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

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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: Stefan Hengelein <ilendir@googlemail.com>
Signed-off-by: Florian Schmaus <fschmaus@gmail.com>
Signed-off-by: Andor Daam <andor.daam@googlemail.com>
Signed-off-by: Dan 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: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Bob 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Dan Magenheimer 2013-04-30 15:26:50 -07:00 committed by Linus Torvalds
parent 421348f1ca
commit 905cd0e1bf
1 changed files with 84 additions and 10 deletions
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94
mm/frontswap.c
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@ -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];
BUG_ON(sis == NULL);
if (sis->frontswap_map == NULL)
return;
frontswap_ops.init(type);
if (backend_registered) {
BUG_ON(sis == NULL);
if (sis->frontswap_map == NULL)
return;
(*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);
@ -226,12 +296,15 @@ 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));
if (backend_registered) {
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));
}
clear_bit(type, need_init);
}
EXPORT_SYMBOL(__frontswap_invalidate_area);
@ -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;
}