[PATCH] Loop device - Tracking page writes made to a loop devicethrough mmap

From: Kandan Venkataraman
Date: Tue Mar 13 2007 - 16:22:16 EST

All comments have been taken care of.

Description:

A file_operations structure variable called loop_fops is initialised with
the default block device file operations (def_blk_fops).
The mmap operation is overriden with a new function called loop_file_mmap.

A vm_operations structure variable called loop_file_vm_ops is initialised
with the default operations for a disk file.
The page_mkwrite operation in this variable is initialised to a new
function called loop_track_pgwrites.

In the function lo_open, the file operations pointer of the device file is
initialised with the address of loop_fops.

The function loop_file_mmap simply calls generic_file_mmap and then
initialises the vm_ops of the vma with address of loop_file_vm_ops.

The function loop_track_pgwrites stores the page offset of the page that
is being written to, in a red-black tree within the loop device.

A flag lo_track_pgwrite has been added to the struct loop_device to turn
on/off tracking of page writes.

Three new ioctls have been added.

The ioctl cmd LOOP_SET_TRACK_PGWRITE sets/unsets the flag lo_track_pgwrite
in the struct loop_device.
The ioctl cmd LOOP_GET_PGWRITES retrieves the page offsets of pages that
have been written to.
The ioctl cmd LOOP_CLR_PGWRITES empties the red-black tree.

The existing interface has been left untouched. Thus applications such as losetup would work as is.

The functionality added would allow us to have a read only version and a write version of memory by doing the following:
Associate a normal file as backing storage for the loop device and mmap
to the loop device. Call this mmapped address space as area1.
Mmap to a normal file of identical size. Call this mmapped address space
as area2.

Changes made to area1 can be periodically copied to area2 using the ioctl
cmds (retreive dirty page offsets and copy the dirty pages from area1 to
area2). This facility would provide a quick way of updating the read only
version.

Motivation for new ioctls:

Imagine a business server application which processes messages from
clients as they come in (say over a TCP connection).
Some of those messages may be transactions, i.e. they cause data changes
in the application.
Rest of those messages may be queries i.e. they get information from the
application.
The application can consist of two processes. One process will handle the
transactions.
The other process will handle the queries. Each process will have its own
copy of the business data.
The process handling transactions can mmap to the loop device for its copy
of the memory. The loop device must have a normal file for its backing
storage.
The process handling queries can mmap to another normal file for its copy
of the memory. Both these memories have identical data at the beginning.
Queries and transactions can now be handled simultaneously by the
respective processes.
The query process can update its memory periodically by obtaining the
changes that have have happened to the loop device.
By using the ioctl call to retrieve the dirty page offsets, only the dirty
pages need to be copied over to the query process's copy of memory. We can
infact have multiple processes to handle queries sharing the same memory.
During this copy over, the transaction process will hold off processing
transactions till the update is complete.

This would be very useful for high speed in-memory transaction systems,
where the query load can be passed of to other processes. Example of such
systems would be a stock trading system, where clients buy and sell
stock(equity, options etc).
At the same time lot of clients would be downloading market data and this
can be done independently of the transactions.

This new facility will provide a way of tracking changes made to business
data, independent of the application domain.


Test program:

Before you run the test program, please create the backing storage file
for the loop device as follows

dd if=/dev/zero of=/root/file bs=4K count=10

Set bs to be whatever pagesize is in your machine. In my machine it was
4K.


#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <string.h>
#include <assert.h>
#include <signal.h>
#include <errno.h>
#include <linux/loop.h>

int main()
{
int maxPages = 10;
char* start = 0;
int fd;
int dfd;
int *array = 0;
int pageSize;
int elemsPerPage;
struct loop_info64 info;
struct loop_pgoff_array pgarray;

pgarray.max = maxPages;
pgarray.pgoff = calloc(maxPages, sizeof(long));

if (pgarray.pgoff == NULL) {
fprintf(stderr, "can't create pgarray\n");
exit(1);
}

pageSize = getpagesize();
elemsPerPage = pageSize/sizeof(int);

/* open the device file */
if ((fd = open ("/dev/loop0", O_RDWR, S_IRWXU)) < 0) {
fprintf(stderr, "can't create device file for writing\n");
goto out5;
}

/* open the disk file to set as backing storage*/
if ((dfd = open ("/root/file", O_RDWR, S_IRWXU)) < 0) {
fprintf(stderr, "can't create device file for writing\n");
goto out4;
}

if (ioctl(fd, LOOP_SET_FD, dfd) < 0) {
perror("ioctl: LOOP_SET_FD");
goto out3;
}

if ((start = mmap(0, maxPages * pageSize, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0)) == MAP_FAILED) {
perror("mmap error");
goto out2;
}


if (ioctl(fd, LOOP_SET_TRACK_PGWRITE, 1) < 0) {
perror("ioctl: LOOP_SET_TRACK_PGWRITE");
goto out1;
}

if (ioctl(fd, LOOP_CLR_PGWRITES, 0) < 0) {
perror("ioctl: LOOP_CLR_PGWRITES");
goto out1;
}

array = (int *)start;

array[0] = 5;

fprintf(stderr, "value = %d\n", array[0]);

array[1] = 9;

fprintf(stderr, "value = %d\n", array[1]);

array[elemsPerPage] = 14;

fprintf(stderr, "value = %d\n", array[elemsPerPage]);

array[3*elemsPerPage+60] = 35;

fprintf(stderr, "value = %d\n", array[3*elemsPerPage+60]);

if (ioctl(fd, LOOP_GET_PGWRITES, &pgarray) < 0) {
perror("ioctl: LOOP_GET_PGWRITES");
goto out1;
}

int i;
for (i= 0; i < pgarray.num; i++)
fprintf(stderr, "offset %ld\n", pgarray.pgoff[i]);

out1:
munmap(start, maxPages * pageSize);
out2:
ioctl(fd, LOOP_CLR_FD, 0);
out3:
close(dfd);
out4:
close(fd);
out5:
return 0;
}



Signed-off-by: Kandan Venkataraman kandan.venkataraman@xxxxxxxxxxxx


diff -uprN linux-2.6.19.2/drivers/block/loop.c linux-2.6.19.2-new/drivers/block/loop.c
--- linux-2.6.19.2/drivers/block/loop.c 2007-01-11 06:10:37.000000000 +1100
+++ linux-2.6.19.2-new/drivers/block/loop.c 2007-03-11 07:31:26.000000000 +1100
@@ -74,12 +74,16 @@
#include <linux/highmem.h>
#include <linux/gfp.h>
#include <linux/kthread.h>
+#include <linux/mm.h>

#include <asm/uaccess.h>

static int max_loop = 8;
static struct loop_device *loop_dev;
static struct gendisk **disks;
+static kmem_cache_t *pgoff_elem_cache;
+static char *cache_name = "loop_pgoff_elem_cache";
+static struct file_operations loop_fops;

/*
* Transfer functions
@@ -646,6 +650,85 @@ static void do_loop_switch(struct loop_d
complete(&p->wait);
}

+static void pgoff_tree_clear(struct rb_root *rb_root)
+{
+ struct rb_node *rb_node = rb_root->rb_node;
+
+ while (rb_node != NULL) {
+
+ rb_erase(rb_node, rb_root);
+ kmem_cache_free(pgoff_elem_cache, rb_entry(rb_node,
+ struct pgoff_elem, node));
+ rb_node = rb_root->rb_node;
+ }
+
+ *rb_root = RB_ROOT;
+}
+
+static int loop_set_track_pgwrite(struct loop_device *lo, unsigned long arg)
+{
+ if (lo->lo_state != Lo_bound)
+ return -ENXIO;
+
+ if (arg)
+ lo->lo_track_pgwrite = 1;
+ else {
+ if (lo->lo_track_pgwrite)
+ pgoff_tree_clear(&lo->pgoff_tree);
+ lo->lo_track_pgwrite = 0;
+ }
+
+ return 0;
+}
+
+static int loop_clr_pgwrites(struct loop_device *lo)
+{
+ struct file *filp = lo->lo_backing_file;
+
+ if (lo->lo_state != Lo_bound)
+ return -ENXIO;
+
+ if (filp == NULL || !lo->lo_track_pgwrite)
+ return -EINVAL;
+
+ pgoff_tree_clear(&lo->pgoff_tree);
+
+ return 0;
+}
+
+static int loop_get_pgwrites(struct loop_device *lo,
+ struct loop_pgoff_array __user *arg)
+{
+ struct file *filp = lo->lo_backing_file;
+ struct loop_pgoff_array array;
+ loff_t i = 0;
+ struct rb_node *rb_node = rb_first(&lo->pgoff_tree);
+
+ if (lo->lo_state != Lo_bound)
+ return -ENXIO;
+
+ if (filp == NULL || !lo->lo_track_pgwrite)
+ return -EINVAL;
+
+ if (copy_from_user(&array, arg, sizeof (struct loop_pgoff_array)))
+ return -EFAULT;
+
+ while (i < array.max && rb_node != NULL) {
+
+ if (put_user(rb_entry(rb_node, struct pgoff_elem, node)->offset,
+ array.pgoff + i))
+ return -EFAULT;
+
+ ++i;
+ rb_node = rb_next(rb_node);
+ }
+ array.num = i;
+
+ if (copy_to_user(arg, &array, sizeof(array)))
+ return -EFAULT;
+
+ return 0;
+}

/*
* loop_change_fd switched the backing store of a loopback device to
@@ -692,6 +775,8 @@ static int loop_change_fd(struct loop_de
if (get_loop_size(lo, file) != get_loop_size(lo, old_file))
goto out_putf;

+ pgoff_tree_clear(&lo->pgoff_tree);
+
/* and ... switch */
error = loop_switch(lo, file);
if (error)
@@ -799,6 +884,8 @@ static int loop_set_fd(struct loop_devic
lo->transfer = transfer_none;
lo->ioctl = NULL;
lo->lo_sizelimit = 0;
+ lo->lo_track_pgwrite = 0;
+ lo->pgoff_tree = RB_ROOT;
lo->old_gfp_mask = mapping_gfp_mask(mapping);
mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));

@@ -913,6 +1000,8 @@ static int loop_clr_fd(struct loop_devic
lo->lo_sizelimit = 0;
lo->lo_encrypt_key_size = 0;
lo->lo_flags = 0;
+ lo->lo_track_pgwrite = 0;
+ pgoff_tree_clear(&lo->pgoff_tree);
lo->lo_thread = NULL;
memset(lo->lo_encrypt_key, 0, LO_KEY_SIZE);
memset(lo->lo_crypt_name, 0, LO_NAME_SIZE);
@@ -1159,6 +1248,15 @@ static int lo_ioctl(struct inode * inode
case LOOP_GET_STATUS64:
err = loop_get_status64(lo, (struct loop_info64 __user *) arg);
break;
+ case LOOP_SET_TRACK_PGWRITE:
+ err = loop_set_track_pgwrite(lo, arg);
+ break;
+ case LOOP_GET_PGWRITES:
+ err = loop_get_pgwrites(lo, (struct loop_pgoff_array __user *) arg);
+ break;
+ case LOOP_CLR_PGWRITES:
+ err = loop_clr_pgwrites(lo);
+ break;
default:
err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
}
@@ -1322,10 +1420,67 @@ static long lo_compat_ioctl(struct file
}
#endif

+static int pgoff_tree_insert(struct rb_root *rb_root, unsigned long offset)
+{
+ struct rb_node **p = &rb_root->rb_node;
+ struct rb_node *parent = NULL;
+ struct pgoff_elem *pgoff_elem;
+
+ while (*p) {
+ parent = *p;
+ pgoff_elem = rb_entry(parent, struct pgoff_elem, node);
+
+ if (offset < pgoff_elem->offset)
+ p = &(*p)->rb_left;
+ else if (offset > pgoff_elem->offset)
+ p = &(*p)->rb_right;
+ else
+ return 0;
+ }
+
+ pgoff_elem = kmem_cache_alloc(pgoff_elem_cache, GFP_KERNEL);
+ if (!pgoff_elem)
+ return -ENOMEM;
+ pgoff_elem->offset = offset;
+
+ rb_link_node(&pgoff_elem->node, parent, p);
+ rb_insert_color(&pgoff_elem->node, rb_root);
+
+ return 0;
+}
+
+static int loop_track_pgwrites(struct vm_area_struct *vma, struct page *page)
+{
+ struct file *file = vma->vm_file;
+ struct inode *inode = file->f_dentry->d_inode;
+ struct loop_device *lo = inode->i_bdev->bd_disk->private_data;
+
+ return pgoff_tree_insert(&lo->pgoff_tree, page->index);
+}
+
+struct vm_operations_struct loop_file_vm_ops = {
+ .nopage = filemap_nopage,
+ .populate = filemap_populate,
+ .page_mkwrite = loop_track_pgwrites
+};
+
+static int loop_file_mmap(struct file * file, struct vm_area_struct * vma)
+{
+ /* This is used for a general mmap of a disk file */
+ int err = generic_file_mmap(file, vma);
+
+ if (err)
+ return err;
+
+ vma->vm_ops = &loop_file_vm_ops;
+ return 0;
+}
+
static int lo_open(struct inode *inode, struct file *file)
{
struct loop_device *lo = inode->i_bdev->bd_disk->private_data;

+ file->f_op = &loop_fops;
mutex_lock(&lo->lo_ctl_mutex);
lo->lo_refcnt++;
mutex_unlock(&lo->lo_ctl_mutex);
@@ -1398,10 +1553,24 @@ int loop_unregister_transfer(int number)
EXPORT_SYMBOL(loop_register_transfer);
EXPORT_SYMBOL(loop_unregister_transfer);

+static const struct file_operations *get_def_blk_fops(void)
+{
+ struct inode inode;
+
+ /* a roundabout way to retrieve def_blk_fops but avoids undefined
+ * reference warning */
+ init_special_inode(&inode, S_IFBLK, 0);
+
+ return inode.i_fop;
+}
+
static int __init loop_init(void)
{
int i;

+ loop_fops = *(get_def_blk_fops());
+ loop_fops.mmap = loop_file_mmap;
+
if (max_loop < 1 || max_loop > 256) {
printk(KERN_WARNING "loop: invalid max_loop (must be between"
" 1 and 256), using default (8)\n");
@@ -1411,6 +1580,12 @@ static int __init loop_init(void)
if (register_blkdev(LOOP_MAJOR, "loop"))
return -EIO;

+ pgoff_elem_cache = kmem_cache_create(cache_name,
+ sizeof(struct pgoff_elem), 0,
+ SLAB_HWCACHE_ALIGN, NULL, NULL);
+ if (!pgoff_elem_cache)
+ goto out_mem0;
+
loop_dev = kmalloc(max_loop * sizeof(struct loop_device), GFP_KERNEL);
if (!loop_dev)
goto out_mem1;
@@ -1464,6 +1639,8 @@ out_mem3:
out_mem2:
kfree(loop_dev);
out_mem1:
+ kmem_cache_destroy(pgoff_elem_cache);
+out_mem0:
unregister_blkdev(LOOP_MAJOR, "loop");
printk(KERN_ERR "loop: ran out of memory\n");
return -ENOMEM;
@@ -1483,6 +1660,7 @@ static void loop_exit(void)

kfree(disks);
kfree(loop_dev);
+ kmem_cache_destroy(pgoff_elem_cache);
}

module_init(loop_init);
diff -uprN linux-2.6.19.2/include/linux/loop.h linux-2.6.19.2-new/include/linux/loop.h
--- linux-2.6.19.2/include/linux/loop.h 2007-01-11 06:10:37.000000000 +1100
+++ linux-2.6.19.2-new/include/linux/loop.h 2007-03-11 07:31:41.000000000 +1100
@@ -18,6 +18,7 @@
#include <linux/blkdev.h>
#include <linux/spinlock.h>
#include <linux/mutex.h>
+#include <linux/rbtree.h>

/* Possible states of device */
enum {
@@ -34,6 +35,8 @@ struct loop_device {
loff_t lo_offset;
loff_t lo_sizelimit;
int lo_flags;
+ int lo_track_pgwrite;
+ struct rb_root pgoff_tree;
int (*transfer)(struct loop_device *, int cmd,
struct page *raw_page, unsigned raw_off,
struct page *loop_page, unsigned loop_off,
@@ -66,6 +69,11 @@ struct loop_device {
request_queue_t *lo_queue;
};

+struct pgoff_elem {
+ struct rb_node node;
+ unsigned long offset;
+};
+
#endif /* __KERNEL__ */

/*
@@ -111,6 +119,13 @@ struct loop_info64 {
__u64 lo_init[2];
};

+struct loop_pgoff_array {
+ __u64 max; /* size of array passed by user */
+ __u64 num; /* number of entries filled in by driver */
+ __u64 *pgoff; /* array of page offsets of pages written to by mmap */
+};
+
+
/*
* Loop filter types
*/
@@ -157,5 +172,8 @@ int loop_unregister_transfer(int number)
#define LOOP_SET_STATUS64 0x4C04
#define LOOP_GET_STATUS64 0x4C05
#define LOOP_CHANGE_FD 0x4C06
+#define LOOP_SET_TRACK_PGWRITE 0x4C07
+#define LOOP_GET_PGWRITES 0x4C08
+#define LOOP_CLR_PGWRITES 0x4C09

#endif