mm: frontswap: core frontswap functionality

/*

 * Frontswap frontend

 *

 * This code provides the generic "frontend" layer to call a matching

 * "backend" driver implementation of frontswap.  See

 * Documentation/vm/frontswap.txt for more information.

 *

 * Copyright (C) 2009-2012 Oracle Corp.  All rights reserved.

 * Author: Dan Magenheimer

 *

 * This work is licensed under the terms of the GNU GPL, version 2.

 */

#include <linux/mm.h>

#include <linux/mman.h>

#include <linux/swap.h>

#include <linux/swapops.h>

#include <linux/proc_fs.h>

#include <linux/security.h>

#include <linux/capability.h>

#include <linux/module.h>

#include <linux/uaccess.h>

#include <linux/debugfs.h>

#include <linux/frontswap.h>

#include <linux/swapfile.h>

/*

 * frontswap_ops is set by frontswap_register_ops to contain the pointers

 * to the frontswap "backend" implementation functions.

 */

static struct frontswap_ops frontswap_ops __read_mostly;

/*

 * This global enablement flag reduces overhead on systems where frontswap_ops

 * has not been registered, so is preferred to the slower alternative: a

 * function call that checks a non-global.

 */

bool frontswap_enabled __read_mostly;

EXPORT_SYMBOL(frontswap_enabled);

/*

 * If enabled, frontswap_put will return failure even on success.  As

 * a result, the swap subsystem will always write the page to swap, in

 * effect converting frontswap into a writethrough cache.  In this mode,

 * there is no direct reduction in swap writes, but a frontswap backend

 * can unilaterally "reclaim" any pages in use with no data loss, thus

 * providing increases control over maximum memory usage due to frontswap.

 */

static bool frontswap_writethrough_enabled __read_mostly;

#ifdef CONFIG_DEBUG_FS

/*

 * Counters available via /sys/kernel/debug/frontswap (if debugfs is

 * properly configured).  These are for information only so are not protected

 * against increment races.

 */

static u64 frontswap_gets;

static u64 frontswap_succ_puts;

static u64 frontswap_failed_puts;

static u64 frontswap_invalidates;

static inline void inc_frontswap_gets(void) {

	frontswap_gets++;

}

static inline void inc_frontswap_succ_puts(void) {

	frontswap_succ_puts++;

}

static inline void inc_frontswap_failed_puts(void) {

	frontswap_failed_puts++;

}

static inline void inc_frontswap_invalidates(void) {

	frontswap_invalidates++;

}

#else

static inline void inc_frontswap_gets(void) { }

static inline void inc_frontswap_succ_puts(void) { }

static inline void inc_frontswap_failed_puts(void) { }

static inline void inc_frontswap_invalidates(void) { }

#endif

/*

 * Register operations for frontswap, returning previous thus allowing

 * detection of multiple backends and possible nesting.

 */

struct frontswap_ops frontswap_register_ops(struct frontswap_ops *ops)

{

	struct frontswap_ops old = frontswap_ops;

	frontswap_ops = *ops;

	frontswap_enabled = true;

	return old;

}

EXPORT_SYMBOL(frontswap_register_ops);

/*

 * Enable/disable frontswap writethrough (see above).

 */

void frontswap_writethrough(bool enable)

{

	frontswap_writethrough_enabled = enable;

}

EXPORT_SYMBOL(frontswap_writethrough);

/*

 * Called when a swap device is swapon'd.

 */

void __frontswap_init(unsigned type)

{

	struct swap_info_struct *sis = swap_info[type];

	BUG_ON(sis == NULL);

	if (sis->frontswap_map == NULL)

		return;

	if (frontswap_enabled)

		(*frontswap_ops.init)(type);

}

EXPORT_SYMBOL(__frontswap_init);

/*

 * "Put" data from a page to frontswap and associate it with the page's

 * swaptype and offset.  Page must be locked and in the swap cache.

 * If frontswap already contains a page with matching swaptype and

 * offset, the frontswap implmentation may either overwrite the data and

 * return success or invalidate the page from frontswap and return failure.

 */

int __frontswap_put_page(struct page *page)

{

	int ret = -1, dup = 0;

	swp_entry_t entry = { .val = page_private(page), };

	int type = swp_type(entry);

	struct swap_info_struct *sis = swap_info[type];

	pgoff_t offset = swp_offset(entry);

	BUG_ON(!PageLocked(page));

	BUG_ON(sis == NULL);

	if (frontswap_test(sis, offset))

		dup = 1;

	ret = (*frontswap_ops.put_page)(type, offset, page);

	if (ret == 0) {

		frontswap_set(sis, offset);

		inc_frontswap_succ_puts();

		if (!dup)

			atomic_inc(&sis->frontswap_pages);

	} else if (dup) {

		/*

		  failed dup always results in automatic invalidate of

		  the (older) page from frontswap

		 */

		frontswap_clear(sis, offset);

		atomic_dec(&sis->frontswap_pages);

		inc_frontswap_failed_puts();

	} else

		inc_frontswap_failed_puts();

	if (frontswap_writethrough_enabled)

		/* report failure so swap also writes to swap device */

		ret = -1;

	return ret;

}

EXPORT_SYMBOL(__frontswap_put_page);

/*

 * "Get" data from frontswap associated with swaptype and offset that were

 * specified when the data was put to frontswap and use it to fill the

 * specified page with data. Page must be locked and in the swap cache.

 */

int __frontswap_get_page(struct page *page)

{

	int ret = -1;

	swp_entry_t entry = { .val = page_private(page), };

	int type = swp_type(entry);

	struct swap_info_struct *sis = swap_info[type];

	pgoff_t offset = swp_offset(entry);

	BUG_ON(!PageLocked(page));

	BUG_ON(sis == NULL);

	if (frontswap_test(sis, offset))

		ret = (*frontswap_ops.get_page)(type, offset, page);

	if (ret == 0)

		inc_frontswap_gets();

	return ret;

}

EXPORT_SYMBOL(__frontswap_get_page);

/*

 * Invalidate any data from frontswap associated with the specified swaptype

 * and offset so that a subsequent "get" will fail.

 */

void __frontswap_invalidate_page(unsigned type, pgoff_t offset)

{

	struct swap_info_struct *sis = swap_info[type];

	BUG_ON(sis == NULL);

	if (frontswap_test(sis, offset)) {

		(*frontswap_ops.invalidate_page)(type, offset);

		atomic_dec(&sis->frontswap_pages);

		frontswap_clear(sis, offset);

		inc_frontswap_invalidates();

	}

}

EXPORT_SYMBOL(__frontswap_invalidate_page);

/*

 * Invalidate all data from frontswap associated with all offsets for the

 * specified swaptype.

 */

void __frontswap_invalidate_area(unsigned type)

{

	struct swap_info_struct *sis = swap_info[type];

	BUG_ON(sis == NULL);

	if (sis->frontswap_map == NULL)

		return;

	(*frontswap_ops.invalidate_area)(type);

	atomic_set(&sis->frontswap_pages, 0);

	memset(sis->frontswap_map, 0, sis->max / sizeof(long));

}

EXPORT_SYMBOL(__frontswap_invalidate_area);

/*

 * Frontswap, like a true swap device, may unnecessarily retain pages

 * under certain circumstances; "shrink" frontswap is essentially a

 * "partial swapoff" and works by calling try_to_unuse to attempt to

 * unuse enough frontswap pages to attempt to -- subject to memory

 * constraints -- reduce the number of pages in frontswap to the

 * number given in the parameter target_pages.

 */

void frontswap_shrink(unsigned long target_pages)

{

	struct swap_info_struct *si = NULL;

	int si_frontswap_pages;

	unsigned long total_pages = 0, total_pages_to_unuse;

	unsigned long pages = 0, pages_to_unuse = 0;

	int type;

	bool locked = false;

	/*

	 * we don't want to hold swap_lock while doing a very

	 * lengthy try_to_unuse, but swap_list may change

	 * so restart scan from swap_list.head each time

	 */

	spin_lock(&swap_lock);

	locked = true;

	total_pages = 0;

	for (type = swap_list.head; type >= 0; type = si->next) {

		si = swap_info[type];

		total_pages += atomic_read(&si->frontswap_pages);

	}

	if (total_pages <= target_pages)

		goto out;

	total_pages_to_unuse = total_pages - target_pages;

	for (type = swap_list.head; type >= 0; type = si->next) {

		si = swap_info[type];

		si_frontswap_pages = atomic_read(&si->frontswap_pages);

		if (total_pages_to_unuse < si_frontswap_pages)

			pages = pages_to_unuse = total_pages_to_unuse;

		else {

			pages = si_frontswap_pages;

			pages_to_unuse = 0; /* unuse all */

		}

		/* ensure there is enough RAM to fetch pages from frontswap */

		if (security_vm_enough_memory_mm(current->mm, pages))

			continue;

		vm_unacct_memory(pages);

		break;

	}

	if (type < 0)

		goto out;

	locked = false;

	spin_unlock(&swap_lock);

	try_to_unuse(type, true, pages_to_unuse);

out:

	if (locked)

		spin_unlock(&swap_lock);

	return;

}

EXPORT_SYMBOL(frontswap_shrink);

/*

 * Count and return the number of frontswap pages across all

 * swap devices.  This is exported so that backend drivers can

 * determine current usage without reading debugfs.

 */

unsigned long frontswap_curr_pages(void)

{

	int type;

	unsigned long totalpages = 0;

	struct swap_info_struct *si = NULL;

	spin_lock(&swap_lock);

	for (type = swap_list.head; type >= 0; type = si->next) {

		si = swap_info[type];

		totalpages += atomic_read(&si->frontswap_pages);

	}

	spin_unlock(&swap_lock);

	return totalpages;

}

EXPORT_SYMBOL(frontswap_curr_pages);

static int __init init_frontswap(void)

{

#ifdef CONFIG_DEBUG_FS

	struct dentry *root = debugfs_create_dir("frontswap", NULL);

	if (root == NULL)

		return -ENXIO;

	debugfs_create_u64("gets", S_IRUGO, root, &frontswap_gets);

	debugfs_create_u64("succ_puts", S_IRUGO, root, &frontswap_succ_puts);

	debugfs_create_u64("failed_puts", S_IRUGO, root,

				&frontswap_failed_puts);

	debugfs_create_u64("invalidates", S_IRUGO,

				root, &frontswap_invalidates);

#endif

	return 0;

}

module_init(init_frontswap);