[PATCH 0/3]: compressed in-memory swapping

[PATCH 0/3]: compressed in-memory swapping

[Nitin Gupta]

Hi,

Project home: http://code.google.com/p/compcache/

It allows creating a RAM based block device which acts as swap disk.
Pages swapped to this device are compressed and stored in memory itself.
This is a big win over swapping to slow hard-disk which are typically used
as swap disk. For flash, these suffer from wear-leveling issues when used
as swap disk - so again its helpful. For swapless systems, it allows more
apps to run.

Its now part of Ubuntu, ALT Linux and some other distros.

Some use cases:
  - Embedded Devices: Memory is scarce and adding more memory increases
    device cost. Also, flash storage suffers from wear-levelling issues.
  - Virtualization: For host/hypervisor, VM/guest memory is all anonymous
    memory. So, compcache can compress any part of guest. So we can host
    more VMs on same amount of memory.
  - Thinclients.

Testing/Performance:
  - Testing for use on Thinclients (contrib: Nai Xia):
     Details: http://code.google.com/p/compcache/wiki/LTSPPerf
     Summary: http://code.google.com/p/compcache/wiki/LTSPPerfSummary

     Summary: after the compcache was loaded:
         - The time of paging down one pdf page was reduced to 1/4~1/100
         - The time of switching from one firefox tab to another was reduced to 1/6
         - The capacity of kpdf was be increased from 2 pdf files to 11 pdf files.
         - The capacity of firefox was increased from 6 web pages to 15 web pages.

  - Testing on x86 and x64 VMs (256/512MB RAM, 1 VCPU, 512MB Swap):
    Started Fedora 10 and some "generic desktop" apps (KDE4, Firefox,
    filemanager, editors) - without compcache, system swapped heavily
    and very unresponsive. With compcache, very noticeable improvement
    in responsiveness.

  - Some reported running on PS3 systems though no performance stats
    available here.

Limitations:
  - Being a "swap disk" it can compress only anonymous memory. But as mentioned
    above, at hypervisor/host level, all guest memory is anonymous so any part
    of guest memory can be compressed.
  - Memory allocator used is not scalable at all (work in progress).
  - No run-time defragmentation, swapping of allocated memory - however,
    incompressible pages are forwarded to physical swap and can set limit
    on amount of memory used by compcache.

I do not claim performance gain for any arbitrary workload but as testing
shows, it certainly helps in some cases :)

Also note that this is a low risk change - it does not modify any part
of kernel. Its just an additional module. In case you find it hurting
performance, just unload this module and maybe use normal disk swap.

Any reviews/comments/suggestions are welcome.

Thanks,
Nitin

[PATCH 1/3]: compressed RAM block device

[Nitin Gupta]

  drivers/block/Kconfig     |   22 +
  drivers/block/Makefile    |    1 +
  drivers/block/compcache.c |  995 +++++++++++++++++++++++++++++++++++++++++++++
  drivers/block/compcache.h |  160 ++++++++
  4 files changed, 1178 insertions(+), 0 deletions(-)

Creates RAM based block device (ramzswap0) which can be used as swap device.
Pages swapped to this are compressed and stored in memory itself.

The module is called compcache.ko. It depends on:
  - xvmalloc.ko: memory allocator
  - lzo_compress.ko
  - lzo_decompress.ko

See Documentation/blockdev/compcache.txt for usage details.

Project home: http://code.google.com/p/compcache/

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
---

diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index 0344a8a..cccf76c 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -348,6 +348,28 @@ config BLK_DEV_RAM_SIZE
  	  The default value is 4096 kilobytes. Only change this if you know
  	  what you are doing.

+config BLK_DEV_COMPCACHE
+	tristate "Compressed RAM swap device"
+	depends on XVMALLOC
+	depends on LZO_COMPRESS
+	depends on LZO_DECOMPRESS
+	help
+	  Saying Y here will allow you to use in-memory compressed swapping.
+	  It creates a pseudo block device (named ramzswap) which acts as
+	  swap device. Pages swapped to this device are compressed and stored
+	  in memory itself.
+
+	  Project home: http://code.google.com/p/compcache/
+	  For details, read <file:Documentation/blockdev/compcache.txt>
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called compcache.
+
+config BLK_DEV_COMPCACHE_STATS
+	bool "Collect statistics"
+	depends on BLK_DEV_COMPCACHE
+	default y
+
  config BLK_DEV_XIP
  	bool "Support XIP filesystems on RAM block device"
  	depends on BLK_DEV_RAM
diff --git a/drivers/block/Makefile b/drivers/block/Makefile
index 204332b..6b2ee9e 100644
--- a/drivers/block/Makefile
+++ b/drivers/block/Makefile
@@ -12,6 +12,7 @@ obj-$(CONFIG_PS3_DISK)		+= ps3disk.o
  obj-$(CONFIG_ATARI_FLOPPY)	+= ataflop.o
  obj-$(CONFIG_AMIGA_Z2RAM)	+= z2ram.o
  obj-$(CONFIG_BLK_DEV_RAM)	+= brd.o
+obj-$(CONFIG_BLK_DEV_COMPCACHE)	+= compcache.o
  obj-$(CONFIG_BLK_DEV_LOOP)	+= loop.o
  obj-$(CONFIG_BLK_DEV_XD)	+= xd.o
  obj-$(CONFIG_BLK_CPQ_DA)	+= cpqarray.o
diff --git a/drivers/block/compcache.c b/drivers/block/compcache.c
new file mode 100644
index 0000000..42bbb52
--- /dev/null
+++ b/drivers/block/compcache.c
@@ -0,0 +1,995 @@
+/*
+ * Compressed RAM based swap device
+ *
+ * Copyright (C) 2008, 2009  Nitin Gupta
+ *
+ * This RAM based block device acts as swap disk.
+ * Pages swapped to this device are compressed and
+ * stored in memory.
+ *
+ * Released under the terms of the GNU General Public
+ * License (version 2). See linux/COPYING for more information.
+ *
+ * Project home: http://code.google.com/p/compcache
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/bitops.h>
+#include <linux/blkdev.h>
+#include <linux/buffer_head.h>
+#include <linux/device.h>
+#include <linux/genhd.h>
+#include <linux/highmem.h>
+#include <linux/lzo.h>
+#include <linux/mutex.h>
+#include <linux/proc_fs.h>
+#include <linux/string.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
+#include <linux/vmalloc.h>
+#include <linux/xvmalloc.h>
+
+#include "compcache.h"
+
+/* Globals */
+static struct compcache compcache;
+static struct compcache_stats stats;
+
+/* Module params (documentation at end) */
+static unsigned long disksize_kb;
+static unsigned long memlimit_kb;
+static char *backing_dev;
+
+/*
+ * Pages that compress to larger than this size are
+ * forwarded to backing swap, if present or stored
+ * uncompressed in memory otherwise.
+ */
+static unsigned int MAX_CPAGE_SIZE;
+
+static int __init compcache_init(void);
+static struct block_device_operations compcache_devops = {
+	.owner = THIS_MODULE,
+};
+
+static void set_page_zero(u32 index)
+{
+	compcache.table[index].flags |= (1 << CC_zero);
+}
+
+static void set_page_uncompressed(u32 index)
+{
+	compcache.table[index].flags |= (1 << CC_uncompressed);
+}
+
+static void clear_page_zero(u32 index)
+{
+	compcache.table[index].flags &= ~(1 << CC_zero);
+}
+
+static void clear_page_uncompressed(u32 index)
+{
+	compcache.table[index].flags &= ~(1 << CC_uncompressed);
+}
+
+static int is_page_zero(u32 index)
+{
+	return compcache.table[index].flags & (1 << CC_zero);
+}
+
+static int is_page_uncompressed(u32 index)
+{
+	return compcache.table[index].flags & (1 << CC_uncompressed);
+}
+
+static int page_zero_filled(void *ptr)
+{
+	u32 pos;
+	u64 *page;
+
+	page = (u64 *)ptr;
+
+	for (pos = 0; pos != PAGE_SIZE / sizeof(*page); pos++) {
+		if (page[pos])
+			return 0;
+	}
+
+	return 1;
+}
+
+/*
+ * Given <pagenum, offset> pair, provide a dereferencable pointer.
+ */
+static void *get_ptr_atomic(u32 pagenum, u16 offset, enum km_type type)
+{
+	unsigned char *page;
+
+	page = kmap_atomic(pfn_to_page(pagenum), type);
+	return page + offset;
+}
+
+static void put_ptr_atomic(void *ptr, enum km_type type)
+{
+	kunmap_atomic(ptr, type);
+}
+
+#if defined(STATS)
+static struct proc_dir_entry *proc;
+
+static int proc_compcache_read(char *page, char **start, off_t off,
+				int count, int *eof, void *data)
+{
+	int len;
+	size_t succ_writes, mem_used;
+	unsigned int good_compress_perc = 0, no_compress_perc = 0;
+
+	mem_used = xvGetTotalSizeBytes(compcache.mem_pool)
+			+ (stats.pages_expand << PAGE_SHIFT);
+
+	if (off > 0) {
+		*eof = 1;
+		return 0;
+	}
+
+#define K(x)	((x) >> 10)
+	/* Basic stats */
+	len = sprintf(page,
+		"DiskSize:	%8zu kB\n",
+		(size_t)(K(compcache.disksize)));
+
+	if (compcache.backing_dev) {
+		/* This must always be less than ComprDataSize */
+		len += sprintf(page + len,
+			"MemLimit:	%8zu kB\n",
+			K(compcache.memlimit));
+	}
+
+	succ_writes = stats.num_writes - stats.failed_writes;
+	if (succ_writes) {
+		good_compress_perc = stats.good_compress * 100
+					/ stats.pages_stored;
+		no_compress_perc = stats.pages_expand * 100
+					/ stats.pages_stored;
+	}
+
+	/* Extended stats */
+	len += sprintf(page + len,
+		"NumReads:	%8llu\n"
+		"NumWrites:	%8llu\n"
+		"FailedReads:	%8llu\n"
+		"FailedWrites:	%8llu\n"
+		"InvalidIO:	%8llu\n"
+		"PagesDiscard:	%8llu\n"
+		"ZeroPages:	%8u\n"
+		"GoodCompress:	%8u %%\n"
+		"NoCompress:	%8u %%\n"
+		"PagesStored:	%8u\n"
+		"PagesUsed:	%8zu\n"
+		"OrigDataSize:	%8zu kB\n"
+		"ComprDataSize:	%8zu kB\n"
+		"MemUsedTotal:	%8zu kB\n",
+		stats.num_reads,
+		stats.num_writes,
+		stats.failed_reads,
+		stats.failed_writes,
+		stats.invalid_io,
+		stats.pages_discard,
+		stats.pages_zero,
+		good_compress_perc,
+		no_compress_perc,
+		stats.pages_stored,
+		mem_used >> PAGE_SHIFT,
+		(size_t)(K(stats.pages_stored << PAGE_SHIFT)),
+		(size_t)(K(stats.compr_size)),
+		(size_t)(K(mem_used)));
+
+	if (compcache.backing_dev) {
+		/* This must always be less than ComprDataSize */
+		len += sprintf(page + len,
+			"BDevNumReads:	%8llu\n"
+			"BDevNumWrites:	%8llu\n",
+			stats.bdev_num_reads,
+			stats.bdev_num_writes);
+	}
+
+
+	return len;
+}
+#endif	/* STATS */
+
+/*
+ * Check if value of backing_dev module param is sane.
+ * Claim this device and set compcache size equal to
+ * size of this block device.
+ */
+static int setup_backing_device(void)
+{
+	int error = 0;
+	struct inode *inode;
+	struct file *swap_file;
+	struct address_space *mapping;
+	struct block_device *bdev = NULL;
+
+	if (backing_dev == NULL) {
+		pr_debug(C "backing_dev param not given\n");
+		goto out;
+	}
+
+	pr_info(C "Using backing swap device: %s\n", backing_dev);
+
+	swap_file = filp_open(backing_dev, O_RDWR | O_LARGEFILE, 0);
+	if (IS_ERR(swap_file)) {
+		pr_err(C "Error opening backing device: %s\n", backing_dev);
+		error = -EINVAL;
+		goto out;
+	}
+
+	mapping = swap_file->f_mapping;
+	inode = mapping->host;
+
+	if (S_ISBLK(inode->i_mode)) {
+		bdev = I_BDEV(inode);
+		error = bd_claim(bdev, compcache_init);
+		if (error < 0) {
+			bdev = NULL;
+			goto bad_param;
+		}
+		compcache.old_block_size = block_size(bdev);
+		error = set_blocksize(bdev, PAGE_SIZE);
+		if (error < 0)
+			goto bad_param;
+	} else {
+		/* TODO: support for regular file as backing swap */
+		pr_info(C "%s is not a block device.\n", backing_dev);
+		error = -EINVAL;
+		goto out;
+	}
+
+	compcache.swap_file = swap_file;
+	compcache.backing_dev = bdev;
+	compcache.disksize = i_size_read(inode);
+	BUG_ON(!compcache.disksize);
+
+	return 0;
+
+bad_param:
+	if (bdev) {
+		set_blocksize(bdev, compcache.old_block_size);
+		bd_release(bdev);
+	}
+	filp_close(swap_file, NULL);
+
+out:
+	compcache.backing_dev = NULL;
+	return error;
+}
+
+/*
+ * Check if request is within bounds and page aligned.
+ */
+static inline int valid_swap_request(struct bio *bio)
+{
+	if (unlikely(
+		(bio->bi_sector >= (compcache.disksize >> SECTOR_SHIFT)) ||
+		(bio->bi_sector & (SECTORS_PER_PAGE - 1)) ||
+		(bio->bi_vcnt != 1) ||
+		(bio->bi_size != PAGE_SIZE) ||
+		(bio->bi_io_vec[0].bv_offset != 0))) {
+
+		return 0;
+	}
+
+	/* swap request is valid*/
+	return 1;
+}
+
+static void compcache_free_page(size_t index)
+{
+
+	u32 clen;
+	void *obj;
+
+
+	if (unlikely(is_page_uncompressed(index))) {
+		clen = PAGE_SIZE;
+		__free_page(pfn_to_page(
+			compcache.table[index].pageNum));
+		clear_page_uncompressed(index);
+		stat_dec(stats.pages_expand);
+	} else {
+		obj = get_ptr_atomic(compcache.table[index].pageNum,
+			compcache.table[index].offset, KM_USER0);
+		clen = xvGetObjectSize(obj) - sizeof(struct zobj_header);
+		put_ptr_atomic(obj, KM_USER0);
+		xvFree(compcache.mem_pool,
+			compcache.table[index].pageNum,
+			compcache.table[index].offset);
+		stat_dec_if_less(stats.good_compress, clen, PAGE_SIZE/2 + 1);
+	}
+
+	stats.compr_size -= clen;
+	stat_dec(stats.pages_stored);
+
+	compcache.table[index].pageNum = 0;
+	compcache.table[index].offset = 0;
+}
+
+static int compcache_prepare_discard(struct request_queue *q,
+					struct request *req)
+{
+	return 0;
+}
+
+/*
+ * Called by main I/O handler function. This helper
+ * function handles 'discard' I/O requests which means
+ * that  some swap pages are no longer required, so
+ * swap device can take needed action -- we free memory
+ * allocated for these pages.
+ */
+static void compcache_discard(struct bio *bio)
+{
+	size_t index, start_page, num_pages;
+
+	start_page = bio->bi_sector >> SECTORS_PER_PAGE_SHIFT;
+	num_pages = bio->bi_size >> (SECTOR_SHIFT + SECTORS_PER_PAGE_SHIFT);
+
+	for (index = start_page; index < start_page + num_pages;
+							index++) {
+		if (compcache.table[index].pageNum) {
+			compcache_free_page(index);
+			stat_inc(stats.pages_discard);
+		}
+	}
+	set_bit(BIO_UPTODATE, &bio->bi_flags);
+	bio_endio(bio, 0);
+	return;
+}
+
+/*
+ * Handler function for all compcache I/O requests.
+ */
+static int compcache_make_request(struct request_queue *queue, struct bio *bio)
+{
+	int ret, fwd_write_request = 0;
+	u32 offset;
+	size_t clen, index;
+	struct zobj_header *zheader;
+	struct page *page, *page_store;
+	unsigned char *user_mem, *cmem, *src;
+
+	if (bio_discard(bio)) {
+		compcache_discard(bio);
+		return 0;
+	}
+
+	if (!valid_swap_request(bio)) {
+		stat_inc(stats.invalid_io);
+		goto out;
+	}
+
+	page = bio->bi_io_vec[0].bv_page;
+	index = bio->bi_sector >> SECTORS_PER_PAGE_SHIFT;
+
+	switch (bio_data_dir(bio)) {
+	case READ:
+		stat_inc(stats.num_reads);
+
+		if (is_page_zero(index)) {
+			user_mem = get_ptr_atomic(page_to_pfn(page), 0,
+						KM_USER0);
+			memset(user_mem, 0, PAGE_SIZE);
+			put_ptr_atomic(user_mem, KM_USER0);
+			set_bit(BIO_UPTODATE, &bio->bi_flags);
+			bio_endio(bio, 0);
+			return 0;
+		}
+
+		/*
+		 * Requested page is not present in compressed area.
+		 * Its either in backing swap device (if present) or
+		 * this is an attempt to read before any previous write
+		 * to this location - this happens due to readahead when
+		 * swap device is read from user-space (e.g. during swapon)
+		 */
+		if (!compcache.table[index].pageNum) {
+			/*
+			 * Always forward such requests to backing swap
+			 * device (if present)
+			 */
+			if (compcache.backing_dev) {
+				stat_dec(stats.num_reads);
+				stat_inc(stats.bdev_num_reads);
+				bio->bi_bdev = compcache.backing_dev;
+				return 1;
+			}
+			/*
+			 * Its unlikely event in case backing dev is
+			 * not present
+			 */
+			pr_debug(C "Read before write on swap device: "
+				"sector=%lu, size=%u, offset=%u\n",
+				(ulong)(bio->bi_sector),
+				bio->bi_size,
+				bio->bi_io_vec[0].bv_offset);
+			user_mem = kmap(page);
+			memset(user_mem, 0, PAGE_SIZE);
+			kunmap(page);
+			set_bit(BIO_UPTODATE, &bio->bi_flags);
+			bio_endio(bio, 0);
+			return 0;
+		}
+
+		user_mem = get_ptr_atomic(page_to_pfn(page), 0, KM_USER0);
+
+		clen = PAGE_SIZE;
+		cmem = get_ptr_atomic(compcache.table[index].pageNum,
+				compcache.table[index].offset, KM_USER1);
+
+		/* Page is stored uncompressed since its incompressible */
+		if (unlikely(is_page_uncompressed(index))) {
+			memcpy(user_mem, cmem, PAGE_SIZE);
+			put_ptr_atomic(user_mem, KM_USER0);
+			put_ptr_atomic(cmem, KM_USER1);
+			set_bit(BIO_UPTODATE, &bio->bi_flags);
+			bio_endio(bio, 0);
+			return 0;
+		}
+
+		ret = lzo1x_decompress_safe(
+			cmem + sizeof(*zheader),
+			xvGetObjectSize(cmem) - sizeof(*zheader),
+			user_mem, &clen);
+
+		put_ptr_atomic(user_mem, KM_USER0);
+		put_ptr_atomic(cmem, KM_USER1);
+
+		/* should NEVER happen */
+		if (unlikely(ret != LZO_E_OK)) {
+			pr_err(C "Decompression failed! "
+				"err=%d, page=%zu\n",
+				ret, index);
+			stat_inc(stats.failed_reads);
+			goto out;
+		}
+
+		set_bit(BIO_UPTODATE, &bio->bi_flags);
+		bio_endio(bio, 0);
+		return 0;
+
+	case WRITE:
+		src = compcache.compress_buffer;
+		stat_inc(stats.num_writes);
+
+		/*
+		 * System swaps to same sector again when the stored page
+		 * is no longer referenced by any process. So, its now safe
+		 * to free the memory that was allocated for this page.
+		 */
+		if (compcache.table[index].pageNum)
+			compcache_free_page(index);
+
+		/*
+		 * No memory ia allocated for zero filled pages.
+		 * Simply clear zero page flag.
+		 */
+		if (is_page_zero(index)) {
+			stat_dec(stats.pages_zero);
+			clear_page_zero(index);
+		}
+
+		mutex_lock(&compcache.lock);
+
+		user_mem = get_ptr_atomic(page_to_pfn(page), 0, KM_USER0);
+		if (page_zero_filled(user_mem)) {
+			put_ptr_atomic(user_mem, KM_USER0);
+			mutex_unlock(&compcache.lock);
+			stat_inc(stats.pages_zero);
+			set_page_zero(index);
+			set_bit(BIO_UPTODATE, &bio->bi_flags);
+			bio_endio(bio, 0);
+			return 0;
+		}
+
+		if (compcache.backing_dev &&
+			(stats.compr_size > compcache.memlimit - PAGE_SIZE)) {
+			put_ptr_atomic(user_mem, KM_USER0);
+			mutex_unlock(&compcache.lock);
+			fwd_write_request = 1;
+			goto out;
+		}
+
+		ret = lzo1x_1_compress(user_mem, PAGE_SIZE,
+			src, &clen, compcache.compress_workmem);
+
+		put_ptr_atomic(user_mem, KM_USER0);
+
+		if (unlikely(ret != LZO_E_OK)) {
+			mutex_unlock(&compcache.lock);
+			pr_err(C "Compression failed! err=%d\n", ret);
+			stat_inc(stats.failed_writes);
+			goto out;
+		}
+
+		/* Page is incompressible - store it as is */
+		if (unlikely(clen > MAX_CPAGE_SIZE)) {
+			if (compcache.backing_dev) {
+				mutex_unlock(&compcache.lock);
+				fwd_write_request = 1;
+				goto out;
+			}
+			clen = PAGE_SIZE;
+			page_store = alloc_page(GFP_NOIO | __GFP_HIGHMEM);
+			if (unlikely(!page_store)) {
+				mutex_unlock(&compcache.lock);
+				stat_inc(stats.failed_writes);
+				goto out;
+			}
+			stat_inc(stats.pages_expand);
+			compcache.table[index].pageNum =
+						page_to_pfn(page_store);
+			set_page_uncompressed(index);
+			src = get_ptr_atomic(page_to_pfn(page), 0, KM_USER0);
+			offset = 0;
+		} else {
+			if (xvMalloc(compcache.mem_pool,
+				clen + sizeof(*zheader),
+				&compcache.table[index].pageNum,
+				&offset)) {
+				mutex_unlock(&compcache.lock);
+				pr_debug(C "Error allocating memory for "
+					"compressed page: %zu, size=%zu \n",
+					index, clen);
+				stat_inc(stats.failed_writes);
+				goto out;
+			}
+		}
+
+		compcache.table[index].offset = offset;
+
+		cmem = get_ptr_atomic(compcache.table[index].pageNum,
+				compcache.table[index].offset, KM_USER1);
+
+		if (!is_page_uncompressed(index)) {
+			zheader = (struct zobj_header *)cmem;
+			zheader->table_idx = index;
+			cmem += sizeof(*zheader);
+		}
+
+		memcpy(cmem, src, clen);
+
+		put_ptr_atomic(cmem, KM_USER1);
+		if (unlikely(is_page_uncompressed(index)))
+			put_ptr_atomic(src, KM_USER0);
+
+		/* Update stats */
+		stats.compr_size += clen;
+		stat_inc(stats.pages_stored);
+		stat_inc_if_less(stats.pages_expand, PAGE_SIZE - 1, clen);
+		stat_inc_if_less(stats.good_compress, clen, PAGE_SIZE/2 + 1);
+
+		mutex_unlock(&compcache.lock);
+
+		set_bit(BIO_UPTODATE, &bio->bi_flags);
+		bio_endio(bio, 0);
+		return 0;
+	}
+
+out:
+	if (fwd_write_request) {
+		stat_inc(stats.bdev_num_writes);
+		bio->bi_bdev = compcache.backing_dev;
+		return 1;
+	}
+
+	bio_io_error(bio);
+	return 0;
+}
+
+/*
+ * Swap header (1st page of swap device) contains information
+ * to indentify it as a swap partition. Prepare such a header
+ * for compcache device (ramzswap) so that swapon can identify
+ * it as swap partition. In case backing swap device is provided,
+ * copy its swap header.
+ */
+static int setup_swap_header(union swap_header *s)
+{
+	int ret = 0;
+	struct page *page;
+	struct address_space *mapping;
+	union swap_header *backing_dev_header;
+
+	/*
+	 * There is no backing swap device. Create a swap header
+	 * that is acceptable by swapon.
+	 */
+	if (compcache.backing_dev == NULL) {
+		s->info.version = 1;
+		s->info.last_page = compcache.disksize >> PAGE_SHIFT;
+		s->info.nr_badpages = 0;
+		memcpy(s->magic.magic, "SWAPSPACE2", 10);
+		return 0;
+	}
+
+	/*
+	 * We have a backing swap device. Copy its swap header
+	 * to compcache swap header. If this header contains
+	 * invalid information (backing device not a swap
+	 * partition, etc.), swapon will fail for compcache
+	 * which is correct behavior - we don't want to
+	 * swap over filesystem partition!
+	 */
+
+	/*
+	 * Read the backing swap header.
+	 * (code from sys_swapon)
+	 */
+
+	mapping = compcache.swap_file->f_mapping;
+	if (!mapping->a_ops->readpage) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	page = read_mapping_page(mapping, 0, compcache.swap_file);
+	if (IS_ERR(page)) {
+		ret = PTR_ERR(page);
+		goto out;
+	}
+
+	backing_dev_header = kmap(page);
+	*s = *backing_dev_header;
+	kunmap(page);
+
+out:
+	return ret;
+}
+
+static void compcache_set_disksize(size_t totalram_bytes)
+{
+	compcache.disksize = disksize_kb << 10;
+
+	if (!disksize_kb) {
+		pr_info(C
+		"disk size not provided. You can use disksize_kb module "
+		"param to specify size.\nUsing default: (%u%% of RAM).\n",
+		DEFAULT_DISKSIZE_PERC_RAM
+		);
+		compcache.disksize = DEFAULT_DISKSIZE_PERC_RAM *
+					(totalram_bytes / 100);
+	}
+
+	if (disksize_kb > 2 * (totalram_bytes >> 10)) {
+		pr_info(C
+		"There is little point creating a compcache of greater than "
+		"twice the size of memory since we expect a 2:1 compression "
+		"ratio. Note that compcache uses about 0.1%% of the size of "
+		"the swap device when not in use so a huge compcache is "
+		"wasteful.\n"
+		"\tMemory Size: %zu kB\n"
+		"\tSize you selected: %lu kB\n"
+		"Continuing anyway ...\n",
+		totalram_bytes >> 10, disksize_kb
+		);
+	}
+
+	compcache.disksize &= PAGE_MASK;
+
+	pr_info(C "disk size set to %zu kB\n", compcache.disksize >> 10);
+}
+
+/*
+ * memlimit cannot be greater than backing disk size.
+ */
+static void compcache_set_memlimit(size_t totalram_bytes)
+{
+	int memlimit_valid = 1;
+	compcache.memlimit = memlimit_kb << 10;
+
+	if (!compcache.memlimit) {
+		pr_info(C "memory limit not set. You can use "
+			"memlimit_kb module param to specify limit.");
+		memlimit_valid = 0;
+	}
+
+	if (compcache.memlimit > compcache.disksize) {
+		pr_info(C "memory limit cannot be greater than "
+			"disksize: limit=%zu, disksize=%zu",
+			compcache.memlimit,
+			compcache.disksize);
+		memlimit_valid = 0;
+	}
+
+	if (!memlimit_valid) {
+		size_t mempart, disksize;
+		pr_info(C "\nUsing default: MIN[(%u%% of RAM), "
+			"(backing disk size)].\n",
+			DEFAULT_MEMLIMIT_PERC_RAM);
+		mempart = DEFAULT_MEMLIMIT_PERC_RAM * (totalram_bytes / 100);
+		disksize = compcache.disksize;
+		compcache.memlimit = mempart > disksize ? disksize : mempart;
+	}
+
+	if (compcache.memlimit > totalram_bytes / 2) {
+		pr_info(C
+		"Its not advisable setting limit more than half of "
+		"size of memory since we expect a 2:1 compression ratio. "
+		"Limit represents amount of *compressed* data we can keep "
+		"in memory!\n"
+		"\tMemory Size: %zu kB\n"
+		"\tLimit you selected: %lu kB\n"
+		"Continuing anyway ...\n",
+		totalram_bytes >> 10, memlimit_kb
+		);
+	}
+
+	compcache.memlimit &= PAGE_MASK;
+	BUG_ON(!compcache.memlimit);
+
+	pr_info(C "memory limit set to %zu kB\n", compcache.memlimit >> 10);
+
+}
+
+static int __init compcache_init(void)
+{
+	int ret;
+	size_t num_pages, totalram_bytes;
+	struct sysinfo i;
+	struct page *page;
+	void *swap_header;
+
+	mutex_init(&compcache.lock);
+
+	ret = setup_backing_device();
+	if (ret)
+		goto fail;
+
+	si_meminfo(&i);
+	/* Here is a trivia: guess unit used for i.totalram !! */
+	totalram_bytes = i.totalram << PAGE_SHIFT;
+
+	if (compcache.backing_dev)
+		compcache_set_memlimit(totalram_bytes);
+	else
+		compcache_set_disksize(totalram_bytes);
+
+	compcache.compress_workmem = kmalloc(LZO1X_MEM_COMPRESS, GFP_KERNEL);
+	if (compcache.compress_workmem == NULL) {
+		pr_err(C "Error allocating compressor working memory\n");
+		ret = -ENOMEM;
+		goto fail;
+	}
+
+	compcache.compress_buffer = kmalloc(2 * PAGE_SIZE, GFP_KERNEL);
+	if (compcache.compress_buffer == NULL) {
+		pr_err(C "Error allocating compressor buffer space\n");
+		ret = -ENOMEM;
+		goto fail;
+	}
+
+	num_pages = compcache.disksize >> PAGE_SHIFT;
+	compcache.table = vmalloc(num_pages * sizeof(*compcache.table));
+	if (compcache.table == NULL) {
+		pr_err(C "Error allocating compcache address table\n");
+		ret = -ENOMEM;
+		goto fail;
+	}
+	memset(compcache.table, 0, num_pages * sizeof(*compcache.table));
+
+	page = alloc_page(__GFP_ZERO);
+	if (page == NULL) {
+		pr_err(C "Error allocating swap header page\n");
+		ret = -ENOMEM;
+		goto fail;
+	}
+	compcache.table[0].pageNum = page_to_pfn(page);
+	set_page_uncompressed(0);
+
+	swap_header = kmap(page);
+	ret = setup_swap_header((union swap_header *)(swap_header));
+	kunmap(page);
+	if (ret) {
+		pr_err(C "Error setting swap header\n");
+		goto fail;
+	}
+
+	compcache.disk = alloc_disk(1);
+	if (compcache.disk == NULL) {
+		pr_err(C "Error allocating disk structure\n");
+		ret = -ENOMEM;
+		goto fail;
+	}
+
+	compcache.disk->first_minor = 0;
+	compcache.disk->fops = &compcache_devops;
+	/*
+	 * It is named like this to prevent distro installers
+	 * from offering compcache as installation target. They
+	 * seem to ignore all devices beginning with 'ram'
+	 */
+	strcpy(compcache.disk->disk_name, "ramzswap0");
+
+	compcache.disk->major = register_blkdev(0, compcache.disk->disk_name);
+	if (compcache.disk->major < 0) {
+		pr_err(C "Cannot register block device\n");
+		ret = -EFAULT;
+		goto fail;
+	}
+
+	compcache.disk->queue = blk_alloc_queue(GFP_KERNEL);
+	if (compcache.disk->queue == NULL) {
+		pr_err(C "Cannot register disk queue\n");
+		ret = -EFAULT;
+		goto fail;
+	}
+
+	set_capacity(compcache.disk, compcache.disksize >> SECTOR_SHIFT);
+	blk_queue_make_request(compcache.disk->queue, compcache_make_request);
+
+	/*
+	 * Assuming backing device is "rotational" type.
+	 * TODO: check if its actually "non-rotational" (SSD).
+	 *
+	 * We have ident mapping of sectors for compcache and
+	 * and the backing swap device. So, this queue flag
+	 * should be according to backing dev.
+	 */
+	if (!compcache.backing_dev) {
+		queue_flag_set_unlocked(QUEUE_FLAG_NONROT,
+					compcache.disk->queue);
+	}
+	blk_queue_set_discard(compcache.disk->queue,
+				compcache_prepare_discard);
+	blk_queue_hardsect_size(compcache.disk->queue, PAGE_SIZE);
+	add_disk(compcache.disk);
+
+	compcache.mem_pool = xvCreateMemPool();
+	if (compcache.mem_pool == INVALID_POOL_ID) {
+		pr_err(C "Error creating memory pool\n");
+		ret = -ENOMEM;
+		goto fail;
+	}
+
+#if defined(STATS)
+	proc = create_proc_entry("compcache", S_IRUGO, NULL);
+	if (proc)
+		proc->read_proc = &proc_compcache_read;
+	else {
+		ret = -ENOMEM;
+		pr_warning(C "Error creating proc entry\n");
+		goto fail;
+	}
+#endif
+
+	/*
+	 * Pages that compress to size greater than this are forwarded
+	 * to physical swap disk (if backing dev is provided)
+	 */
+	if (compcache.backing_dev)
+		MAX_CPAGE_SIZE = MAX_CPAGE_SIZE_BDEV;
+	else
+		MAX_CPAGE_SIZE = MAX_CPAGE_SIZE_NOBDEV;
+
+	pr_debug(C "Max compressed page size: %u bytes\n", MAX_CPAGE_SIZE);
+
+	pr_debug(C "Initialization done!\n");
+	return 0;
+
+fail:
+	if (compcache.disk != NULL) {
+		if (compcache.disk->major > 0)
+			unregister_blkdev(compcache.disk->major,
+					compcache.disk->disk_name);
+		del_gendisk(compcache.disk);
+	}
+
+	if (compcache.table && compcache.table[0].pageNum)
+		__free_page(pfn_to_page(compcache.table[0].pageNum));
+	kfree(compcache.compress_workmem);
+	kfree(compcache.compress_buffer);
+	vfree(compcache.table);
+	xvDestroyMemPool(compcache.mem_pool);
+#if defined(STATS)
+	if (proc)
+		remove_proc_entry("compcache", proc->parent);
+#endif
+	pr_err(C "Initialization failed: err=%d\n", ret);
+	return ret;
+}
+
+static void __exit compcache_exit(void)
+{
+	size_t index, num_pages;
+	num_pages = compcache.disksize >> PAGE_SHIFT;
+
+	unregister_blkdev(compcache.disk->major, compcache.disk->disk_name);
+	del_gendisk(compcache.disk);
+
+	/* Close backing swap device (if present) */
+	if (compcache.backing_dev) {
+		set_blocksize(compcache.backing_dev, compcache.old_block_size);
+		bd_release(compcache.backing_dev);
+		filp_close(compcache.swap_file, NULL);
+	}
+
+	__free_page(pfn_to_page(compcache.table[0].pageNum));
+	kfree(compcache.compress_workmem);
+	kfree(compcache.compress_buffer);
+
+	/* Free all pages that are still in compcache */
+	for (index = 1; index < num_pages; index++) {
+		if (!compcache.table[index].pageNum)
+			continue;
+
+		if (unlikely(is_page_uncompressed(index))) {
+			__free_page(pfn_to_page(
+				compcache.table[index].pageNum));
+		} else {
+			xvFree(compcache.mem_pool,
+				compcache.table[index].pageNum,
+				compcache.table[index].offset);
+		}
+	}
+
+	vfree(compcache.table);
+	xvDestroyMemPool(compcache.mem_pool);
+
+#if defined(STATS)
+	remove_proc_entry("compcache", proc->parent);
+#endif
+	pr_debug(C "cleanup done!\n");
+}
+
+/*
+ * This param is applicable only when there is no backing swap device.
+ * We ignore this param in case backing dev is provided since then its
+ * always equal to size of the backing swap device.
+ *
+ * This size refers to amount of (uncompressed) data it can hold.
+ * For e.g. disksize_kb=1024 means it can hold 1024kb worth of
+ * uncompressed data even if this data compresses to just, say, 100kb.
+ *
+ * Default value is used if this param is missing or 0 (if its applicable).
+ * Default: [DEFAULT_DISKSIZE_PERC_RAM]% of RAM
+ */
+module_param(disksize_kb, ulong, 0);
+MODULE_PARM_DESC(disksize_kb, "compcache device size (kB)");
+
+/*
+ * This param is applicable only when backing swap device is provided.
+ * This refers to limit on amount of (compressed) data it can hold in
+ * memory. Note that total amount of memory used (MemUsedTotal) can
+ * exceed this memlimit since that includes memory wastage due to
+ * fragmentation and metadata overhead.
+ *
+ * Any additional data beyond this limit is forwarded to backing
+ * swap device. TODO: allow changing memlimit at runtime.
+ *
+ * Default value is used if this param is missing or 0 (if its applicable).
+ * Default: MIN([DEFAULT_MEMLIMIT_PERC_RAM]% of RAM, Backing Device Size)
+ */
+module_param(memlimit_kb, ulong, 0);
+MODULE_PARM_DESC(memlimit_kb, "compcache memory limit (kB)");
+
+/*
+ * This is block device to be used as backing store for compcache.
+ * When pages more than memlimit_kb as swapped to compcache, we store
+ * any additional pages in this device. We may also move some pages
+ * from compcache to this device in case system is really low on
+ * memory (TODO).
+ *
+ * This device is not directly visible to kernel as a swap device
+ * (/proc/swaps will only show /dev/ramzswap0 and not this device).
+ * Managing this backing device is the job of compcache module.
+ */
+module_param(backing_dev, charp, 0);
+MODULE_PARM_DESC(backing_dev, "Backing swap partition");
+
+module_init(compcache_init);
+module_exit(compcache_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");
+MODULE_DESCRIPTION("Compressed RAM Based Swap Device");
diff --git a/drivers/block/compcache.h b/drivers/block/compcache.h
new file mode 100644
index 0000000..23a58e7
--- /dev/null
+++ b/drivers/block/compcache.h
@@ -0,0 +1,160 @@
+/*
+ * Compressed RAM based swap device
+ *
+ * Copyright (C) 2008, 2009  Nitin Gupta
+ *
+ * This RAM based block device acts as swap disk.
+ * Pages swapped to this device are compressed and
+ * stored in memory.
+ *
+ * Released under the terms of the GNU General Public
+ * License (version 2). See linux/COPYING for more information.
+ *
+ * Project home: http://code.google.com/p/compcache
+ */
+
+#ifndef _COMPCACHE_H_
+#define _COMPCACHE_H_
+
+#include <linux/xvmalloc.h>
+
+/*
+ * Stored at beginning of each compressed object.
+ *
+ * It stores back-reference to table entry which points
+ * to this object. This will required when we implement
+ * memory defragmentation or migrating compressed pages
+ * to swap disk.
+ */
+struct zobj_header {
+	u32 table_idx;
+};
+
+/*-- Configurable parameters */
+
+/* Default compcache disk size: 25% of total RAM */
+#define DEFAULT_DISKSIZE_PERC_RAM	25
+#define DEFAULT_MEMLIMIT_PERC_RAM	15
+
+/*
+ * Max compressed page size when backing device is provided.
+ * Pages that compress to size greater than this are sent to
+ * physical swap disk.
+ */
+#define MAX_CPAGE_SIZE_BDEV	(PAGE_SIZE / 2)
+
+/*
+ * Max compressed page size when there is no backing dev.
+ * Pages that compress to size greater than this are stored
+ * uncompressed in memory.
+ */
+#define MAX_CPAGE_SIZE_NOBDEV	(PAGE_SIZE / 4 * 3)
+
+/*
+ * NOTE: MAX_CPAGE_SIZE_{BDEV,NOBDEV} sizes must be
+ * less than or equal to:
+ *   XV_MAX_ALLOC_SIZE - sizeof(struct zobj_header)
+ * since otherwise xvMalloc would always return failure.
+ */
+
+/*-- End of configurable params */
+
+#define SECTOR_SHIFT		9
+#define SECTOR_SIZE		(1 << SECTOR_SHIFT)
+#define SECTORS_PER_PAGE_SHIFT	(PAGE_SHIFT - SECTOR_SHIFT)
+#define SECTORS_PER_PAGE	(1 << SECTORS_PER_PAGE_SHIFT)
+
+/* Message prefix */
+#define C "compcache: "
+
+/* Debugging and Stats */
+#define NOP	do { } while (0)
+
+#if defined(CONFIG_BLK_DEV_COMPCACHE_STATS)
+#define STATS
+#endif
+
+#if defined(STATS)
+#define stat_inc(stat)			((stat)++)
+#define stat_dec(stat)			((stat)--)
+#define stat_inc_if_less(stat, val1, val2) \
+				((stat) += ((val1) < (val2) ? 1 : 0))
+#define stat_dec_if_less(stat, val1, val2) \
+				((stat) -= ((val1) < (val2) ? 1 : 0))
+#else	/* STATS */
+#define stat_inc(x)			NOP
+#define stat_dec(x)			NOP
+#define stat_inc_if_less(x, v1, v2)	NOP
+#define stat_dec_if_less(x, v1, v2)	NOP
+#endif	/* STATS */
+
+/* Flags for compcache pages (table[page_no].flags) */
+enum cc_pageflags {
+	/* Page is stored uncompressed */
+	CC_uncompressed,
+
+	/* Page consists entirely of zeros */
+	CC_zero,
+
+	__NR_CC_PAGEFLAGS,
+};
+
+/*-- Data structures */
+
+/* Indexed by page no. */
+struct table {
+	u32 pageNum;
+	u16 offset;
+	u8 count;	/* object ref count (not yet used) */
+	u8 flags;
+};
+
+struct compcache {
+	struct Pool *mem_pool;
+	void *compress_workmem;
+	void *compress_buffer;
+	struct table *table;
+	struct mutex lock;
+	struct gendisk *disk;
+	/*
+	 * This is limit on compressed data size (stats.compr_size)
+	 * Its applicable only when backing swap device is present.
+	 */
+	size_t memlimit;	/* bytes */
+	/*
+	 * This is limit on amount of *uncompressed* worth of data
+	 * we can hold. When backing swap device is provided, it is
+	 * set equal to device size.
+	 */
+	size_t disksize;	/* bytes */
+
+
+	/* backing swap device info */
+	struct block_device *backing_dev;
+	struct file *swap_file;
+	int old_block_size;
+};
+
+struct compcache_stats {
+	/* basic stats */
+	size_t compr_size;	/* compressed size of pages stored -
+				 * needed to enforce memlimit */
+	/* more stats */
+#if defined(STATS)
+	u64 num_reads;		/* failed + successful */
+	u64 num_writes;		/* --do-- */
+	u64 failed_reads;	/* can happen when memory is too low */
+	u64 failed_writes;	/* should NEVER! happen */
+	u64 invalid_io;		/* non-swap I/O requests */
+	u64 pages_discard;	/* no. of pages freed by discard callback */
+	u32 pages_zero;		/* no. of zero filled pages */
+	u32 pages_stored;	/* no. of pages currently stored */
+	u32 good_compress;	/* no. of pages with compression ratio<=50% */
+	u32 pages_expand;	/* no. of incompressible pages */
+	u64 bdev_num_reads;	/* no. of reads on backing dev */
+	u64 bdev_num_writes;	/* no. of writes on backing dev */
+#endif
+};
+/*-- */
+
+#endif

Re: [PATCH 1/3]: compressed RAM block device

[Nick Piggin]

On Tuesday 17 March 2009 22:36:46 Nitin Gupta wrote:
>   drivers/block/Kconfig     |   22 +
>   drivers/block/Makefile    |    1 +
>   drivers/block/compcache.c |  995
> +++++++++++++++++++++++++++++++++++++++++++++ drivers/block/compcache.h | 
> 160 ++++++++
>   4 files changed, 1178 insertions(+), 0 deletions(-)
>
> Creates RAM based block device (ramzswap0) which can be used as swap
> device. Pages swapped to this are compressed and stored in memory itself.
>
> The module is called compcache.ko. It depends on:
>   - xvmalloc.ko: memory allocator
>   - lzo_compress.ko
>   - lzo_decompress.ko
>
> See Documentation/blockdev/compcache.txt for usage details.
>
> Project home: http://code.google.com/p/compcache/

I wonder how hard it would be to make the compression code
use an arbitrary file or device for the storage backend rather
than make a new block device? Then you could make a new ram
block device that can swap its pages out (or even extend brd.c
with that functionality, or use loop on tmpfs etc).

Re: [PATCH 1/3]: compressed RAM block device

[Nitin Gupta]

Nick Piggin wrote:
> On Tuesday 17 March 2009 22:36:46 Nitin Gupta wrote:
>>   drivers/block/Kconfig     |   22 +
>>   drivers/block/Makefile    |    1 +
>>   drivers/block/compcache.c |  995
>> +++++++++++++++++++++++++++++++++++++++++++++ drivers/block/compcache.h | 
>> 160 ++++++++
>>   4 files changed, 1178 insertions(+), 0 deletions(-)
>>
>> Creates RAM based block device (ramzswap0) which can be used as swap
>> device. Pages swapped to this are compressed and stored in memory itself.
>>
>> The module is called compcache.ko. It depends on:
>>   - xvmalloc.ko: memory allocator
>>   - lzo_compress.ko
>>   - lzo_decompress.ko
>>
>> See Documentation/blockdev/compcache.txt for usage details.
>>
>> Project home: http://code.google.com/p/compcache/
> 

> I wonder how hard it would be to make the compression code
> use an arbitrary file or device for the storage backend rather
> than make a new block device? Then you could make a new ram
> block device that can swap its pages out (or even extend brd.c
> with that functionality, or use loop on tmpfs etc).
> 

Its already done as mentioned in Documentation/blockdev/compcache.txt
(patch 3/3) but I should have added it here also: compcache accepts
"backing_dev" parameter for backing swap partition - this allows us to
forward any R/W request to backing swap device. Currently only poorly
compressible pages (compress length > PAGE_SIZE/2) are forwarded to
backing_dev.

Thanks,
Nitin

Re: [PATCH 1/3]: compressed RAM block device

[Nick Piggin]

On Wednesday 18 March 2009 23:38:30 Nitin Gupta wrote:
> Nick Piggin wrote:
> > On Tuesday 17 March 2009 22:36:46 Nitin Gupta wrote:
> >>   drivers/block/Kconfig     |   22 +
> >>   drivers/block/Makefile    |    1 +
> >>   drivers/block/compcache.c |  995
> >> +++++++++++++++++++++++++++++++++++++++++++++ drivers/block/compcache.h
> >> | 160 ++++++++
> >>   4 files changed, 1178 insertions(+), 0 deletions(-)
> >>
> >> Creates RAM based block device (ramzswap0) which can be used as swap
> >> device. Pages swapped to this are compressed and stored in memory
> >> itself.
> >>
> >> The module is called compcache.ko. It depends on:
> >>   - xvmalloc.ko: memory allocator
> >>   - lzo_compress.ko
> >>   - lzo_decompress.ko
> >>
> >> See Documentation/blockdev/compcache.txt for usage details.
> >>
> >> Project home: http://code.google.com/p/compcache/
> >
> > I wonder how hard it would be to make the compression code
> > use an arbitrary file or device for the storage backend rather
> > than make a new block device? Then you could make a new ram
> > block device that can swap its pages out (or even extend brd.c
> > with that functionality, or use loop on tmpfs etc).
>
> Its already done as mentioned in Documentation/blockdev/compcache.txt
> (patch 3/3) but I should have added it here also: compcache accepts
> "backing_dev" parameter for backing swap partition - this allows us to

I meant more like compressing the data then passing it down to
another device. I guess you still need to make a block device to
operate on the requests, however, so it would get more complicated
because you'd need to pass your own down.... yeah it would probably
end up being just as complex and less efficient. Ignore me then :)


> forward any R/W request to backing swap device. Currently only poorly
> compressible pages (compress length > PAGE_SIZE/2) are forwarded to
> backing_dev.

Re: [PATCH 1/3]: compressed RAM block device

[Pavel Machek]

On Tue 2009-03-17 17:06:46, Nitin Gupta wrote:
>  drivers/block/Kconfig     |   22 +
>  drivers/block/Makefile    |    1 +
>  drivers/block/compcache.c |  995 +++++++++++++++++++++++++++++++++++++++++++++
>  drivers/block/compcache.h |  160 ++++++++
>  4 files changed, 1178 insertions(+), 0 deletions(-)
>
> Creates RAM based block device (ramzswap0) which can be used as swap device.
> Pages swapped to this are compressed and stored in memory itself.
>
> The module is called compcache.ko. It depends on:
>  - xvmalloc.ko: memory allocator
>  - lzo_compress.ko
>  - lzo_decompress.ko
>
> See Documentation/blockdev/compcache.txt for usage details.
>
> Project home: http://code.google.com/p/compcache/


Compcache is really bad name for this. zramdisk? gzrd?

Is the block device useful for general filesystem storage?

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Re: [PATCH 1/3]: compressed RAM block device

[Nitin Gupta]

Pavel Machek wrote:
> On Tue 2009-03-17 17:06:46, Nitin Gupta wrote:
>>  drivers/block/Kconfig     |   22 +
>>  drivers/block/Makefile    |    1 +
>>  drivers/block/compcache.c |  995 +++++++++++++++++++++++++++++++++++++++++++++
>>  drivers/block/compcache.h |  160 ++++++++
>>  4 files changed, 1178 insertions(+), 0 deletions(-)
>>
>> Creates RAM based block device (ramzswap0) which can be used as swap device.
>> Pages swapped to this are compressed and stored in memory itself.
>>
>> The module is called compcache.ko. It depends on:
>>  - xvmalloc.ko: memory allocator
>>  - lzo_compress.ko
>>  - lzo_decompress.ko
>>
>> See Documentation/blockdev/compcache.txt for usage details.
>>
>> Project home: http://code.google.com/p/compcache/
> 
> 
> Compcache is really bad name for this. zramdisk? gzrd?
> 
It was named compcache according to its original goal:
compressed caching for anonymous _and_ filesystem caches. This ram block
device is for handling anonymous memory only. Anyway, actual block device
is called ramzswap which you might like more :)

> Is the block device useful for general filesystem storage?
> 
No. It can only handle page aligned I/O. But it shouldn't be too hard
to make it generic compressed ram disk with physical backing device support.
But unfortunately I do not have bandwidth to do this myself.

Thanks,
Nitin

[PATCH 2/3]: xvmalloc memory allocator

[Nitin Gupta]

  include/linux/xvmalloc.h |   30 +++
  init/Kconfig             |   14 +
  mm/Makefile              |    1 +
  mm/xvmalloc.c            |  605 ++++++++++++++++++++++++++++++++++++++++++++++
  mm/xvmalloc_int.h        |   91 +++++++
  5 files changed, 741 insertions(+), 0 deletions(-)

xvMalloc is a memory allocator designed specifically for compcache project.

* Features:
  - Low metadata overhead (just 4 bytes per object)
  - O(1) Alloc/Free - except when we have to call system page allocator to
	get additional memory.
  - Very low fragmentation: In all tests, xvMalloc memory usage is within 12%
	of "Ideal".

One of the main highlights is that it maps pages only when required.
So, it does not hog vmalloc area which is very small on 32-bit systems.

Slub allocator could not be used due to fragmentation issues:
http://code.google.com/p/compcache/wiki/AllocatorsComparison
Data here shows kmalloc using ~43% more memory than TLSF and xvMalloc
is showed ~2% more space efficiency than TLSF (due to smaller metadata).

* Implementation:
It uses two-level bitmap search to find free list containing block of
correct size. This idea is taken from TLSF (Two-Level Segregate Fit)
allocator and is well explained in its paper (see [Links] below).
Highlights:
  - Pool based allocator: each pool can grow/shrink.
  - Immediate coalescing of free blocks.
  - Maps/Unmaps memory pages only when required.

* Limitations:
  - Poor scalability: No per-cpu data structures (work in progress).

[Links]
1. Details and Performance data:
http://code.google.com/p/compcache/wiki/xvMalloc
http://code.google.com/p/compcache/wiki/xvMallocPerformance

2. TLSF memory allocator:
home: http://rtportal.upv.es/rtmalloc/
paper: http://rtportal.upv.es/rtmalloc/files/MRBC_2008.pdf

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
---

diff --git a/include/linux/xvmalloc.h b/include/linux/xvmalloc.h
new file mode 100755
index 0000000..92366bd
--- /dev/null
+++ b/include/linux/xvmalloc.h
@@ -0,0 +1,30 @@
+/*
+ * xvmalloc.h
+ *
+ * Copyright (C) 2008, 2009  Nitin Gupta
+ *
+ * This code is released using a dual license strategy: GPL/LGPL
+ * You can choose the licence that better fits your requirements.
+ *
+ * Released under the terms of the GNU General Public License Version 2.0
+ * Released under the terms of the GNU Lesser General Public License Version 2.1
+ */
+
+#ifndef _XVMALLOC_H_
+#define _XVMALLOC_H_
+
+#define INVALID_POOL_ID	NULL
+#define INVALID_PGNUM	((u32)(-1))
+
+struct Pool;
+
+struct Pool *xvCreateMemPool(void);
+void xvDestroyMemPool(struct Pool *pool);
+
+int xvMalloc(struct Pool *pool, u32 size, u32 *pageNum, u32 *offset);
+void xvFree(struct Pool *pool, u32 pageNum, u32 offset);
+
+u32 xvGetObjectSize(void *obj);
+u64 xvGetTotalSizeBytes(struct Pool *pool);
+
+#endif
diff --git a/init/Kconfig b/init/Kconfig
index 6a5c5fe..5e722cc 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -930,6 +930,20 @@ config SLOB

  endchoice

+config XVMALLOC
+	tristate "xvMalloc memory allocator"
+	help
+	   This is a simple, low fragmentation, O(1) allocator.
+	   Details: http://code.google.com/p/compcache/wiki/xvMalloc
+
+config XVMALLOC_STATS
+	depends on XVMALLOC
+	bool "Collect statistics"
+	default y
+	help
+	  Collect basic allocator statistics with minimal overhead.
+	  In unsure, say Y.
+
  config PROFILING
  	bool "Profiling support (EXPERIMENTAL)"
  	help
diff --git a/mm/Makefile b/mm/Makefile
index 72255be..b6b705f 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -26,6 +26,7 @@ obj-$(CONFIG_SLOB) += slob.o
  obj-$(CONFIG_MMU_NOTIFIER) += mmu_notifier.o
  obj-$(CONFIG_SLAB) += slab.o
  obj-$(CONFIG_SLUB) += slub.o
+obj-$(CONFIG_XVMALLOC) += xvmalloc.o
  obj-$(CONFIG_FAILSLAB) += failslab.o
  obj-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
  obj-$(CONFIG_FS_XIP) += filemap_xip.o
diff --git a/mm/xvmalloc.c b/mm/xvmalloc.c
new file mode 100755
index 0000000..3fcc012
--- /dev/null
+++ b/mm/xvmalloc.c
@@ -0,0 +1,605 @@
+/*
+ * xvmalloc.c
+ *
+ * Copyright (C) 2008, 2009  Nitin Gupta
+ *
+ * This code is released using a dual license strategy: GPL/LGPL
+ * You can choose the licence that better fits your requirements.
+ *
+ * Released under the terms of the GNU General Public License Version 2.0
+ * Released under the terms of the GNU Lesser General Public License Version 2.1
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/bitops.h>
+#include <linux/errno.h>
+#include <linux/highmem.h>
+#include <linux/init.h>
+#include <linux/string.h>
+#include <linux/slab.h>
+#include <linux/xvmalloc.h>
+
+#include "xvmalloc_int.h"
+
+#ifdef CONFIG_XVMALLOC_STATS
+static void statInc(u64 *value)
+{
+	*value = *value + 1;
+}
+
+static void statDec(u64 *value)
+{
+	*value = *value - 1;
+}
+#else
+#define statInc(x) do { } while (0)
+#define statDec(x) do { } while (0)
+#endif
+
+static void SetBit(u32 *value, u32 bitIdx)
+{
+	*value |= (u32)(1 << bitIdx);
+}
+
+static void ClearBit(u32 *value, u32 bitIdx)
+{
+	*value &= (u32)(~(1 << bitIdx));
+}
+
+static u32 IsBlockFree(struct BlockHeader *block)
+{
+	return block->prev & BLOCK_FREE;
+}
+
+static u32 IsPrevBlockFree(struct BlockHeader *block)
+{
+	return block->prev & PREV_FREE;
+}
+
+static void SetBlockFree(struct BlockHeader *block)
+{
+	block->prev |= BLOCK_FREE;
+}
+
+static void SetBlockPrevFree(struct BlockHeader *block)
+{
+	block->prev |= PREV_FREE;
+}
+
+static void SetBlockUsed(struct BlockHeader *block)
+{
+	block->prev &= ~BLOCK_FREE;
+}
+
+static void SetBlockPrevUsed(struct BlockHeader *block)
+{
+	block->prev &= ~PREV_FREE;
+}
+
+static u16 GetBlockPrev(struct BlockHeader *block)
+{
+	return block->prev & PREV_MASK;
+}
+
+static void SetBlockPrev(struct BlockHeader *block, u16 newOffset)
+{
+	block->prev = newOffset | (block->prev & FLAGS_MASK);
+}
+
+/*
+ * Get index of free list containing blocks of maximum size
+ * which is less than or equal to given size.
+ */
+static u32 GetIndexForInsert(u32 size)
+{
+	size = size > XV_MAX_ALLOC_SIZE ? XV_MAX_ALLOC_SIZE : size;
+	size &= ~FL_DELTA_MASK;
+	return (size - XV_MIN_ALLOC_SIZE) >> FL_DELTA_SHIFT;
+}
+
+/*
+ * Get index of free list having blocks of size greater than
+ * or equal to requested size.
+ */
+static u32 GetIndex(u32 size)
+{
+	size = (size + FL_DELTA_MASK) & ~FL_DELTA_MASK;
+	return (size - XV_MIN_ALLOC_SIZE) >> FL_DELTA_SHIFT;
+}
+
+/*
+ * Given <pageNum, offset> pair, provide a derefrencable pointer.
+ * This is called from xvMalloc/xvFree path, so it needs to be fast.
+ */
+static void *GetPtrAtomic(u32 pageNum, u16 offset, enum km_type type)
+{
+	unsigned char *pagePtr;
+
+	pagePtr = kmap_atomic(pfn_to_page(pageNum), type);
+	return pagePtr + offset;
+}
+
+static void PutPtrAtomic(void *ptr, enum km_type type)
+{
+	kunmap_atomic(ptr, type);
+}
+
+/*
+ * Allocate a memory page. Called when a pool needs to grow.
+ */
+static u32 AllocPage(void)
+{
+	struct page *page;
+
+	page = alloc_page(GFP_NOIO | __GFP_HIGHMEM);
+
+	if (unlikely(!page))
+		return INVALID_PGNUM;
+
+	return page_to_pfn(page);
+}
+
+/*
+ * Called when all objects in a page are freed.
+ */
+static void FreePage(u32 pageNum)
+{
+	__free_page(pfn_to_page(pageNum));
+}
+
+/**
+ * FindBlock - find block of at least given size
+ * @pool: memory pool to search from
+ * @size: size of block required
+ * @pageNum: page no. containing required block
+ * @offset: offset within the page where block is located.
+ *
+ * Searches two level bitmap to locate block of at least
+ * the given size. If such a block is found, it provides
+ * <pageNum, offset> to identify this block and returns index
+ * in FreeList where we found this block.
+ * Otherwise, returns 0 and <pageNum, offset> params are not touched.
+ */
+static u32 FindBlock(struct Pool *pool, u32 size, u32 *pageNum, u32 *offset)
+{
+	u32 flBitmap, slBitmap;
+	u32 flIndex, slIndex, slBitStart;
+
+	/* There are no free blocks in this pool */
+	if (!pool->flBitmap)
+		return 0;
+
+	if (unlikely(size < XV_MIN_ALLOC_SIZE))
+		size = XV_MIN_ALLOC_SIZE;
+
+	/* Get FreeList index correspoding to this size */
+	slIndex = GetIndex(size);
+	slBitmap = pool->slBitmap[slIndex >> BITMAP_SHIFT];
+	slBitStart = slIndex & BITMAP_MASK;
+
+	/*
+	 * If FreeList is not empty at this index, we found the
+	 * block - head of this list. This is approximate best-fit match.
+	 */
+	if (slBitmap & (1 << slBitStart)) {
+		*pageNum = pool->FreeList[slIndex].pageNum;
+		*offset = pool->FreeList[slIndex].offset;
+		return slIndex;
+	}
+
+	/*
+	 * No best-fit found. Search a bit further in bitmap for a free block.
+	 * Second level bitmap consists of series of 32-bit chunks. Search
+	 * further in the chunk where we expected a best-fit, starting from
+	 * index location found above.
+	 */
+	slBitStart++;
+	slBitmap >>= slBitStart;
+
+	/* Skip this search if we were already at end of this bitmap chunk */
+	if ((slBitStart != BITMAP_BITS) && slBitmap) {
+		slIndex += ffs(slBitmap);
+		*pageNum = pool->FreeList[slIndex].pageNum;
+		*offset = pool->FreeList[slIndex].offset;
+		return slIndex;
+	}
+
+	/* Now do a full two-level bitmap search to find next nearest fit */
+	flIndex = slIndex >> BITMAP_SHIFT;
+
+	flBitmap = (pool->flBitmap) >> (flIndex + 1);
+	if (!flBitmap)
+		return 0;
+
+	flIndex += ffs(flBitmap);
+	slBitmap = pool->slBitmap[flIndex];
+	slIndex = (flIndex << BITMAP_SHIFT) + ffs(slBitmap) - 1;
+	*pageNum = pool->FreeList[slIndex].pageNum;
+	*offset = pool->FreeList[slIndex].offset;
+
+	return slIndex;
+}
+
+/*
+ * Insert block at <pageNum, offset> in freelist of given pool.
+ * FreeList used depends on block size.
+ */
+static void InsertBlock(struct Pool *pool, u32 pageNum, u32 offset,
+			struct BlockHeader *block)
+{
+	u32 flIndex, slIndex;
+	struct BlockHeader *nextBlock;
+
+	slIndex = GetIndexForInsert(block->size);
+	flIndex = slIndex >> BITMAP_SHIFT;
+
+	block->link.prevPageNum = INVALID_PGNUM;
+	block->link.prevOffset = 0;
+	block->link.nextPageNum = pool->FreeList[slIndex].pageNum;
+	block->link.nextOffset = pool->FreeList[slIndex].offset;
+	pool->FreeList[slIndex].pageNum = pageNum;
+	pool->FreeList[slIndex].offset = offset;
+
+	if (block->link.nextPageNum != INVALID_PGNUM) {
+		nextBlock = (struct BlockHeader *)GetPtrAtomic(
+				block->link.nextPageNum,
+				block->link.nextOffset, KM_USER1
+			);
+		nextBlock->link.prevPageNum = pageNum;
+		nextBlock->link.prevOffset = offset;
+		PutPtrAtomic(nextBlock, KM_USER1);
+	}
+
+	SetBit(&pool->slBitmap[flIndex], slIndex & BITMAP_MASK);
+	SetBit(&pool->flBitmap, flIndex);
+}
+
+/*
+ * Remove block from head of freelist. Index 'slIndex' identifies the FreeList.
+ */
+static void RemoveBlockHead(struct Pool *pool, struct BlockHeader *block,
+			    u32 slIndex)
+{
+	struct BlockHeader *tmpBlock;
+	u32 flIndex = slIndex >> BITMAP_SHIFT;
+
+	pool->FreeList[slIndex].pageNum = block->link.nextPageNum;
+	pool->FreeList[slIndex].offset = block->link.nextOffset;
+	block->link.prevPageNum = INVALID_PGNUM;
+	block->link.prevOffset = 0;
+
+	if (pool->FreeList[slIndex].pageNum == INVALID_PGNUM) {
+		ClearBit(&pool->slBitmap[flIndex], slIndex & BITMAP_MASK);
+		if (!pool->slBitmap[flIndex])
+			ClearBit(&pool->flBitmap, flIndex);
+	} else {
+		/*
+		 * DEBUG ONLY: We need not reinitialize freelist head previous
+		 * pointer to INVALID_PGNUM - we never depend on its value.
+		 * But just for sanity, lets keep it.
+		 */
+		tmpBlock = (struct BlockHeader *)GetPtrAtomic(
+				pool->FreeList[slIndex].pageNum,
+				pool->FreeList[slIndex].offset, KM_USER1
+			);
+		tmpBlock->link.prevPageNum = INVALID_PGNUM;
+		tmpBlock->link.prevOffset = 0;
+		PutPtrAtomic(tmpBlock, KM_USER1);
+	}
+}
+
+/*
+ * Remove block from freelist. Index 'slIndex' identifies the FreeList.
+ */
+static void RemoveBlock(struct Pool *pool, u32 pageNum, u32 offset,
+			struct BlockHeader *block, u32 slIndex)
+{
+	u32 flIndex;
+	struct BlockHeader *tmpBlock;
+
+	if (pool->FreeList[slIndex].pageNum == pageNum
+	   && pool->FreeList[slIndex].offset == offset) {
+		RemoveBlockHead(pool, block, slIndex);
+		return;
+	}
+
+	flIndex = slIndex >> BITMAP_SHIFT;
+
+	if (block->link.prevPageNum != INVALID_PGNUM) {
+		tmpBlock = (struct BlockHeader *)GetPtrAtomic(
+				block->link.prevPageNum,
+				block->link.prevOffset, KM_USER1
+			);
+		tmpBlock->link.nextPageNum = block->link.nextPageNum;
+		tmpBlock->link.nextOffset = block->link.nextOffset;
+		PutPtrAtomic(tmpBlock, KM_USER1);
+	}
+
+	if (block->link.nextPageNum != INVALID_PGNUM) {
+		tmpBlock = (struct BlockHeader *)GetPtrAtomic(
+				block->link.nextPageNum,
+				block->link.nextOffset, KM_USER1
+			);
+		tmpBlock->link.prevPageNum = block->link.prevPageNum;
+		tmpBlock->link.prevOffset = block->link.prevOffset;
+		PutPtrAtomic(tmpBlock, KM_USER1);
+	}
+
+	return;
+}
+
+/*
+ * Allocate a page and add it FreeList of given pool.
+ */
+static int GrowPool(struct Pool *pool)
+{
+	u32 pageNum;
+	struct BlockHeader *block;
+
+	pageNum = AllocPage();
+	if (unlikely(pageNum == INVALID_PGNUM))
+		return -ENOMEM;
+
+	statInc(&pool->totalPages);
+
+	spin_lock(&pool->lock);
+	block = GetPtrAtomic(pageNum, 0, KM_USER0);
+
+	block->size = PAGE_SIZE - XV_ALIGN;
+	SetBlockFree(block);
+	SetBlockPrevUsed(block);
+	SetBlockPrev(block, 0);
+
+	InsertBlock(pool, pageNum, 0, block);
+
+	PutPtrAtomic(block, KM_USER0);
+	spin_unlock(&pool->lock);
+
+	return 0;
+}
+
+/*
+ * Create a memory pool. Allocates FreeList, bitmaps and other
+ * per-pool metadata.
+ */
+struct Pool *xvCreateMemPool(void)
+{
+	int i;
+	void *ovhdMem;
+	struct Pool *pool;
+	u32 poolOvhd;
+
+	poolOvhd = ROUNDUP(sizeof(*pool), PAGE_SIZE);
+	ovhdMem = kmalloc(poolOvhd, GFP_KERNEL);
+	if (!ovhdMem)
+		return INVALID_POOL_ID;
+
+	pool = ovhdMem;
+	memset(pool, 0, poolOvhd);
+
+	for (i = 0; i < NUM_FREE_LISTS; i++)
+		pool->FreeList[i].pageNum = INVALID_PGNUM;
+
+	spin_lock_init(&pool->lock);
+
+	return pool;
+}
+EXPORT_SYMBOL_GPL(xvCreateMemPool);
+
+void xvDestroyMemPool(struct Pool *pool)
+{
+	kfree(pool);
+}
+EXPORT_SYMBOL_GPL(xvDestroyMemPool);
+
+/**
+ * xvMalloc - Allocate block of given size from pool.
+ * @pool: pool to allocate from
+ * @size: size of block to allocate
+ * @pageNum: page no. that holds the object
+ * @offset: location of object within pageNum
+ *
+ * On success, <pageNum, offset> identifies block allocated
+ * and 0 is returned. On failure, <pageNum, offset> is not touched
+ * and -ENOMEM is returned.
+ *
+ * Allocation requests with size > XV_MAX_ALLOC_SIZE will fail.
+ */
+int xvMalloc(struct Pool *pool, u32 size, u32 *pageNum, u32 *offset)
+{
+	int error;
+	u32 index, tmpSize, origSize, tmpOffset;
+	struct BlockHeader *block, *tmpBlock = NULL;
+
+	*pageNum = INVALID_PGNUM;
+	*offset = 0;
+	origSize = size;
+
+	if (unlikely(!size || size > XV_MAX_ALLOC_SIZE))
+		return -ENOMEM;
+
+	if (unlikely(size < XV_MIN_ALLOC_SIZE))
+		size = XV_MIN_ALLOC_SIZE;
+	else
+		size = ROUNDUP_ALIGN(size);
+
+	spin_lock(&pool->lock);
+
+	index = FindBlock(pool, size, pageNum, offset);
+
+	if (*pageNum == INVALID_PGNUM) {
+		spin_unlock(&pool->lock);
+		error = GrowPool(pool);
+		if (unlikely(error))
+			return -ENOMEM;
+
+		spin_lock(&pool->lock);
+		index = FindBlock(pool, size, pageNum, offset);
+	}
+
+	if (*pageNum == INVALID_PGNUM) {
+		spin_unlock(&pool->lock);
+		return -ENOMEM;
+	}
+
+	block = (struct BlockHeader *)GetPtrAtomic(
+				*pageNum, *offset, KM_USER0);
+
+	RemoveBlockHead(pool, block, index);
+
+	/* Split the block if required */
+	tmpOffset = *offset + size + XV_ALIGN;
+	tmpSize = block->size - size;
+	tmpBlock = (struct BlockHeader *)((char *)block + size + XV_ALIGN);
+	if (tmpSize) {
+		tmpBlock->size = tmpSize - XV_ALIGN;
+		SetBlockFree(tmpBlock);
+		SetBlockPrevUsed(tmpBlock);
+
+		SetBlockPrev(tmpBlock, *offset);
+		if (tmpBlock->size >= XV_MIN_ALLOC_SIZE)
+			InsertBlock(pool, *pageNum, tmpOffset, tmpBlock);
+
+		if (tmpOffset + XV_ALIGN + tmpBlock->size < PAGE_SIZE) {
+			tmpBlock = (struct BlockHeader *)((char *)tmpBlock
+						+ tmpBlock->size + XV_ALIGN);
+
+			SetBlockPrev(tmpBlock, tmpOffset);
+		}
+	} else {
+		/* This block is exact fit */
+		if (tmpOffset < PAGE_SIZE)
+			SetBlockPrevUsed(tmpBlock);
+	}
+
+	block->size = origSize;
+	SetBlockUsed(block);
+
+	PutPtrAtomic(block, KM_USER0);
+	spin_unlock(&pool->lock);
+
+	*offset += XV_ALIGN;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(xvMalloc);
+
+/*
+ * Free block identified with <pageNum, offset>
+ */
+void xvFree(struct Pool *pool, u32 pageNum, u32 offset)
+{
+	void *page;
+	struct BlockHeader *block, *tmpBlock;
+
+	offset -= XV_ALIGN;
+
+	spin_lock(&pool->lock);
+
+	page = GetPtrAtomic(pageNum, 0, KM_USER0);
+	block = (struct BlockHeader *)((char *)page + offset);
+
+	if (unlikely(block->size < XV_MIN_ALLOC_SIZE))
+		block->size = XV_MIN_ALLOC_SIZE;
+	else
+		block->size = ROUNDUP_ALIGN(block->size);
+
+	tmpBlock = (struct BlockHeader *)((char *)block +
+					block->size + XV_ALIGN);
+	if (offset + block->size + XV_ALIGN == PAGE_SIZE)
+		tmpBlock = NULL;
+
+	/* Merge next block if its free */
+	if (tmpBlock && IsBlockFree(tmpBlock)) {
+		/*
+		 * Blocks smaller than XV_MIN_ALLOC_SIZE
+		 * are not inserted in any free list.
+		 */
+		if (tmpBlock->size >= XV_MIN_ALLOC_SIZE) {
+			RemoveBlock(pool, pageNum,
+				    offset + block->size + XV_ALIGN, tmpBlock,
+				    GetIndexForInsert(tmpBlock->size));
+		}
+		block->size += tmpBlock->size + XV_ALIGN;
+	}
+
+	/* Merge previous block if its free */
+	if (IsPrevBlockFree(block)) {
+		tmpBlock = (struct BlockHeader *)((char *)(page) +
+						GetBlockPrev(block));
+		offset = offset - tmpBlock->size - XV_ALIGN;
+
+		if (tmpBlock->size >= XV_MIN_ALLOC_SIZE)
+			RemoveBlock(pool, pageNum, offset, tmpBlock,
+				    GetIndexForInsert(tmpBlock->size));
+
+		tmpBlock->size += block->size + XV_ALIGN;
+		block = tmpBlock;
+	}
+
+	/* No used objects in this page. Free it. */
+	if (block->size == PAGE_SIZE - XV_ALIGN) {
+		PutPtrAtomic(page, KM_USER0);
+		spin_unlock(&pool->lock);
+
+		FreePage(pageNum);
+		statDec(&pool->totalPages);
+		return;
+	}
+
+	SetBlockFree(block);
+	InsertBlock(pool, pageNum, offset, block);
+
+	if (offset + block->size < PAGE_SIZE - XV_ALIGN) {
+		tmpBlock = (struct BlockHeader *)((char *)(block) +
+						block->size + XV_ALIGN);
+		SetBlockPrevFree(tmpBlock);
+		SetBlockPrev(tmpBlock, offset);
+	}
+
+	PutPtrAtomic(page, KM_USER0);
+	spin_unlock(&pool->lock);
+
+	return;
+}
+EXPORT_SYMBOL_GPL(xvFree);
+
+u32 xvGetObjectSize(void *obj)
+{
+	struct BlockHeader *blk;
+
+	blk = (struct BlockHeader *)((char *)(obj) - XV_ALIGN);
+	return blk->size;
+}
+EXPORT_SYMBOL_GPL(xvGetObjectSize);
+
+/*
+ * Returns total memory used by allocator (userdata + metadata)
+ */
+u64 xvGetTotalSizeBytes(struct Pool *pool)
+{
+#ifdef CONFIG_XVMALLOC_STATS
+	return pool->totalPages << PAGE_SHIFT;
+#else
+	return 0;
+#endif
+}
+EXPORT_SYMBOL_GPL(xvGetTotalSizeBytes);
+
+static int __init xvMallocInit(void)
+{
+	return 0;
+}
+
+static void __exit xvMallocExit(void)
+{
+	return;
+}
+
+module_init(xvMallocInit);
+module_exit(xvMallocExit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");
+MODULE_DESCRIPTION("xvMalloc Memory Allocator");
diff --git a/mm/xvmalloc_int.h b/mm/xvmalloc_int.h
new file mode 100755
index 0000000..5c648ee
--- /dev/null
+++ b/mm/xvmalloc_int.h
@@ -0,0 +1,91 @@
+/*
+ * xvmalloc_int.c
+ *
+ * Copyright (C) 2008, 2009  Nitin Gupta
+ *
+ * This code is released using a dual license strategy: GPL/LGPL
+ * You can choose the licence that better fits your requirements.
+ *
+ * Released under the terms of the GNU General Public License Version 2.0
+ * Released under the terms of the GNU Lesser General Public License Version 2.1
+ */
+
+#ifndef _XVMALLOC_INT_H_
+#define _XVMALLOC_INT_H_
+
+#include <linux/types.h>
+
+#define ROUNDUP(x, y)	(((x) + (y) - 1) / (y) * (y))
+/* Each individual bitmap is 32-bit */
+#define BITMAP_BITS	32
+#define BITMAP_SHIFT	5
+#define BITMAP_MASK	(BITMAP_BITS - 1)
+
+/* User configurable params */
+
+/* This must be greater than sizeof(LinkFree) */
+#define XV_MIN_ALLOC_SIZE       32
+#define XV_MAX_ALLOC_SIZE       (4096 - XV_MIN_ALLOC_SIZE)
+
+/* Must be power of two */
+#define XV_ALIGN_SHIFT	2
+#define XV_ALIGN	(1 << XV_ALIGN_SHIFT)
+#define XV_ALIGN_MASK	(XV_ALIGN - 1)
+
+/* Free lists are separated by FL_DELTA bytes */
+#define FL_DELTA_SHIFT	3
+#define FL_DELTA	(1 << FL_DELTA_SHIFT)
+#define FL_DELTA_MASK	(FL_DELTA - 1)
+#define NUM_FREE_LISTS	((XV_MAX_ALLOC_SIZE - XV_MIN_ALLOC_SIZE) \
+				/ FL_DELTA + 1)
+
+#define MAX_FLI		(ROUNDUP(NUM_FREE_LISTS, 32) / 32)
+
+/* End of user params */
+
+#define ROUNDUP_ALIGN(x)	(((x) + XV_ALIGN_MASK) & ~XV_ALIGN_MASK)
+
+#define BLOCK_FREE	0x1
+#define PREV_FREE	0x2
+#define FLAGS_MASK	XV_ALIGN_MASK
+#define PREV_MASK	(~FLAGS_MASK)
+
+struct FreeListEntry {
+	u32 pageNum;
+	u16 offset;
+	u16 pad;
+};
+
+struct LinkFree {
+	u32 prevPageNum;
+	u32 nextPageNum;
+	u16 prevOffset;
+	u16 nextOffset;
+};
+
+struct BlockHeader {
+	union {
+		/* This common header must be ALIGN bytes */
+		u8 common[XV_ALIGN];
+		struct {
+			u16 size;
+			u16 prev;
+		};
+	};
+	struct LinkFree link;
+};
+
+struct Pool {
+	u32 flBitmap;
+	u32 slBitmap[MAX_FLI];
+	spinlock_t lock;
+
+	struct FreeListEntry FreeList[NUM_FREE_LISTS];
+
+	/* stats */
+#ifdef CONFIG_XVMALLOC_STATS
+	u64 totalPages;
+#endif
+};
+
+#endif

Re: [PATCH 2/3]: xvmalloc memory allocator

[Christoph Lameter]

On Tue, 17 Mar 2009, Nitin Gupta wrote:

> Slub allocator could not be used due to fragmentation issues:
> http://code.google.com/p/compcache/wiki/AllocatorsComparison
> Data here shows kmalloc using ~43% more memory than TLSF and xvMalloc
> is showed ~2% more space efficiency than TLSF (due to smaller metadata).

Well this looks like the use of 2^n sizes of the kmalloc arrayse cause a
lot of wastage here. Creating slab caches that fit your needs may help?
Have you had a look at the SLOB approach?

Re: [PATCH 2/3]: xvmalloc memory allocator

[Nitin Gupta]

(sending again since it was blocked by lkml last time. Sorry Christoph for duplicate email).

Christoph Lameter wrote:
> On Tue, 17 Mar 2009, Nitin Gupta wrote:
> 
>> Slub allocator could not be used due to fragmentation issues:
>> http://code.google.com/p/compcache/wiki/AllocatorsComparison
>> Data here shows kmalloc using ~43% more memory than TLSF and xvMalloc
>> is showed ~2% more space efficiency than TLSF (due to smaller metadata).
> 
> Well this looks like the use of 2^n sizes of the kmalloc arrayse cause a
> lot of wastage here. Creating slab caches that fit your needs may help?

Creating slabs for sizes in range, say, [32, 3/4*PAGE_SIZE] separated by 64bytes
will require 48 slabs! Then for slab of each size class will have wastage due to
unused slab objects in each class.
Larger difference in slab sizes (and thus small no. of them), will surely cause too much
wastage due to internal fragmentation.

Another (more important) point to consider is that, use of slabs will eat-up vmalloc
area to keep slab memory backed by VA space. On 32-bit systems, vmalloc area
is small and limits amount of memory that can be allocated for compressed pages.
With xvmalloc we map/unmap pages on demand thus removing dependence on
vmalloc VA area.

 > Have you had a look at the SLOB approach?
 >

Nope. I will see how this may help.

Thanks,
Nitin

Re: [PATCH 2/3]: xvmalloc memory allocator

[Christoph Lameter]

On Tue, 17 Mar 2009, Nitin Gupta wrote:

> Creating slabs for sizes in range, say, [32, 3/4*PAGE_SIZE] separated by
> 64bytes
> will require 48 slabs! Then for slab of each size class will have wastage
> due to
> unused slab objects in each class.
> Larger difference in slab sizes (and thus small no. of them), will surely
> cause too much
> wastage due to internal fragmentation.

The slabs that match existing other slabs of similar sizes will be aliased
and not created. Create the 48 slabs and you likely will only use 10 real
additional ones. The rest will just be pointing to existing ones.

> Another (more important) point to consider is that, use of slabs will
> eat-up vmalloc area to keep slab memory backed by VA space. On 32-bit
> systems, vmalloc area is small and limits amount of memory that can be
> allocated for compressed pages. With xvmalloc we map/unmap pages on
> demand thus removing dependence on vmalloc VA area.

Slab memory is not backed by vmalloc space.

> > Have you had a look at the SLOB approach?
> Nope. I will see how this may help.

Slob is another attempt to reduce wastage due to the rounding up of
object sizes to 2^N in SLAB/SLUB.

Re: [PATCH 2/3]: xvmalloc memory allocator

[Pekka Enberg]

On Tue, 17 Mar 2009, Nitin Gupta wrote:
>> Creating slabs for sizes in range, say, [32, 3/4*PAGE_SIZE] separated by
>> 64bytes
>> will require 48 slabs! Then for slab of each size class will have wastage
>> due to
>> unused slab objects in each class.
>> Larger difference in slab sizes (and thus small no. of them), will surely
>> cause too much
>> wastage due to internal fragmentation.

On Tue, Mar 17, 2009 at 10:58 AM, Christoph Lameter
<cl@linux-foundation.org> wrote:
> The slabs that match existing other slabs of similar sizes will be aliased
> and not created. Create the 48 slabs and you likely will only use 10 real
> additional ones. The rest will just be pointing to existing ones.

Yup. One thing I don't quite understand is why you need all the 48
caches in the first place. Allocation sizes tend to be clustered and I
would have imagined you'd see that when compressing page sized chunks
as well. Using kmemtrace to analyze the exact reason for the bad
fragmentation would probably be helpful.

Re: [PATCH 2/3]: xvmalloc memory allocator

[Nitin Gupta]

Pekka Enberg wrote:
> On Tue, 17 Mar 2009, Nitin Gupta wrote:
>>> Creating slabs for sizes in range, say, [32, 3/4*PAGE_SIZE] separated by
>>> 64bytes
>>> will require 48 slabs! Then for slab of each size class will have wastage
>>> due to
>>> unused slab objects in each class.
>>> Larger difference in slab sizes (and thus small no. of them), will surely
>>> cause too much
>>> wastage due to internal fragmentation.
> 
> On Tue, Mar 17, 2009 at 10:58 AM, Christoph Lameter
> <cl@linux-foundation.org> wrote:
>> The slabs that match existing other slabs of similar sizes will be aliased
>> and not created. Create the 48 slabs and you likely will only use 10 real
>> additional ones. The rest will just be pointing to existing ones.
> 
> Yup. One thing I don't quite understand is why you need all the 48
> caches in the first place. Allocation sizes tend to be clustered and I
> would have imagined you'd see that when compressing page sized chunks
> as well.

Compressed page lengths sometimes do tend to cluster within somewhat
small range. However, the range of where majority of pages will lie depends
highly of workload - sometimes range is not clear and sometime there is no
preferred range at all. Please refer this data:
http://code.google.com/p/compcache/wiki/CompressedLengthDistribution

It shows compressed page size distribution for various workloads.

> Using kmemtrace to analyze the exact reason for the bad
> fragmentation would probably be helpful.
> 

That was purely internal fragmentation.
Wastage per obj = ksize(obj) - actual_size.

Code used for testing:
http://code.google.com/p/compcache/source/browse/trunk/sub-projects/testing/kmalloc_test/kmalloc_test.c
This is a "SwapReplay client". Please see:
http://code.google.com/p/compcache/wiki/SwapReplayDesign

Thanks,
Nitin

Re: [PATCH 2/3]: xvmalloc memory allocator

[Nitin Gupta]

Christoph Lameter wrote:
> On Tue, 17 Mar 2009, Nitin Gupta wrote:
> 
>> Creating slabs for sizes in range, say, [32, 3/4*PAGE_SIZE] separated by
>> 64bytes
>> will require 48 slabs! Then for slab of each size class will have wastage
>> due to
>> unused slab objects in each class.
>> Larger difference in slab sizes (and thus small no. of them), will surely
>> cause too much
>> wastage due to internal fragmentation.
> 
> The slabs that match existing other slabs of similar sizes will be aliased
> and not created. Create the 48 slabs and you likely will only use 10 real
> additional ones. The rest will just be pointing to existing ones.
> 
>> Another (more important) point to consider is that, use of slabs will
>> eat-up vmalloc area to keep slab memory backed by VA space. On 32-bit
>> systems, vmalloc area is small and limits amount of memory that can be
>> allocated for compressed pages. With xvmalloc we map/unmap pages on
>> demand thus removing dependence on vmalloc VA area.
> 
> Slab memory is not backed by vmalloc space.
> 

Oh, it uses "low memory". Still not good for compcache :)

>>> Have you had a look at the SLOB approach?
>> Nope. I will see how this may help.
> 
> Slob is another attempt to reduce wastage due to the rounding up of
> object sizes to 2^N in SLAB/SLUB.
> 
> 

I had detailed look at SLOB allocator and found it unacceptable to be
used for compcache.

To begin with, SLOB maintains just 3 freelists:
  - for size < 256     - free_slob_small
  - [256, 1024)        - free_slob_medium
  - [1024, PAGE_SIZE)  - free_slob_large

and allocates from one of these lists depending on size requested. No need to
create 50+ caches, we only get to use these 3 lists.

Why SLOB is bad:

1) O(n) allocation:
To find block of given size, it _linearaly_ scans corresponding free list to
find a page with _total_ free space >= requested size. This free space might not
be contiguous. So it runs through free blocks within each such candidate
page until it finally finds some page with free contiguous area >= requested
size.

2) When growing SLOB cache, page is added to one of 3 freelists (depending on
what size we are currently allocating). After this, this page can never move to
any other list - even if its free space drops down to fall in next range below
or vice versa. This has two problems:
   - Potentially large wastage due to "page internal fragmentation": e.g.:
alloc(3096) is satisfied from a page in 'large free list'. Now it has
1000b free (assuming 4k page) which will now never be used.
   - It can trigger unnecessary cache grows: e.g.: even though we have such
unfilled pages in 'large' list, allocation in 'small' range can still cause
cache grow if 'small free list' is empty.

3) It only allocates from "low memory". This is clearly not acceptable for
compcache.

In contrast xvmalloc is O(1): do a simple search in two-level bitmap to find
freelist containing block of required size. Obviously not O(1) in case it has
to go to system page allocator to grow pool.

Also, xvmalloc doesn't dedicate a page to any single size class - so it doesn't
suffer from above problems. Note that this might not be good in general - say,
in cases where majority of alloc requests are for some select sizes only. But
for compcache, this is not true. Also, in our case, there is no correlation
between object sizes and object lifetime - so no benefit keeping similar sized
objects together. Considering these, there's no point dedicating pages to size
classes. Instead, better go for freely mixing these objects to get maximum
packing.

...and xvmalloc is not restricted to "low memory".

Thanks,
Nitin

Re: [PATCH 2/3]: xvmalloc memory allocator

[Christoph Lameter]

On Wed, 18 Mar 2009, Nitin Gupta wrote:

> > Slab memory is not backed by vmalloc space.
> Oh, it uses "low memory". Still not good for compcache :)

Only if you use 32 bit.

Re: [PATCH 2/3]: xvmalloc memory allocator

[Nitin Gupta]

On Wed, Mar 18, 2009 at 10:28 PM, Christoph Lameter
<cl@linux-foundation.org> wrote:
> On Wed, 18 Mar 2009, Nitin Gupta wrote:
>
>> > Slab memory is not backed by vmalloc space.
>> Oh, it uses "low memory". Still not good for compcache :)
>
> Only if you use 32 bit.
>

Yes, this point is for 32-bit only - many users were unhappy with
compcache size limit due to small vmalloc area on x86-32.

Nitin

Re: [PATCH 2/3]: xvmalloc memory allocator

[Pekka Enberg]

On Wed, 18 Mar 2009, Nitin Gupta wrote:
>>> > Slab memory is not backed by vmalloc space.
>>> Oh, it uses "low memory". Still not good for compcache :)

On Wed, Mar 18, 2009 at 10:28 PM, Christoph Lameter
<cl@linux-foundation.org> wrote:
>> Only if you use 32 bit.

On Wed, Mar 18, 2009 at 10:29 AM, Nitin Gupta <ngupta@vflare.org> wrote:
> Yes, this point is for 32-bit only - many users were unhappy with
> compcache size limit due to small vmalloc area on x86-32.

Yeah, I can see the point in having a custom allocator for this. But
quite frankly the xvmalloc code is just too ugly to live with. You
might want to make it look like kernel code as per CodingStyle and
change the name to something less generic.

Re: [PATCH 2/3]: xvmalloc memory allocator

[Nitin Gupta]

Pekka Enberg wrote:
> Yeah, I can see the point in having a custom allocator for this. But
> quite frankly the xvmalloc code is just too ugly to live with. You
> might want to make it look like kernel code as per CodingStyle and
> change the name to something less generic.
> 

In beginning I also thought CamelCase coding would be a problem.
But quite frankly, this is best I could do to beautify xvmalloc code.

I will change it to kernel_style. Maybe that helps.

Thanks,
Nitin

Re: [PATCH 2/3]: xvmalloc memory allocator

[Nitin Gupta]

On Thu, Mar 19, 2009 at 12:51 AM, Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> On Wed, 18 Mar 2009, Nitin Gupta wrote:
>>>> > Slab memory is not backed by vmalloc space.
>>>> Oh, it uses "low memory". Still not good for compcache :)
>
> On Wed, Mar 18, 2009 at 10:28 PM, Christoph Lameter
> <cl@linux-foundation.org> wrote:
>>> Only if you use 32 bit.
>
> On Wed, Mar 18, 2009 at 10:29 AM, Nitin Gupta <ngupta@vflare.org> wrote:
>> Yes, this point is for 32-bit only - many users were unhappy with
>> compcache size limit due to small vmalloc area on x86-32.
>
> Yeah, I can see the point in having a custom allocator for this. But
> quite frankly the xvmalloc code is just too ugly to live with. You
> might want to make it look like kernel code as per CodingStyle and
> change the name to something less generic.
>

I can see quite some places for cleanups (like macros for
reaching next free block etc). Do you think CamelCase will be acceptable
for now? and also keeping xvmalloc name? Other than these, I will
check for potential cleanups and send updated diffs.

Thanks,
Nitin

Re: [PATCH 2/3]: xvmalloc memory allocator

[Pekka Enberg]

On 3/18/09, Nitin Gupta <ngupta@vflare.org> wrote:
> I can see quite some places for cleanups (like macros for
> reaching next free block etc). Do you think CamelCase will be acceptable
> for now? and also keeping xvmalloc name? Other than these, I will
> check for potential cleanups and send updated diffs.

No, I do not think it is acceptable for core kernel code.

[PATCH 3/3]: documentation

[Nitin Gupta]

  Documentation/blockdev/00-INDEX      |    2 +
  Documentation/blockdev/compcache.txt |   67 ++++++++++++++++++++++++++++++++++
  Documentation/kernel-parameters.txt  |    9 +++++
  3 files changed, 78 insertions(+), 0 deletions(-)

Documentation on how to use compcache module.

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
---

diff --git a/Documentation/blockdev/00-INDEX b/Documentation/blockdev/00-INDEX
index 86f054c..d53dec0 100644
--- a/Documentation/blockdev/00-INDEX
+++ b/Documentation/blockdev/00-INDEX
@@ -4,6 +4,8 @@ README.DAC960
  	- info on Mylex DAC960/DAC1100 PCI RAID Controller Driver for Linux.
  cciss.txt
  	- info, major/minor #'s for Compaq's SMART Array Controllers.
+compcache.txt
+	- short guide on how to setup compressed RAM swap device.
  cpqarray.txt
  	- info on using Compaq's SMART2 Intelligent Disk Array Controllers.
  floppy.txt
diff --git a/Documentation/blockdev/compcache.txt b/Documentation/blockdev/compcache.txt
new file mode 100644
index 0000000..46c33a5
--- /dev/null
+++ b/Documentation/blockdev/compcache.txt
@@ -0,0 +1,67 @@
+compcache: Compressed RAM swap device
+-------------------------------------
+
+Project home: http://code.google.com/p/compcache/
+
+This module creates RAM based block device (named ramzswap0) which acts
+as swap disk. Pages swapped to this disk are compressed and stored in
+memory itself.
+
+It uses these components:
+ - xvMalloc: memory allocator (xvmalloc.ko)
+ - LZO1X: de/compressor: (lzo_compress.ko, lzo_decompress.ko)
+
+Usage:
+ - modprobe compcache [memlimit_kb=<val>|disksize_kb=<val>] [backing_dev=<dev>]
+
+   memlimit_kb: This param is applicable only when backing_dev is given.
+	It is limit on amount compressed data stored in memory. Any
+	additional data is forwarded to backing_dev. It cannot be greater
+	than backing device size. If missing or 0, default value is used:
+	15% of RAM or backing device size, whichever is smaller.
+
+   disksize_kb: This param is applicable only when backing_dev is not given.
+	It is limit on amount of *uncompressed* worth of data stored in
+	memory. For e.g. disksize_kb=1024 means it can hold 1024kb worth of
+	uncompressed data even if this data compresses to just, say, 100kb.
+	If missing or 0, default value is used: 25% of RAM.
+
+   backing_dev: This is block device to be used as backing store for compcache.
+	It must be a valid swap partition. We move data to this device when we
+	encounter incompressible page or memlimit is reached. TODO: we may also
+	move some pages from compcache to this device in case system is really
+	low on memory.
+	This device is not directly visible to kernel as a swap device
+	(/proc/swaps will only show /dev/ramzswap0 and not this device).
+	Managing this backing device is the job of compcache module.
+
+Examples:
+	1) modprobe compcache memlimit_kb=10240 backing_dev=/dev/sda2
+	sets compcache limit as 10MB and /dev/sda2 as backing swap device.
+	NOTE: here /dev/sda2 is a valid swap partition.
+
+	2) modprobe compcache backing_dev=/dev/sda2
+	same as (1) but memlimit is set to default: 15% of RAM or size of
+	backing swap device, whichever is smaller.
+
+	3) modprobe compcache disksize_kb=10240
+	sets compcache disk size as 10MB.
+
+	4) modprobe compcache.ko
+	same as (3) but compcache disk size will be set to default:
+	25% of RAM size.
+
+	Once module is loaded, activate this swap with highest priority:
+	swapon /dev/ramzswap0 -p 100
+	(-p param set swap priority)
+
+Notes:
+ - compcache stats are exported via /proc/compcache
+ - If you give non-swap partition as backing_dev, nothing bad will happen -
+   swapon will simply fail to recognize /dev/ramzswap0 as swap partition.
+   So, in this case, unload the module and reload with correct backing_dev.
+
+Please report any problems to:
+
+Nitin Gupta
+ngupta@vflare.org
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 54f21a5..a23f0ae 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -414,6 +414,15 @@ and is between 256 and 4096 characters. It is defined in the file
  			possible to determine what the correct size should be.
  			This option provides an override for these situations.

+	compcache.memlimit_kb=
+			See Documentation/blockdev/compcache.txt.
+
+	compcache.disksize_kb=
+			See Documentation/blockdev/compcache.txt.
+
+	compcache.backing_dev=
+			See Documentation/blockdev/compcache.txt.
+
  	security=	[SECURITY] Choose a security module to enable at boot.
  			If this boot parameter is not specified, only the first
  			security module asking for security registration will be

Re: [PATCH 0/3]: compressed in-memory swapping

[Russ Dill]

Nitin Gupta <ngupta <at> vflare.org> writes:

> 
> Hi,
> 
> Project home: http://code.google.com/p/compcache/
> 
> It allows creating a RAM based block device which acts as swap disk.
> Pages swapped to this device are compressed and stored in memory itself.
> This is a big win over swapping to slow hard-disk which are typically used
> as swap disk. For flash, these suffer from wear-leveling issues when used
> as swap disk - so again its helpful. For swapless systems, it allows more
> apps to run.
> 

I would recommend making xvmalloc the 1/3 instead of 2/3 so that the commit is
bisectable.

I really like this project and I'm especially impressed with how the SSD ATAPI
discard command fell in nicely with the discard requirement for compcache.