Date:	Tue, 7 Jun 2011 13:53:20 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, jeremy@...p.org,
	hughd@...gle.com, ngupta@...are.org, JBeulich@...ell.com,
	kurt.hackel@...cle.com, npiggin@...e.de, akpm@...ux-foundation.org,
	riel@...hat.com, hannes@...xchg.org, matthew@....cx,
	chris.mason@...cle.com
Subject: Re: [PATCH V4 2/4] mm: frontswap: core code

On Fri, May 27, 2011 at 12:48:45PM -0700, Dan Magenheimer wrote:
> [PATCH V4 2/4] mm: frontswap: core code
> 
> Core frontswap interface between swap subsystem hooks and
> frontswap backend via frontswap_ops.
> 
> Credits: Frontswap_ops design derived from Jeremy Fitzhardinge
> design for tmem; sysfs code modelled after mm/ksm.c
> 
> Signed-off-by: Dan Magenheimer <dan.magenheimer@...cle.com>
> 
> Diffstat:
>  include/linux/frontswap.h                |   86 +++++
>  mm/frontswap.c                           |  331 +++++++++++++++++++++
>  2 files changed, 417 insertions(+)
> 
> --- linux-2.6.39/include/linux/frontswap.h	1969-12-31 17:00:00.000000000 -0700
> +++ linux-2.6.39-frontswap/include/linux/frontswap.h	2011-05-26 15:37:25.198065401 -0600
> @@ -0,0 +1,86 @@
> +#ifndef _LINUX_FRONTSWAP_H
> +#define _LINUX_FRONTSWAP_H
> +
> +#include <linux/swap.h>
> +#include <linux/mm.h>
> +
> +struct frontswap_ops {
> +	void (*init)(unsigned);
> +	int (*put_page)(unsigned, pgoff_t, struct page *);
> +	int (*get_page)(unsigned, pgoff_t, struct page *);
> +	void (*flush_page)(unsigned, pgoff_t);
> +	void (*flush_area)(unsigned);
> +};
> +
> +extern int frontswap_enabled;
> +extern struct frontswap_ops
> +	frontswap_register_ops(struct frontswap_ops *ops);
> +extern void frontswap_shrink(unsigned long);
> +extern unsigned long frontswap_curr_pages(void);
> +
> +extern void frontswap_init(unsigned type);
> +extern int __frontswap_put_page(struct page *page);
> +extern int __frontswap_get_page(struct page *page);
> +extern void __frontswap_flush_page(unsigned, pgoff_t);
> +extern void __frontswap_flush_area(unsigned);
> +
> +#ifndef CONFIG_FRONTSWAP
> +/* all inline routines become no-ops and all externs are ignored */
> +#define frontswap_enabled (0)
> +#endif
> +
> +static inline int frontswap_test(struct swap_info_struct *sis, pgoff_t offset)
> +{
> +	int ret = 0;
> +
> +	if (frontswap_enabled && sis->frontswap_map)
> +		ret = test_bit(offset % BITS_PER_LONG,
> +			&sis->frontswap_map[offset/BITS_PER_LONG]);
> +	return ret;
> +}
> +
> +static inline void frontswap_set(struct swap_info_struct *sis, pgoff_t offset)
> +{
> +	if (frontswap_enabled && sis->frontswap_map)
> +		set_bit(offset % BITS_PER_LONG,
> +			&sis->frontswap_map[offset/BITS_PER_LONG]);
> +}
> +
> +static inline void frontswap_clear(struct swap_info_struct *sis, pgoff_t offset)
> +{
> +	if (frontswap_enabled && sis->frontswap_map)
> +		clear_bit(offset % BITS_PER_LONG,
> +			&sis->frontswap_map[offset/BITS_PER_LONG]);
> +}
> +
> +static inline int frontswap_put_page(struct page *page)
> +{
> +	int ret = -1;
> +
> +	if (frontswap_enabled)
> +		ret = __frontswap_put_page(page);
> +	return ret;
> +}
> +
> +static inline int frontswap_get_page(struct page *page)
> +{
> +	int ret = -1;
> +
> +	if (frontswap_enabled)
> +		ret = __frontswap_get_page(page);
> +	return ret;
> +}
> +
> +static inline void frontswap_flush_page(unsigned type, pgoff_t offset)
> +{
> +	if (frontswap_enabled)
> +		__frontswap_flush_page(type, offset);
> +}
> +
> +static inline void frontswap_flush_area(unsigned type)
> +{
> +	if (frontswap_enabled)
> +		__frontswap_flush_area(type);
> +}
> +
> +#endif /* _LINUX_FRONTSWAP_H */
> --- linux-2.6.39/mm/frontswap.c	1969-12-31 17:00:00.000000000 -0700
> +++ linux-2.6.39-frontswap/mm/frontswap.c	2011-05-26 15:37:25.254816281 -0600
> @@ -0,0 +1,331 @@
> +/*
> + * 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) 2009-2010 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/sysctl.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/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;
> +
> +/*
> + * 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.
> + */
> +int frontswap_enabled;
> +EXPORT_SYMBOL(frontswap_enabled);
> +
> +/* useful stats available in /sys/kernel/mm/frontswap */
> +static unsigned long frontswap_gets;
> +static unsigned long frontswap_succ_puts;
> +static unsigned long frontswap_failed_puts;
> +static unsigned long frontswap_flushes;
> +
> +/*
> + * 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 = 1;
> +	return old;
> +}
> +EXPORT_SYMBOL(frontswap_register_ops);
> +
> +/* Called when a swap device is swapon'd */
> +void frontswap_init(unsigned type)
> +{
> +	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 flush 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];

There is no danger that swap_info[type] returns NULL?
Should you get the get the swap_lock lock? Or is that not
neccessary b/c the caller already holds the lock? If so it might
be better to name the function __frontswap_put_page_locked(..)?


> +	pgoff_t offset = swp_offset(entry);
> +
> +	BUG_ON(!PageLocked(page));
> +	if (frontswap_test(sis, offset))
> +		dup = 1;
> +	ret = (*frontswap_ops.put_page)(type, offset, page);
> +	if (ret == 0) {
> +		frontswap_set(sis, offset);
> +		frontswap_succ_puts++;
> +		if (!dup)
> +			sis->frontswap_pages++;
> +	} else if (dup) {
> +		/*
> +		  failed dup always results in automatic flush of
> +		  the (older) page from frontswap
> +		 */
> +		frontswap_clear(sis, offset);
> +		sis->frontswap_pages--;
> +		frontswap_failed_puts++;
> +	} else
> +		frontswap_failed_puts++;
> +	return ret;
> +}
> +
> +/*
> + * "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));

Looks like swap_readpage already does this?

> +	if (frontswap_test(sis, offset))

So no checking if sis == NULL b/c the caller already does that? You might
want to stick a comment here.
> +		ret = (*frontswap_ops.get_page)(type, offset, page);
> +	if (ret == 0)
> +		frontswap_gets++;
> +	return ret;
> +}
> +
> +/*
> + * Flush any data from frontswap associated with the specified swaptype
> + * and offset so that a subsequent "get" will fail.
> + */
> +void __frontswap_flush_page(unsigned type, pgoff_t offset)
> +{
> +	struct swap_info_struct *sis = swap_info[type];
> +
> +	if (frontswap_test(sis, offset)) {
> +		(*frontswap_ops.flush_page)(type, offset);
> +		sis->frontswap_pages--;
> +		frontswap_clear(sis, offset);
> +		frontswap_flushes++;
> +	}
> +}
> +
> +/*
> + * Flush all data from frontswap associated with all offsets for the
> + * specified swaptype.
> + */
> +void __frontswap_flush_area(unsigned type)
> +{
> +	struct swap_info_struct *sis = swap_info[type];
> +
> +	(*frontswap_ops.flush_area)(type);
> +	sis->frontswap_pages = 0;
> +	memset(sis->frontswap_map, 0, sis->max / sizeof(long));
> +}
> +
> +/*
> + * 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
> + */
> +void frontswap_shrink(unsigned long target_pages)
> +{
> +	int wrapped = 0;
> +	bool locked = false;
> +
> +	for (wrapped = 0; wrapped <= 3; wrapped++) {

3? Is this value special? You might want to provide a comment
of why '3' is _the_ choice.

> +
> +		struct swap_info_struct *si = NULL;
> +		unsigned long total_pages = 0, total_pages_to_unuse;
> +		unsigned long pages = 0, unuse_pages = 0;
> +		int type;
> +
> +		/*
> +		 * 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 += 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];
> +			if (total_pages_to_unuse < si->frontswap_pages)
> +				pages = unuse_pages = total_pages_to_unuse;
> +			else {
> +				pages = si->frontswap_pages;

unused_pages sounds better than 'pages'.

> +				unuse_pages = 0; /* unuse all */

pages_unuse?  pages_to_unuse? Yeah, that sounds better.
> +			}
> +			if (security_vm_enough_memory_kern(pages))
> +				continue;
> +			vm_unacct_memory(pages);
> +			break;
> +		}
> +		if (type < 0)
> +			goto out;
> +		locked = false;
> +		spin_unlock(&swap_lock);
> +		try_to_unuse(type, true, unuse_pages);
> +	}
> +
> +out:
> +	if (locked)
> +		spin_unlock(&swap_lock);
> +	return;
> +}
> +EXPORT_SYMBOL(frontswap_shrink);
> +
> +/*
> + * count and return the number of pages frontswap pages across all
> + * swap devices.  This is exported so that a kernel module can
> + * determine current usage without reading sysfs.
> + */
> +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 += si->frontswap_pages;

Uh, you don't need to check 'si' for NULL?
> +	}
> +	spin_unlock(&swap_lock);
> +	return totalpages;
> +}
> +EXPORT_SYMBOL(frontswap_curr_pages);
> +
> +#ifdef CONFIG_SYSFS
> +
> +/* see Documentation/ABI/xxx/sysfs-kernel-mm-frontswap */
> +
> +#define FRONTSWAP_ATTR_RO(_name) \
> +	static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
> +#define FRONTSWAP_ATTR(_name) \
> +	static struct kobj_attribute _name##_attr = \
> +		__ATTR(_name, 0644, _name##_show, _name##_store)
> +
> +static ssize_t curr_pages_show(struct kobject *kobj,
> +			       struct kobj_attribute *attr, char *buf)
> +{
> +	return sprintf(buf, "%lu\n", frontswap_curr_pages());
> +}
> +
> +static ssize_t curr_pages_store(struct kobject *kobj,
> +			       struct kobj_attribute *attr,
> +			       const char *buf, size_t count)
> +{
> +	unsigned long target_pages;
> +	int err;
> +
> +	err = strict_strtoul(buf, 10, &target_pages);
> +	if (err)
> +		return -EINVAL;

How about just "if (strict..) return -EINVAL" ) and ditch the 'int err'?

> +
> +	frontswap_shrink(target_pages);
> +
> +	return count;
> +}
> +FRONTSWAP_ATTR(curr_pages);
> +
> +static ssize_t succ_puts_show(struct kobject *kobj,
> +			       struct kobj_attribute *attr, char *buf)
> +{
> +	return sprintf(buf, "%lu\n", frontswap_succ_puts);
> +}
> +FRONTSWAP_ATTR_RO(succ_puts);
> +
> +static ssize_t failed_puts_show(struct kobject *kobj,
> +			       struct kobj_attribute *attr, char *buf)
> +{
> +	return sprintf(buf, "%lu\n", frontswap_failed_puts);
> +}
> +FRONTSWAP_ATTR_RO(failed_puts);
> +
> +static ssize_t gets_show(struct kobject *kobj,
> +			       struct kobj_attribute *attr, char *buf)
> +{
> +	return sprintf(buf, "%lu\n", frontswap_gets);
> +}
> +FRONTSWAP_ATTR_RO(gets);
> +
> +static ssize_t flushes_show(struct kobject *kobj,
> +			       struct kobj_attribute *attr, char *buf)
> +{
> +	return sprintf(buf, "%lu\n", frontswap_flushes);
> +}
> +FRONTSWAP_ATTR_RO(flushes);
> +
> +static struct attribute *frontswap_attrs[] = {
> +	&curr_pages_attr.attr,
> +	&succ_puts_attr.attr,
> +	&failed_puts_attr.attr,
> +	&gets_attr.attr,
> +	&flushes_attr.attr,
> +	NULL,
> +};
> +
> +static struct attribute_group frontswap_attr_group = {
> +	.attrs = frontswap_attrs,
> +	.name = "frontswap",
> +};
> +
> +#endif /* CONFIG_SYSFS */
> +
> +static int __init init_frontswap(void)
> +{
> +#ifdef CONFIG_SYSFS
> +	int err;
> +
> +	err = sysfs_create_group(mm_kobj, &frontswap_attr_group);

So shouldn't you return 'err'?

> +#endif /* CONFIG_SYSFS */
> +	return 0;
> +}
> +
> +static void __exit exit_frontswap(void)
> +{
> +	frontswap_shrink(0UL);
> +}
> +
> +module_init(init_frontswap);
> +module_exit(exit_frontswap);
--
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