mm: frontswap: lazy initialization to allow tmem backends to build/run as modules (905cd0e1) · Commits · 戴 / test

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;
		}

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