FreeBSD/Linux Kernel Cross Reference
sys/mm/frontswap.c

     /*
      * Frontswap frontend
      *
      * This code provides the generic "frontend" layer to call a matching
      * "backend" driver implementation of frontswap.  See
      * Documentation/vm/frontswap.txt for more information.
      *
      * Copyright (C) 2009-2012 Oracle Corp.  All rights reserved.
      * Author: Dan Magenheimer
     *
     * This work is licensed under the terms of the GNU GPL, version 2.
     */
    
    #include <linux/mman.h>
    #include <linux/swap.h>
    #include <linux/swapops.h>
    #include <linux/security.h>
    #include <linux/module.h>
    #include <linux/debugfs.h>
    #include <linux/frontswap.h>
    #include <linux/swapfile.h>
    
    /*
     * frontswap_ops is set by frontswap_register_ops to contain the pointers
     * to the frontswap "backend" implementation functions.
     */
    static struct frontswap_ops frontswap_ops __read_mostly;
    
    /*
     * This global enablement flag reduces overhead on systems where frontswap_ops
     * has not been registered, so is preferred to the slower alternative: a
     * function call that checks a non-global.
     */
    bool frontswap_enabled __read_mostly;
    EXPORT_SYMBOL(frontswap_enabled);
    
    /*
     * If enabled, frontswap_store will return failure even on success.  As
     * a result, the swap subsystem will always write the page to swap, in
     * effect converting frontswap into a writethrough cache.  In this mode,
     * there is no direct reduction in swap writes, but a frontswap backend
     * can unilaterally "reclaim" any pages in use with no data loss, thus
     * providing increases control over maximum memory usage due to frontswap.
     */
    static bool frontswap_writethrough_enabled __read_mostly;
    
    /*
     * If enabled, the underlying tmem implementation is capable of doing
     * exclusive gets, so frontswap_load, on a successful tmem_get must
     * mark the page as no longer in frontswap AND mark it dirty.
     */
    static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;
    
    #ifdef CONFIG_DEBUG_FS
    /*
     * Counters available via /sys/kernel/debug/frontswap (if debugfs is
     * properly configured).  These are for information only so are not protected
     * against increment races.
     */
    static u64 frontswap_loads;
    static u64 frontswap_succ_stores;
    static u64 frontswap_failed_stores;
    static u64 frontswap_invalidates;
    
    static inline void inc_frontswap_loads(void) {
            frontswap_loads++;
    }
    static inline void inc_frontswap_succ_stores(void) {
            frontswap_succ_stores++;
    }
    static inline void inc_frontswap_failed_stores(void) {
            frontswap_failed_stores++;
    }
    static inline void inc_frontswap_invalidates(void) {
            frontswap_invalidates++;
    }
    #else
    static inline void inc_frontswap_loads(void) { }
    static inline void inc_frontswap_succ_stores(void) { }
    static inline void inc_frontswap_failed_stores(void) { }
    static inline void inc_frontswap_invalidates(void) { }
    #endif
    /*
     * Register operations for frontswap, returning previous thus allowing
     * detection of multiple backends and possible nesting.
     */
    struct frontswap_ops frontswap_register_ops(struct frontswap_ops *ops)
    {
            struct frontswap_ops old = frontswap_ops;
    
            frontswap_ops = *ops;
            frontswap_enabled = true;
            return old;
    }
    EXPORT_SYMBOL(frontswap_register_ops);
    
    /*
     * Enable/disable frontswap writethrough (see above).
     */
   void frontswap_writethrough(bool enable)
   {
           frontswap_writethrough_enabled = enable;
   }
   EXPORT_SYMBOL(frontswap_writethrough);
   
   /*
    * Enable/disable frontswap exclusive gets (see above).
    */
   void frontswap_tmem_exclusive_gets(bool enable)
   {
           frontswap_tmem_exclusive_gets_enabled = enable;
   }
   EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
   
   /*
    * Called when a swap device is swapon'd.
    */
   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);
   }
   EXPORT_SYMBOL(__frontswap_init);
   
   static inline void __frontswap_clear(struct swap_info_struct *sis, pgoff_t offset)
   {
           frontswap_clear(sis, offset);
           atomic_dec(&sis->frontswap_pages);
   }
   
   /*
    * "Store" data from a page to frontswap and associate it with the page's
    * swaptype and offset.  Page must be locked and in the swap cache.
    * If frontswap already contains a page with matching swaptype and
    * offset, the frontswap implementation may either overwrite the data and
    * return success or invalidate the page from frontswap and return failure.
    */
   int __frontswap_store(struct page *page)
   {
           int ret = -1, dup = 0;
           swp_entry_t entry = { .val = page_private(page), };
           int type = swp_type(entry);
           struct swap_info_struct *sis = swap_info[type];
           pgoff_t offset = swp_offset(entry);
   
           BUG_ON(!PageLocked(page));
           BUG_ON(sis == NULL);
           if (frontswap_test(sis, offset))
                   dup = 1;
           ret = frontswap_ops.store(type, offset, page);
           if (ret == 0) {
                   frontswap_set(sis, offset);
                   inc_frontswap_succ_stores();
                   if (!dup)
                           atomic_inc(&sis->frontswap_pages);
           } else {
                   /*
                     failed dup always results in automatic invalidate of
                     the (older) page from frontswap
                    */
                   inc_frontswap_failed_stores();
                   if (dup)
                           __frontswap_clear(sis, offset);
           }
           if (frontswap_writethrough_enabled)
                   /* report failure so swap also writes to swap device */
                   ret = -1;
           return ret;
   }
   EXPORT_SYMBOL(__frontswap_store);
   
   /*
    * "Get" data from frontswap associated with swaptype and offset that were
    * specified when the data was put to frontswap and use it to fill the
    * specified page with data. Page must be locked and in the swap cache.
    */
   int __frontswap_load(struct page *page)
   {
           int ret = -1;
           swp_entry_t entry = { .val = page_private(page), };
           int type = swp_type(entry);
           struct swap_info_struct *sis = swap_info[type];
           pgoff_t offset = swp_offset(entry);
   
           BUG_ON(!PageLocked(page));
           BUG_ON(sis == NULL);
           if (frontswap_test(sis, offset))
                   ret = frontswap_ops.load(type, offset, page);
           if (ret == 0) {
                   inc_frontswap_loads();
                   if (frontswap_tmem_exclusive_gets_enabled) {
                           SetPageDirty(page);
                           frontswap_clear(sis, offset);
                   }
           }
           return ret;
   }
   EXPORT_SYMBOL(__frontswap_load);
   
   /*
    * Invalidate any data from frontswap associated with the specified swaptype
    * and offset so that a subsequent "get" will fail.
    */
   void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
   {
           struct swap_info_struct *sis = swap_info[type];
   
           BUG_ON(sis == NULL);
           if (frontswap_test(sis, offset)) {
                   frontswap_ops.invalidate_page(type, offset);
                   __frontswap_clear(sis, offset);
                   inc_frontswap_invalidates();
           }
   }
   EXPORT_SYMBOL(__frontswap_invalidate_page);
   
   /*
    * Invalidate all data from frontswap associated with all offsets for the
    * specified swaptype.
    */
   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));
   }
   EXPORT_SYMBOL(__frontswap_invalidate_area);
   
   static unsigned long __frontswap_curr_pages(void)
   {
           int type;
           unsigned long totalpages = 0;
           struct swap_info_struct *si = NULL;
   
           assert_spin_locked(&swap_lock);
           for (type = swap_list.head; type >= 0; type = si->next) {
                   si = swap_info[type];
                   totalpages += atomic_read(&si->frontswap_pages);
           }
           return totalpages;
   }
   
   static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
                                           int *swapid)
   {
           int ret = -EINVAL;
           struct swap_info_struct *si = NULL;
           int si_frontswap_pages;
           unsigned long total_pages_to_unuse = total;
           unsigned long pages = 0, pages_to_unuse = 0;
           int type;
   
           assert_spin_locked(&swap_lock);
           for (type = swap_list.head; type >= 0; type = si->next) {
                   si = swap_info[type];
                   si_frontswap_pages = atomic_read(&si->frontswap_pages);
                   if (total_pages_to_unuse < si_frontswap_pages) {
                           pages = pages_to_unuse = total_pages_to_unuse;
                   } else {
                           pages = si_frontswap_pages;
                           pages_to_unuse = 0; /* unuse all */
                   }
                   /* ensure there is enough RAM to fetch pages from frontswap */
                   if (security_vm_enough_memory_mm(current->mm, pages)) {
                           ret = -ENOMEM;
                           continue;
                   }
                   vm_unacct_memory(pages);
                   *unused = pages_to_unuse;
                   *swapid = type;
                   ret = 0;
                   break;
           }
   
           return ret;
   }
   
   /*
    * Used to check if it's necessory and feasible to unuse pages.
    * Return 1 when nothing to do, 0 when need to shink pages,
    * error code when there is an error.
    */
   static int __frontswap_shrink(unsigned long target_pages,
                                   unsigned long *pages_to_unuse,
                                   int *type)
   {
           unsigned long total_pages = 0, total_pages_to_unuse;
   
           assert_spin_locked(&swap_lock);
   
           total_pages = __frontswap_curr_pages();
           if (total_pages <= target_pages) {
                   /* Nothing to do */
                   *pages_to_unuse = 0;
                   return 1;
           }
           total_pages_to_unuse = total_pages - target_pages;
           return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
   }
   
   /*
    * Frontswap, like a true swap device, may unnecessarily retain pages
    * under certain circumstances; "shrink" frontswap is essentially a
    * "partial swapoff" and works by calling try_to_unuse to attempt to
    * unuse enough frontswap pages to attempt to -- subject to memory
    * constraints -- reduce the number of pages in frontswap to the
    * number given in the parameter target_pages.
    */
   void frontswap_shrink(unsigned long target_pages)
   {
           unsigned long pages_to_unuse = 0;
           int uninitialized_var(type), ret;
   
           /*
            * we don't want to hold swap_lock while doing a very
            * lengthy try_to_unuse, but swap_list may change
            * so restart scan from swap_list.head each time
            */
           spin_lock(&swap_lock);
           ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
           spin_unlock(&swap_lock);
           if (ret == 0)
                   try_to_unuse(type, true, pages_to_unuse);
           return;
   }
   EXPORT_SYMBOL(frontswap_shrink);
   
   /*
    * Count and return the number of frontswap pages across all
    * swap devices.  This is exported so that backend drivers can
    * determine current usage without reading debugfs.
    */
   unsigned long frontswap_curr_pages(void)
   {
           unsigned long totalpages = 0;
   
           spin_lock(&swap_lock);
           totalpages = __frontswap_curr_pages();
           spin_unlock(&swap_lock);
   
           return totalpages;
   }
   EXPORT_SYMBOL(frontswap_curr_pages);
   
   static int __init init_frontswap(void)
   {
   #ifdef CONFIG_DEBUG_FS
           struct dentry *root = debugfs_create_dir("frontswap", NULL);
           if (root == NULL)
                   return -ENXIO;
           debugfs_create_u64("loads", S_IRUGO, root, &frontswap_loads);
           debugfs_create_u64("succ_stores", S_IRUGO, root, &frontswap_succ_stores);
           debugfs_create_u64("failed_stores", S_IRUGO, root,
                                   &frontswap_failed_stores);
           debugfs_create_u64("invalidates", S_IRUGO,
                                   root, &frontswap_invalidates);
   #endif
           return 0;
   }
   
   module_init(init_frontswap);

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