_add_tail(&brd->brd_list, &brd_devices);
 	}
 	*new = true;
 out:
 	return brd;
 }

 static void brd_del_one(struct brd_device *brd)
 {
 	list_del(&brd->brd_list);
 	del_gendisk(brd->brd_disk);
 	brd_free(brd);
 }

 static struct kobject *brd_probe(dev_t dev, int *part, void *data)
 {
 	struct brd_device *brd;
 	struct kobject *kobj;
 	bool new;

 	mutex_lock(&brd_devices_mutex);
 	brd = brd_init_one(MINOR(dev) / max_part, &new);
 	kobj = brd ? get_disk_and_module(brd->brd_disk) : NULL;
 	mutex_unlock(&brd_devices_mutex);

 	if (new)
 		*part = 0;

 	return kobj;
 }

 static inline void brd_check_and_reset_par(void)
 {
 	if (unlikely(!max_part))
 		max_part = 1;

 	/*
 	 * make sure 'max_part' can be divided exactly by (1U << MINORBITS),
 	 * otherwise, it is possiable to get same dev_t when adding partitions.
 	 */
 	if ((1U << MINORBITS) % max_part != 0)
 		max_part = 1UL << fls(max_part);

 	if (max_part > DISK_MAX_PARTS) {
 		pr_info("brd: max_part can't be larger than %d, reset max_part = %d.\n",
 			DISK_MAX_PARTS, DISK_MAX_PARTS);
 		max_part = DISK_MAX_PARTS;
 	}
 }

 static int __init brd_init(void)
 {
 	struct brd_device *brd, *next;
 	int i;

 	/*
 	 * brd module now has a feature to instantiate underlying device
 	 * structure on-demand, provided that there is an access dev node.
 	 *
 	 * (1) if rd_nr is specified, create that many upfront. else
 	 *     it defaults to CONFIG_BLK_DEV_RAM_COUNT
 	 * (2) User can further extend brd devices by create dev node themselves
 	 *     and have kernel automatically instantiate actual device
 	 *     on-demand. Example:
 	 *		mknod /path/devnod_name b 1 X	# 1 is the rd major
 	 *		fdisk -l /path/devnod_name
 	 *	If (X / max_part) was not already created it will be created
 	 *	dynamically.
 	 */

 	if (register_blkdev(RAMDISK_MAJOR, "ramdisk"))
 		return -EIO;

 	brd_check_and_reset_par();

 	for (i = 0; i < rd_nr; i++) {
 		brd = brd_alloc(i);
 		if (!brd)
 			goto out_free;
 		list_add_tail(&brd->brd_list, &brd_devices);
 	}

 	/* point of no return */

 	list_for_each_entry(brd, &brd_devices, brd_list) {
 		/*
 		 * associate with queue just before adding disk for
 		 * avoiding to mess up failure path
 		 */
 		brd->brd_disk->queue = brd->brd_queue;
 		add_disk(brd->brd_disk);
 	}

 	blk_register_region(MKDEV(RAMDISK_MAJOR, 0), 1UL << MINORBITS,
 				  THIS_MODULE, brd_probe, NULL, NULL);

 	pr_info("brd: module loaded\n");
 	return 0;

 out_free:
 	list_for_each_entry_safe(brd, next, &brd_devices, brd_list) {
 		list_del(&brd->brd_list);
 		brd_free(brd);
 	}
 	unregister_blkdev(RAMDISK_MAJOR, "ramdisk");

 	pr_info("brd: module NOT loaded !!!\n");
 	return -ENOMEM;
 }

 static void __exit brd_exit(void)
 {
 	struct brd_device *brd, *next;

 	list_for_each_entry_safe(brd, next, &brd_devices, brd_list)
 		brd_del_one(brd);

 	blk_unregister_region(MKDEV(RAMDISK_MAJOR, 0), 1UL << MINORBITS);
 	unregister_blkdev(RAMDISK_MAJOR, "ramdisk");

 	pr_info("brd: module unloaded\n");
 }

 module_init(brd_init);
 module_exit(brd_exit);
	// SPDX-License-Identifier: GPL-2.0-only
	/*
	* Ram backed block device driver.
	*
	* Copyright (C) 2007 Nick Piggin
	* Copyright (C) 2007 Novell Inc.
	*
	* Parts derived from drivers/block/rd.c, and drivers/block/loop.c, copyright
	* of their respective owners.
	*/

	#include <linux/init.h>
	#include <linux/initrd.h>
	#include <linux/module.h>
	#include <linux/moduleparam.h>
	#include <linux/major.h>
	#include <linux/blkdev.h>
	#include <linux/bio.h>
	#include <linux/highmem.h>
	#include <linux/mutex.h>
	#include <linux/radix-tree.h>
	#include <linux/fs.h>
	#include <linux/slab.h>
	#include <linux/backing-dev.h>

	#include <linux/uaccess.h>

	#define PAGE_SECTORS_SHIFT (PAGE_SHIFT - SECTOR_SHIFT)
	#define PAGE_SECTORS (1 << PAGE_SECTORS_SHIFT)

	/*
	* Each block ramdisk device has a radix_tree brd_pages of pages that stores
	* the pages containing the block device's contents. A brd page's ->index is
	* its offset in PAGE_SIZE units. This is similar to, but in no way connected
	* with, the kernel's pagecache or buffer cache (which sit above our block
	* device).
	*/
	struct brd_device {
	int brd_number;

	struct request_queue *brd_queue;
	struct gendisk *brd_disk;
	struct list_head brd_list;

	/*
	* Backing store of pages and lock to protect it. This is the contents
	* of the block device.
	*/
	spinlock_t brd_lock;
	struct radix_tree_root brd_pages;
	};

	/*
	* Look up and return a brd's page for a given sector.
	*/
	static struct page brd_lookup_page(struct brd_device brd, sector_t sector)
	{
	pgoff_t idx;
	struct page *page;

	/*
	* The page lifetime is protected by the fact that we have opened the
	* device node -- brd pages will never be deleted under us, so we
	* don't need any further locking or refcounting.
	*
	* This is strictly true for the radix-tree nodes as well (ie. we
	* don't actually need the rcu_read_lock()), however that is not a
	* documented feature of the radix-tree API so it is better to be
	* safe here (we don't have total exclusion from radix tree updates
	* here, only deletes).
	*/
	rcu_read_lock();
	idx = sector >> PAGE_SECTORS_SHIFT; /* sector to page index */
	page = radix_tree_lookup(&brd->brd_pages, idx);
	rcu_read_unlock();

	BUG_ON(page && page->index != idx);

	return page;
	}

	/*
	* Look up and return a brd's page for a given sector.
	* If one does not exist, allocate an empty page, and insert that. Then
	* return it.
	*/
	static struct page brd_insert_page(struct brd_device brd, sector_t sector)
	{
	pgoff_t idx;
	struct page *page;
	gfp_t gfp_flags;

	page = brd_lookup_page(brd, sector);
	if (page)
	return page;

	/*
	* Must use NOIO because we don't want to recurse back into the
	* block or filesystem layers from page reclaim.
	*/
	gfp_flags = GFP_NOIO \| __GFP_ZERO \| __GFP_HIGHMEM;
	page = alloc_page(gfp_flags);
	if (!page)
	return NULL;

	if (radix_tree_preload(GFP_NOIO)) {
	__free_page(page);
	return NULL;
	}

	spin_lock(&brd->brd_lock);
	idx = sector >> PAGE_SECTORS_SHIFT;
	page->index = idx;
	if (radix_tree_insert(&brd->brd_pages, idx, page)) {
	__free_page(page);
	page = radix_tree_lookup(&brd->brd_pages, idx);
	BUG_ON(!page);
	BUG_ON(page->index != idx);
	}
	spin_unlock(&brd->brd_lock);

	radix_tree_preload_end();

	return page;
	}

	/*
	* Free all backing store pages and radix tree. This must only be called when
	* there are no other users of the device.
	*/
	#define FREE_BATCH 16
	static void brd_free_pages(struct brd_device *brd)
	{
	unsigned long pos = 0;
	struct page *pages[FREE_BATCH];
	int nr_pages;

	do {
	int i;

	nr_pages = radix_tree_gang_lookup(&brd->brd_pages,
	(void **)pages, pos, FREE_BATCH);

	for (i = 0; i < nr_pages; i++) {
	void *ret;

	BUG_ON(pages[i]->index < pos);
	pos = pages[i]->index;
	ret = radix_tree_delete(&brd->brd_pages, pos);
	BUG_ON(!ret \|\| ret != pages[i]);
	__free_page(pages[i]);
	}

	pos++;

	/*
	* It takes 3.4 seconds to remove 80GiB ramdisk.
	* So, we need cond_resched to avoid stalling the CPU.
	*/
	cond_resched();

	/*
	* This assumes radix_tree_gang_lookup always returns as
	* many pages as possible. If the radix-tree code changes,
	* so will this have to.
	*/
	} while (nr_pages == FREE_BATCH);
	}

	/*
	* copy_to_brd_setup must be called before copy_to_brd. It may sleep.
	*/
	static int copy_to_brd_setup(struct brd_device *brd, sector_t sector, size_t n)
	{
	unsigned int offset = (sector & (PAGE_SECTORS-1)) << SECTOR_SHIFT;
	size_t copy;

	copy = min_t(size_t, n, PAGE_SIZE - offset);
	if (!brd_insert_page(brd, sector))
	return -ENOSPC;
	if (copy < n) {
	sector += copy >> SECTOR_SHIFT;
	if (!brd_insert_page(brd, sector))
	return -ENOSPC;
	}
	return 0;
	}

	/*
	* Copy n bytes from src to the brd starting at sector. Does not sleep.
	*/
	static void copy_to_brd(struct brd_device brd, const void src,
	sector_t sector, size_t n)
	{
	struct page *page;
	void *dst;
	unsigned int offset = (sector & (PAGE_SECTORS-1)) << SECTOR_SHIFT;
	size_t copy;

	copy = min_t(size_t, n, PAGE_SIZE - offset);
	page = brd_lookup_page(brd, sector);
	BUG_ON(!page);

	dst = kmap_atomic(page);
	memcpy(dst + offset, src, copy);
	kunmap_atomic(dst);

	if (copy < n) {
	src += copy;
	sector += copy >> SECTOR_SHIFT;
	copy = n - copy;
	page = brd_lookup_page(brd, sector);
	BUG_ON(!page);

	dst = kmap_atomic(page);
	memcpy(dst, src, copy);
	kunmap_atomic(dst);
	}
	}

	/*
	* Copy n bytes to dst from the brd starting at sector. Does not sleep.
	*/
	static void copy_from_brd(void dst, struct brd_device brd,
	sector_t sector, size_t n)
	{
	struct page *page;
	void *src;
	unsigned int offset = (sector & (PAGE_SECTORS-1)) << SECTOR_SHIFT;
	size_t copy;

	copy = min_t(size_t, n, PAGE_SIZE - offset);
	page = brd_lookup_page(brd, sector);
	if (page) {
	src = kmap_atomic(page);
	memcpy(dst, src + offset, copy);
	kunmap_atomic(src);
	} else
	memset(dst, 0, copy);

	if (copy < n) {
	dst += copy;
	sector += copy >> SECTOR_SHIFT;
	copy = n - copy;
	page = brd_lookup_page(brd, sector);
	if (page) {
	src = kmap_atomic(page);
	memcpy(dst, src, copy);
	kunmap_atomic(src);
	} else
	memset(dst, 0, copy);
	}
	}

	/*
	* Process a single bvec of a bio.
	*/
	static int brd_do_bvec(struct brd_device brd, struct page page,
	unsigned int len, unsigned int off, unsigned int op,
	sector_t sector)
	{
	void *mem;
	int err = 0;

	if (op_is_write(op)) {
	err = copy_to_brd_setup(brd, sector, len);
	if (err)
	goto out;
	}

	mem = kmap_atomic(page);
	if (!op_is_write(op)) {
	copy_from_brd(mem + off, brd, sector, len);
	flush_dcache_page(page);
	} else {
	flush_dcache_page(page);
	copy_to_brd(brd, mem + off, sector, len);
	}
	kunmap_atomic(mem);

	out:
	return err;
	}

	static blk_qc_t brd_submit_bio(struct bio *bio)
	{
	struct brd_device *brd = bio->bi_disk->private_data;
	struct bio_vec bvec;
	sector_t sector;
	struct bvec_iter iter;

	sector = bio->bi_iter.bi_sector;
	if (bio_end_sector(bio) > get_capacity(bio->bi_disk))
	goto io_error;

	bio_for_each_segment(bvec, bio, iter) {
	unsigned int len = bvec.bv_len;
	int err;

	/* Don't support un-aligned buffer */
	WARN_ON_ONCE((bvec.bv_offset & (SECTOR_SIZE - 1)) \|\|
	(len & (SECTOR_SIZE - 1)));

	err = brd_do_bvec(brd, bvec.bv_page, len, bvec.bv_offset,
	bio_op(bio), sector);
	if (err)
	goto io_error;
	sector += len >> SECTOR_SHIFT;
	}

	bio_endio(bio);
	return BLK_QC_T_NONE;
	io_error:
	bio_io_error(bio);
	return BLK_QC_T_NONE;
	}

	static int brd_rw_page(struct block_device *bdev, sector_t sector,
	struct page *page, unsigned int op)
	{
	struct brd_device *brd = bdev->bd_disk->private_data;
	int err;

	if (PageTransHuge(page))
	return -ENOTSUPP;
	err = brd_do_bvec(brd, page, PAGE_SIZE, 0, op, sector);
	page_endio(page, op_is_write(op), err);
	return err;
	}

	static const struct block_device_operations brd_fops = {
	.owner = THIS_MODULE,
	.submit_bio = brd_submit_bio,
	.rw_page = brd_rw_page,
	};

	/*
	* And now the modules code and kernel interface.
	*/
	static int rd_nr = CONFIG_BLK_DEV_RAM_COUNT;
	module_param(rd_nr, int, 0444);
	MODULE_PARM_DESC(rd_nr, "Maximum number of brd devices");

	unsigned long rd_size = CONFIG_BLK_DEV_RAM_SIZE;
	module_param(rd_size, ulong, 0444);
	MODULE_PARM_DESC(rd_size, "Size of each RAM disk in kbytes.");

	static int max_part = 1;
	module_param(max_part, int, 0444);
	MODULE_PARM_DESC(max_part, "Num Minors to reserve between devices");

	MODULE_LICENSE("GPL");
	MODULE_ALIAS_BLOCKDEV_MAJOR(RAMDISK_MAJOR);
	MODULE_ALIAS("rd");

	#ifndef MODULE
	/* Legacy boot options - nonmodular */
	static int __init ramdisk_size(char *str)
	{
	rd_size = simple_strtol(str, NULL, 0);
	return 1;
	}
	__setup("ramdisk_size=", ramdisk_size);
	#endif

	/*
	* The device scheme is derived from loop.c. Keep them in synch where possible
	* (should share code eventually).
	*/
	static LIST_HEAD(brd_devices);
	static DEFINE_MUTEX(brd_devices_mutex);

	static struct brd_device *brd_alloc(int i)
	{
	struct brd_device *brd;
	struct gendisk *disk;

	brd = kzalloc(sizeof(*brd), GFP_KERNEL);
	if (!brd)
	goto out;
	brd->brd_number = i;
	spin_lock_init(&brd->brd_lock);
	INIT_RADIX_TREE(&brd->brd_pages, GFP_ATOMIC);

	brd->brd_queue = blk_alloc_queue(NUMA_NO_NODE);
	if (!brd->brd_queue)
	goto out_free_dev;

	/* This is so fdisk will align partitions on 4k, because of
	* direct_access API needing 4k alignment, returning a PFN
	* (This is only a problem on very small devices <= 4M,
	* otherwise fdisk will align on 1M. Regardless this call
	* is harmless)
	*/
	blk_queue_physical_block_size(brd->brd_queue, PAGE_SIZE);
	disk = brd->brd_disk = alloc_disk(max_part);
	if (!disk)
	goto out_free_queue;
	disk->major = RAMDISK_MAJOR;
	disk->first_minor = i * max_part;
	disk->fops = &brd_fops;
	disk->private_data = brd;
	disk->flags = GENHD_FL_EXT_DEVT;
	sprintf(disk->disk_name, "ram%d", i);
	set_capacity(disk, rd_size * 2);
	brd->brd_queue->backing_dev_info->capabilities \|= BDI_CAP_SYNCHRONOUS_IO;

	/* Tell the block layer that this is not a rotational device */
	blk_queue_flag_set(QUEUE_FLAG_NONROT, brd->brd_queue);
	blk_queue_flag_clear(QUEUE_FLAG_ADD_RANDOM, brd->brd_queue);

	return brd;

	out_free_queue:
	blk_cleanup_queue(brd->brd_queue);
	out_free_dev:
	kfree(brd);
	out:
	return NULL;
	}

	static void brd_free(struct brd_device *brd)
	{
	put_disk(brd->brd_disk);
	blk_cleanup_queue(brd->brd_queue);
	brd_free_pages(brd);
	kfree(brd);
	}

	static struct brd_device brd_init_one(int i, bool new)
	{
	struct brd_device *brd;

	*new = false;
	list_for_each_entry(brd, &brd_devices, brd_list) {
	if (brd->brd_number == i)
	goto out;
	}

	brd = brd_alloc(i);
	if (brd) {
	brd->brd_disk->queue = brd->brd_queue;
	add_disk(brd->brd_disk);
	list_add_tail(&brd->brd_list, &brd_devices);
	}
	*new = true;
	out:
	return brd;
	}

	static void brd_del_one(struct brd_device *brd)
	{
	list_del(&brd->brd_list);
	del_gendisk(brd->brd_disk);
	brd_free(brd);
	}

	static struct kobject brd_probe(dev_t dev, int part, void *data)
	{
	struct brd_device *brd;
	struct kobject *kobj;
	bool new;

	mutex_lock(&brd_devices_mutex);
	brd = brd_init_one(MINOR(dev) / max_part, &new);
	kobj = brd ? get_disk_and_module(brd->brd_disk) : NULL;
	mutex_unlock(&brd_devices_mutex);

	if (new)
	*part = 0;

	return kobj;
	}

	static inline void brd_check_and_reset_par(void)
	{
	if (unlikely(!max_part))
	max_part = 1;

	/*
	* make sure 'max_part' can be divided exactly by (1U << MINORBITS),
	* otherwise, it is possiable to get same dev_t when adding partitions.
	*/
	if ((1U << MINORBITS) % max_part != 0)
	max_part = 1UL << fls(max_part);

	if (max_part > DISK_MAX_PARTS) {
	pr_info("brd: max_part can't be larger than %d, reset max_part = %d.\n",
	DISK_MAX_PARTS, DISK_MAX_PARTS);
	max_part = DISK_MAX_PARTS;
	}
	}

	static int __init brd_init(void)
	{
	struct brd_device brd, next;
	int i;

	/*
	* brd module now has a feature to instantiate underlying device
	* structure on-demand, provided that there is an access dev node.
	*
	* (1) if rd_nr is specified, create that many upfront. else
	* it defaults to CONFIG_BLK_DEV_RAM_COUNT
	* (2) User can further extend brd devices by create dev node themselves
	* and have kernel automatically instantiate actual device
	* on-demand. Example:
	* mknod /path/devnod_name b 1 X # 1 is the rd major
	* fdisk -l /path/devnod_name
	* If (X / max_part) was not already created it will be created
	* dynamically.
	*/

	if (register_blkdev(RAMDISK_MAJOR, "ramdisk"))
	return -EIO;

	brd_check_and_reset_par();

	for (i = 0; i < rd_nr; i++) {
	brd = brd_alloc(i);
	if (!brd)
	goto out_free;
	list_add_tail(&brd->brd_list, &brd_devices);
	}

	/* point of no return */

	list_for_each_entry(brd, &brd_devices, brd_list) {
	/*
	* associate with queue just before adding disk for
	* avoiding to mess up failure path
	*/
	brd->brd_disk->queue = brd->brd_queue;
	add_disk(brd->brd_disk);
	}

	blk_register_region(MKDEV(RAMDISK_MAJOR, 0), 1UL << MINORBITS,
	THIS_MODULE, brd_probe, NULL, NULL);

	pr_info("brd: module loaded\n");
	return 0;

	out_free:
	list_for_each_entry_safe(brd, next, &brd_devices, brd_list) {
	list_del(&brd->brd_list);
	brd_free(brd);
	}
	unregister_blkdev(RAMDISK_MAJOR, "ramdisk");

	pr_info("brd: module NOT loaded !!!\n");
	return -ENOMEM;
	}

	static void __exit brd_exit(void)
	{
	struct brd_device brd, next;

	list_for_each_entry_safe(brd, next, &brd_devices, brd_list)
	brd_del_one(brd);

	blk_unregister_region(MKDEV(RAMDISK_MAJOR, 0), 1UL << MINORBITS);
	unregister_blkdev(RAMDISK_MAJOR, "ramdisk");

	pr_info("brd: module unloaded\n");
	}

	module_init(brd_init);
	module_exit(brd_exit);