Man page 9 : truncate_inode_pages

truncate_inode_pages - truncate *all* the pages from an offset

void truncate_inode_pages(struct address_space *  mapping , loff_t  lstart );

mapping
    mapping to truncate

lstart
    offset from which to truncate

Called under (and serialised by) inode->i_mutex.

When this function returns, there can be a page in the process of deletion (inside __delete_from_page_cache) in the specified range. Thus mapping->nrpages can be non-zero when this function returns even after truncation of the whole mapping.