4 * Copyright (C) 1991, 1992, 1993, 1994 Linus Torvalds
6 * Swap reorganised 29.12.95,
7 * Asynchronous swapping added 30.12.95. Stephen Tweedie
8 * Removed race in async swapping. 14.4.1996. Bruno Haible
9 * Add swap of shared pages through the page cache. 20.2.1998. Stephen Tweedie
10 * Always use brw_page, life becomes simpler. 12 May 1998 Eric Biederman
14 #include <linux/kernel_stat.h>
15 #include <linux/pagemap.h>
16 #include <linux/swap.h>
17 #include <linux/swapctl.h>
19 #include <asm/pgtable.h>
22 * Reads or writes a swap page.
23 * wait=1: start I/O and wait for completion. wait=0: start asynchronous I/O.
25 * Important prevention of race condition: the caller *must* atomically
26 * create a unique swap cache entry for this swap page before calling
27 * rw_swap_page, and must lock that page. By ensuring that there is a
28 * single page of memory reserved for the swap entry, the normal VM page
29 * lock on that page also doubles as a lock on swap entries. Having only
30 * one lock to deal with per swap entry (rather than locking swap and memory
31 * independently) also makes it easier to make certain swapping operations
32 * atomic, which is particularly important when we are trying to ensure
33 * that shared pages stay shared while being swapped.
36 static int rw_swap_page_base(int rw, swp_entry_t entry, struct page *page)
39 sector_t zones[PAGE_SIZE/512];
42 struct inode *swapf = 0;
43 struct block_device *bdev;
46 ClearPageUptodate(page);
51 get_swaphandle_info(entry, &offset, &swapf);
56 block_size = PAGE_SIZE;
59 unsigned int block = offset
60 << (PAGE_SHIFT - swapf->i_sb->s_blocksize_bits);
62 block_size = swapf->i_sb->s_blocksize;
63 for (i=0, j=0; j< PAGE_SIZE ; i++, j += block_size)
64 if (!(zones[i] = bmap(swapf,block++))) {
65 printk("rw_swap_page: bad swap file\n");
69 bdev = swapf->i_sb->s_bdev;
72 /* block_size == PAGE_SIZE/zones_used */
73 brw_page(rw, page, bdev, zones, block_size);
75 /* Note! For consistency we do all of the logic,
76 * decrementing the page count, and unlocking the page in the
77 * swap lock map - in the IO completion handler.
83 * A simple wrapper so the base function doesn't need to enforce
84 * that all swap pages go through the swap cache! We verify that:
85 * - the page is locked
86 * - it's marked as being swap-cache
87 * - it's associated with the swap inode
89 void rw_swap_page(int rw, struct page *page)
93 entry.val = page->index;
95 if (!PageLocked(page))
97 if (!PageSwapCache(page))
99 if (!rw_swap_page_base(rw, entry, page))
104 * The swap lock map insists that pages be in the page cache!
105 * Therefore we can't use it. Later when we can remove the need for the
106 * lock map and we can reduce the number of functions exported.
108 void rw_swap_page_nolock(int rw, swp_entry_t entry, char *buf)
110 struct page *page = virt_to_page(buf);
112 if (!PageLocked(page))
116 /* needs sync_page to wait I/O completation */
117 page->mapping = &swapper_space;
118 if (!rw_swap_page_base(rw, entry, page))
120 wait_on_page_locked(page);
121 page->mapping = NULL;