frontswap: s/put_page/store/g s/get_page/load

conform to certain policies as follows:

An "init" prepares the device to receive frontswap pages associated

with the specified swap device number (aka "type").  A "put_page" will

with the specified swap device number (aka "type").  A "store" will

copy the page to transcendent memory and associate it with the type and

offset associated with the page. A "get_page" will copy the page, if found,

offset associated with the page. A "load" will copy the page, if found,

from transcendent memory into kernel memory, but will NOT remove the page

from from transcendent memory.  An "invalidate_page" will remove the page

from transcendent memory and an "invalidate_area" will remove ALL pages

associated with the swap type (e.g., like swapoff) and notify the "device"

to refuse further puts with that swap type.

to refuse further stores with that swap type.

Once a page is successfully put, a matching get on the page will normally

Once a page is successfully stored, a matching load on the page will normally

succeed.  So when the kernel finds itself in a situation where it needs

to swap out a page, it first attempts to use frontswap.  If the put returns

to swap out a page, it first attempts to use frontswap.  If the store returns

success, the data has been successfully saved to transcendent memory and

a disk write and, if the data is later read back, a disk read are avoided.

If a put returns failure, transcendent memory has rejected the data, and the

If a store returns failure, transcendent memory has rejected the data, and the

page can be written to swap as usual.

If a backend chooses, frontswap can be configured as a "writethrough

in order to allow the backend to arbitrarily "reclaim" space used to

store frontswap pages to more completely manage its memory usage.

Note that if a page is put and the page already exists in transcendent memory

(a "duplicate" put), either the put succeeds and the data is overwritten,

or the put fails AND the page is invalidated.  This ensures stale data may

Note that if a page is stored and the page already exists in transcendent memory

(a "duplicate" store), either the store succeeds and the data is overwritten,

or the store fails AND the page is invalidated.  This ensures stale data may

never be obtained from frontswap.

If properly configured, monitoring of frontswap is done via debugfs in

the /sys/kernel/debug/frontswap directory.  The effectiveness of

frontswap can be measured (across all swap devices) with:

failed_puts	- how many put attempts have failed

gets		- how many gets were attempted (all should succeed)

succ_puts	- how many put attempts have succeeded

failed_stores	- how many store attempts have failed

loads		- how many loads were attempted (all should succeed)

succ_stores	- how many store attempts have succeeded

invalidates	- how many invalidates were attempted

A backend implementation may provide additional metrics.

swap device.  If CONFIG_FRONTSWAP is enabled but no frontswap "backend"

registers, there is one extra global variable compared to zero for

every swap page read or written.  If CONFIG_FRONTSWAP is enabled

AND a frontswap backend registers AND the backend fails every "put"

AND a frontswap backend registers AND the backend fails every "store"

request (i.e. provides no memory despite claiming it might),

CPU overhead is still negligible -- and since every frontswap fail

precedes a swap page write-to-disk, the system is highly likely

Whenever a swap-device is swapon'd frontswap_init() is called,

passing the swap device number (aka "type") as a parameter.

This notifies frontswap to expect attempts to "put" swap pages

This notifies frontswap to expect attempts to "store" swap pages

associated with that number.

Whenever the swap subsystem is readying a page to write to a swap

device (c.f swap_writepage()), frontswap_put_page is called.  Frontswap

device (c.f swap_writepage()), frontswap_store is called.  Frontswap

consults with the frontswap backend and if the backend says it does NOT

have room, frontswap_put_page returns -1 and the kernel swaps the page

have room, frontswap_store returns -1 and the kernel swaps the page

to the swap device as normal.  Note that the response from the frontswap

backend is unpredictable to the kernel; it may choose to never accept a

page, it could accept every ninth page, or it might accept every

otherwise have written the data.

When the swap subsystem needs to swap-in a page (swap_readpage()),

it first calls frontswap_get_page() which checks the frontswap_map to

it first calls frontswap_load() which checks the frontswap_map to

see if the page was earlier accepted by the frontswap backend.  If

it was, the page of data is filled from the frontswap backend and

the swap-in is complete.  If not, the normal swap-in code is

So every time the frontswap backend accepts a page, a swap device read

and (potentially) a swap device write are replaced by a "frontswap backend

put" and (possibly) a "frontswap backend get", which are presumably much

store" and (possibly) a "frontswap backend loads", which are presumably much

faster.

4) Can't frontswap be configured as a "special" swap device that is

the write of some pages for a significant amount of time.  Synchrony is

required to ensure the dynamicity of the backend and to avoid thorny race

conditions that would unnecessarily and greatly complicate frontswap

and/or the block I/O subsystem.  That said, only the initial "put"

and "get" operations need be synchronous.  A separate asynchronous thread

and/or the block I/O subsystem.  That said, only the initial "store"

and "load" operations need be synchronous.  A separate asynchronous thread

is free to manipulate the pages stored by frontswap.  For example,

the "remotification" thread in RAMster uses standard asynchronous

kernel sockets to move compressed frontswap pages to a remote machine.

then force guests to do their own swapping.

There is a downside to the transcendent memory specifications for

frontswap:  Since any "put" might fail, there must always be a real

frontswap:  Since any "store" might fail, there must always be a real

slot on a real swap device to swap the page.  Thus frontswap must be

implemented as a "shadow" to every swapon'd device with the potential

capability of holding every page that the swap device might have held

can still use frontswap but a backend for such devices must configure

some kind of "ghost" swap device and ensure that it is never used.

5) Why this weird definition about "duplicate puts"?  If a page

   has been previously successfully put, can't it always be

5) Why this weird definition about "duplicate stores"?  If a page

   has been previously successfully stored, can't it always be

   successfully overwritten?

Nearly always it can, but no, sometimes it cannot.  Consider an example

where data is compressed and the original 4K page has been compressed

to 1K.  Now an attempt is made to overwrite the page with data that

is non-compressible and so would take the entire 4K.  But the backend

has no more space.  In this case, the put must be rejected.  Whenever

frontswap rejects a put that would overwrite, it also must invalidate

has no more space.  In this case, the store must be rejected.  Whenever

frontswap rejects a store that would overwrite, it also must invalidate

the old data and ensure that it is no longer accessible.  Since the

swap subsystem then writes the new data to the read swap device,

this is the correct course of action to ensure coherency.

	return oid;

}

static int zcache_frontswap_put_page(unsigned type, pgoff_t offset,

static int zcache_frontswap_store(unsigned type, pgoff_t offset,

				   struct page *page)

{

	u64 ind64 = (u64)offset;

/* returns 0 if the page was successfully gotten from frontswap, -1 if

 * was not present (should never happen!) */

static int zcache_frontswap_get_page(unsigned type, pgoff_t offset,

static int zcache_frontswap_load(unsigned type, pgoff_t offset,

				   struct page *page)

{

	u64 ind64 = (u64)offset;

}

static struct frontswap_ops zcache_frontswap_ops = {

	.put_page = zcache_frontswap_put_page,

	.get_page = zcache_frontswap_get_page,

	.store = zcache_frontswap_store,

	.load = zcache_frontswap_load,

	.invalidate_page = zcache_frontswap_flush_page,

	.invalidate_area = zcache_frontswap_flush_area,

	.init = zcache_frontswap_init

 * Swizzling increases objects per swaptype, increasing tmem concurrency

 * for heavy swaploads.  Later, larger nr_cpus -> larger SWIZ_BITS

 * Setting SWIZ_BITS to 27 basically reconstructs the swap entry from

 * frontswap_get_page(), but has side-effects. Hence using 8.

 * frontswap_load(), but has side-effects. Hence using 8.

 */

#define SWIZ_BITS		8

#define SWIZ_MASK		((1 << SWIZ_BITS) - 1)

	return oid;

}

static int zcache_frontswap_put_page(unsigned type, pgoff_t offset,

static int zcache_frontswap_store(unsigned type, pgoff_t offset,

				   struct page *page)

{

	u64 ind64 = (u64)offset;

/* returns 0 if the page was successfully gotten from frontswap, -1 if

 * was not present (should never happen!) */

static int zcache_frontswap_get_page(unsigned type, pgoff_t offset,

static int zcache_frontswap_load(unsigned type, pgoff_t offset,

				   struct page *page)

{

	u64 ind64 = (u64)offset;

}

static struct frontswap_ops zcache_frontswap_ops = {

	.put_page = zcache_frontswap_put_page,

	.get_page = zcache_frontswap_get_page,

	.store = zcache_frontswap_store,

	.load = zcache_frontswap_load,

	.invalidate_page = zcache_frontswap_flush_page,

	.invalidate_area = zcache_frontswap_flush_area,

	.init = zcache_frontswap_init

}

/* returns 0 if the page was successfully put into frontswap, -1 if not */

static int tmem_frontswap_put_page(unsigned type, pgoff_t offset,

static int tmem_frontswap_store(unsigned type, pgoff_t offset,

				   struct page *page)

{

	u64 ind64 = (u64)offset;

 * returns 0 if the page was successfully gotten from frontswap, -1 if

 * was not present (should never happen!)

 */

static int tmem_frontswap_get_page(unsigned type, pgoff_t offset,

static int tmem_frontswap_load(unsigned type, pgoff_t offset,

				   struct page *page)

{

	u64 ind64 = (u64)offset;

__setup("nofrontswap", no_frontswap);

static struct frontswap_ops __initdata tmem_frontswap_ops = {

	.put_page = tmem_frontswap_put_page,

	.get_page = tmem_frontswap_get_page,

	.store = tmem_frontswap_store,

	.load = tmem_frontswap_load,

	.invalidate_page = tmem_frontswap_flush_page,

	.invalidate_area = tmem_frontswap_flush_area,

	.init = tmem_frontswap_init

struct frontswap_ops {

	void (*init)(unsigned);

	int (*put_page)(unsigned, pgoff_t, struct page *);

	int (*get_page)(unsigned, pgoff_t, struct page *);

	int (*store)(unsigned, pgoff_t, struct page *);

	int (*load)(unsigned, pgoff_t, struct page *);

	void (*invalidate_page)(unsigned, pgoff_t);

	void (*invalidate_area)(unsigned);

};

extern void frontswap_writethrough(bool);

extern void __frontswap_init(unsigned type);

extern int __frontswap_put_page(struct page *page);

extern int __frontswap_get_page(struct page *page);

extern int __frontswap_store(struct page *page);

extern int __frontswap_load(struct page *page);

extern void __frontswap_invalidate_page(unsigned, pgoff_t);

extern void __frontswap_invalidate_area(unsigned);

}

#endif

static inline int frontswap_put_page(struct page *page)

static inline int frontswap_store(struct page *page)

{

	int ret = -1;

	if (frontswap_enabled)

		ret = __frontswap_put_page(page);

		ret = __frontswap_store(page);

	return ret;

}

static inline int frontswap_get_page(struct page *page)

static inline int frontswap_load(struct page *page)

{

	int ret = -1;

	if (frontswap_enabled)

		ret = __frontswap_get_page(page);

		ret = __frontswap_load(page);

	return ret;

}