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Commit 905cd0e1 authored by Dan Magenheimer's avatar Dan Magenheimer Committed by Linus Torvalds
Browse files

mm: frontswap: lazy initialization to allow tmem backends to build/run as modules 



With the goal of allowing tmem backends (zcache, ramster, Xen tmem) to
be built/loaded as modules rather than built-in and enabled by a boot
parameter, this patch provides "lazy initialization", allowing backends
to register to frontswap even after swapon was run.  Before a backend
registers all calls to init are recorded and the creation of tmem_pools
delayed until a backend registers or until a frontswap store is
attempted.

Signed-off-by: default avatarStefan Hengelein <ilendir@googlemail.com>
Signed-off-by: default avatarFlorian Schmaus <fschmaus@gmail.com>
Signed-off-by: default avatarAndor Daam <andor.daam@googlemail.com>
Signed-off-by: default avatarDan Magenheimer <dan.magenheimer@oracle.com>
[v1: Fixes per Seth Jennings suggestions]
[v2: Removed FRONTSWAP_HAS_.. ]
[v3: Fix up per Bob Liu <lliubbo@gmail.com> recommendations]
[v4: Fix up per Andrew's comments]
Signed-off-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: default avatarBob Liu <lliubbo@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 421348f1

mm/frontswap.c

+84 −10
Original line number Diff line number Diff line
@@ -80,6 +80,46 @@ static inline void inc_frontswap_succ_stores(void) { } 
static inline void inc_frontswap_failed_stores(void) { }

static inline void inc_frontswap_invalidates(void) { }

#endif



/*

 * Due to the asynchronous nature of the backends loading potentially

 * _after_ the swap system has been activated, we have chokepoints

 * on all frontswap functions to not call the backend until the backend

 * has registered.

 *

 * Specifically when no backend is registered (nobody called

 * frontswap_register_ops) all calls to frontswap_init (which is done via

 * swapon -> enable_swap_info -> frontswap_init) are registered and remembered

 * (via the setting of need_init bitmap) but fail to create tmem_pools. When a

 * backend registers with frontswap at some later point the previous

 * calls to frontswap_init are executed (by iterating over the need_init

 * bitmap) to create tmem_pools and set the respective poolids. All of that is

 * guarded by us using atomic bit operations on the 'need_init' bitmap.

 *

 * This would not guards us against the user deciding to call swapoff right as

 * we are calling the backend to initialize (so swapon is in action).

 * Fortunatly for us, the swapon_mutex has been taked by the callee so we are

 * OK. The other scenario where calls to frontswap_store (called via

 * swap_writepage) is racing with frontswap_invalidate_area (called via

 * swapoff) is again guarded by the swap subsystem.

 *

 * While no backend is registered all calls to frontswap_[store|load|

 * invalidate_area|invalidate_page] are ignored or fail.

 *

 * The time between the backend being registered and the swap file system

 * calling the backend (via the frontswap_* functions) is indeterminate as

 * backend_registered is not atomic_t (or a value guarded by a spinlock).

 * That is OK as we are comfortable missing some of these calls to the newly

 * registered backend.

 *

 * Obviously the opposite (unloading the backend) must be done after all

 * the frontswap_[store|load|invalidate_area|invalidate_page] start

 * ignorning or failing the requests - at which point backend_registered

 * would have to be made in some fashion atomic.

 */

static DECLARE_BITMAP(need_init, MAX_SWAPFILES);

static bool backend_registered __read_mostly;



/*

 * Register operations for frontswap, returning previous thus allowing

 * detection of multiple backends and possible nesting.
@@ -87,9 +127,22 @@ static inline void inc_frontswap_invalidates(void) { } 
struct frontswap_ops frontswap_register_ops(struct frontswap_ops *ops)

{

	struct frontswap_ops old = frontswap_ops;

	int i;



	frontswap_ops = *ops;

	frontswap_enabled = true;



	for (i = 0; i < MAX_SWAPFILES; i++) {

		if (test_and_clear_bit(i, need_init))

			(*frontswap_ops.init)(i);

	}

	/*

	 * We MUST have backend_registered set _after_ the frontswap_init's

	 * have been called. Otherwise __frontswap_store might fail. Hence

	 * the barrier to make sure compiler does not re-order us.

	 */

	barrier();

	backend_registered = true;

	return old;

}

EXPORT_SYMBOL(frontswap_register_ops);
@@ -119,10 +172,16 @@ void __frontswap_init(unsigned type) 
{

	struct swap_info_struct *sis = swap_info[type];



	if (backend_registered) {

		BUG_ON(sis == NULL);

		if (sis->frontswap_map == NULL)

			return;

	frontswap_ops.init(type);

		(*frontswap_ops.init)(type);

	} else {

		BUG_ON(type > MAX_SWAPFILES);

		set_bit(type, need_init);

	}



}

EXPORT_SYMBOL(__frontswap_init);
@@ -147,6 +206,11 @@ int __frontswap_store(struct page *page) 
	struct swap_info_struct *sis = swap_info[type];

	pgoff_t offset = swp_offset(entry);



	if (!backend_registered) {

		inc_frontswap_failed_stores();

		return ret;

	}



	BUG_ON(!PageLocked(page));

	BUG_ON(sis == NULL);

	if (frontswap_test(sis, offset))
@@ -186,6 +250,9 @@ int __frontswap_load(struct page *page) 
	struct swap_info_struct *sis = swap_info[type];

	pgoff_t offset = swp_offset(entry);



	if (!backend_registered)

		return ret;



	BUG_ON(!PageLocked(page));

	BUG_ON(sis == NULL);

	if (frontswap_test(sis, offset))
@@ -209,6 +276,9 @@ void __frontswap_invalidate_page(unsigned type, pgoff_t offset) 
{

	struct swap_info_struct *sis = swap_info[type];



	if (!backend_registered)

		return;



	BUG_ON(sis == NULL);

	if (frontswap_test(sis, offset)) {

		frontswap_ops.invalidate_page(type, offset);
@@ -226,13 +296,16 @@ void __frontswap_invalidate_area(unsigned type) 
{

	struct swap_info_struct *sis = swap_info[type];



	if (backend_registered) {

		BUG_ON(sis == NULL);

		if (sis->frontswap_map == NULL)

			return;

	frontswap_ops.invalidate_area(type);

		(*frontswap_ops.invalidate_area)(type);

		atomic_set(&sis->frontswap_pages, 0);

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

	}

	clear_bit(type, need_init);

}

EXPORT_SYMBOL(__frontswap_invalidate_area);



static unsigned long __frontswap_curr_pages(void)
@@ -364,6 +437,7 @@ static int __init init_frontswap(void) 
	debugfs_create_u64("invalidates", S_IRUGO,

				root, &frontswap_invalidates);

#endif

	frontswap_enabled = 1;

	return 0;

}