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SubjectRe: [PATCH 02/15] mm: frontswap: lazy initialization to allow tmem backends to build/run as modules
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On Sun, Feb 3, 2013 at 3:07 PM, Ric Mason <ric.masonn@gmail.com> wrote:
> Hi Konrad,
> On Fri, 2013-02-01 at 15:22 -0500, Konrad Rzeszutek Wilk wrote:
>> From: Dan Magenheimer <dan.magenheimer@oracle.com>
>>
>> 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 put is attempted.
>
> Do you mean __frontswap_store? It seems that just add fail count if
> backend doesn't register, why you said that the creation of tmem_pools
> will delay until this time?
>
>>
>> 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>
>> ---
>> mm/frontswap.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++-------
>> 1 file changed, 85 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/frontswap.c b/mm/frontswap.c
>> index 2890e67..c05a9db 100644
>> --- a/mm/frontswap.c
>> +++ b/mm/frontswap.c
>> @@ -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 +172,17 @@ 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 +207,11 @@ 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 +251,9 @@ 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 +277,9 @@ 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 +297,15 @@ 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 +438,7 @@ static int __init init_frontswap(void)
>> debugfs_create_u64("invalidates", S_IRUGO,
>> root, &frontswap_invalidates);
>> #endif
>> + frontswap_enabled = 1;
>
> Why has this change?
>

If don't set frontswap_enabled to 1, frontswap_init() will return
without record the swap type.

--
Regards,
--Bob


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