谁能解释一下address_space里的truncate_count的作用？

白痴弟弟2

struct address_space {
        struct inode                *host;                /* owner: inode, block_device */
        struct radix_tree_root        page_tree;        /* radix tree of all pages */
        spinlock_t                tree_lock;        /* and spinlock protecting it */
        unsigned int                i_mmap_writable;/* count VM_SHARED mappings */
        struct prio_tree_root        i_mmap;                /* tree of private and shared mappings */
        struct list_head        i_mmap_nonlinear;/*list VM_NONLINEAR mappings */
        spinlock_t                i_mmap_lock;        /* protect tree, count, list */
        unsigned int                truncate_count;        /* Cover race condition with truncate */
        unsigned long                nrpages;        /* number of total pages */
        pgoff_t                        writeback_index;/* writeback starts here */
        struct address_space_operations *a_ops;        /* methods */
        unsigned long                flags;                /* error bits/gfp mask */
        struct backing_dev_info *backing_dev_info; /* device readahead, etc */
        spinlock_t                private_lock;        /* for use by the address_space */
        struct list_head        private_list;        /* ditto */
        struct address_space        *assoc_mapping;        /* ditto */
} __attribute__((aligned(sizeof(long))));