setup_swap_extents

/*
 * A `swap extent' is a simple thing which maps a contiguous range of pages
 * onto a contiguous range of disk blocks.  An ordered list of swap extents
 * is built at swapon time and is then used at swap_writepage/swap_readpage
 * time for locating where on disk a page belongs.
 *
 * If the swapfile is an S_ISBLK block device, a single extent is installed.
 * This is done so that the main operating code can treat S_ISBLK and S_ISREG
 * swap files identically.
 *
 * Whether the swapdev is an S_ISREG file or an S_ISBLK blockdev, the swap
 * extent list operates in PAGE_SIZE disk blocks.  Both S_ISREG and S_ISBLK
 * swapfiles are handled *identically* after swapon time.
 *
 * For S_ISREG swapfiles, setup_swap_extents() will walk all the file's blocks
 * and will parse them into an ordered extent list, in PAGE_SIZE chunks.  If
 * some stray blocks are found which do not fall within the PAGE_SIZE alignment
 * requirements, they are simply tossed out - we will never use those blocks
 * for swapping.
 *
 * For all swap devices we set S_SWAPFILE across the life of the swapon.  This
 * prevents users from writing to the swap device, which will corrupt memory.
 *
 * The amount of disk space which a single swap extent represents varies.
 * Typically it is in the 1-4 megabyte range.  So we can have hundreds of
 * extents in the list.  To avoid much list walking, we cache the previous
 * search location in `curr_swap_extent', and start new searches from there.
 * This is extremely effective.  The average number of iterations in
 * map_swap_page() has been measured at about 0.3 per page.  - akpm.
 */
static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
{
    struct file *swap_file = sis->swap_file;
    struct address_space *mapping = swap_file->f_mapping;
    struct inode *inode = mapping->host;
    int ret;
 
    if (S_ISBLK(inode->i_mode)) {
        ret = add_swap_extent(sis, 0, sis->max, 0);
        *span = sis->pages;
        return ret;
    }
 
    if (mapping->a_ops->swap_activate) {
        ret = mapping->a_ops->swap_activate(sis, swap_file, span);
        if (ret >= 0)
            sis->flags |= SWP_ACTIVATED;
        if (!ret) {
            sis->flags |= SWP_FS;
            ret = add_swap_extent(sis, 0, sis->max, 0);
            *span = sis->pages;
        }
        return ret;
    }
 
    return generic_swapfile_activate(sis, swap_file, span);
}
函数源码
