Re: 2.6.17-rc5-mm1

[Christoph Lameter]

On Tue, 6 Jun 2006, Martin Schwidefsky wrote:

> swp_type(pte_to_swp_entry(swp_entry_to_pte(swp_entry(~0UL,0))))
> evaluates to 31 for s390. The idea has been that the creation of a swap
> entry with ~0UL as the type should mask off all bits that can not be
> represented in a swap entry. Convert it back and you have the maximum
> number of swap devices. That should be true for all architectures.

It evaluates to 31 it seems for all platforms. Could we replace this 
expression with MAX_SWAPFILES? While we at it get rid of the "type" 
variable because we are usually using pointers to swap_info.

Note that there is  still another similar check in there for an arch 
specific test of the number of pages available per swap device.


Index: linux-2.6.17-rc5-mm2/mm/swapfile.c
===================================================================
--- linux-2.6.17-rc5-mm2.orig/mm/swapfile.c	2006-06-01 10:03:07.127259731 -0700
+++ linux-2.6.17-rc5-mm2/mm/swapfile.c	2006-06-06 10:50:09.692165312 -0700
@@ -1363,12 +1363,11 @@ __initcall(procswaps_init);
  */
 asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
 {
-	struct swap_info_struct * p;
+	struct swap_info_struct *p,*q;
 	char *name = NULL;
 	struct block_device *bdev = NULL;
 	struct file *swap_file = NULL;
 	struct address_space *mapping;
-	unsigned int type;
 	int i, prev;
 	int error;
 	static int least_priority;
@@ -1387,29 +1386,19 @@ asmlinkage long sys_swapon(const char __
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 	spin_lock(&swap_lock);
-	p = swap_info;
-	for (type = 0 ; type < nr_swapfiles ; type++,p++)
+
+	for (p = swap_info; p < swap_info + nr_swapfiles; p++)
 		if (!(p->flags & SWP_USED))
 			break;
 	error = -EPERM;
-	/*
-	 * Test if adding another swap device is possible. There are
-	 * two limiting factors: 1) the number of bits for the swap
-	 * type swp_entry_t definition and 2) the number of bits for
-	 * the swap type in the swap ptes as defined by the different
-	 * architectures. To honor both limitations a swap entry
-	 * with swap offset 0 and swap type ~0UL is created, encoded
-	 * to a swap pte, decoded to a swp_entry_t again and finally
-	 * the swap type part is extracted. This will mask all bits
-	 * from the initial ~0UL that can't be encoded in either the
-	 * swp_entry_t or the architecture definition of a swap pte.
-	 */
-	if (type > swp_type(pte_to_swp_entry(swp_entry_to_pte(swp_entry(~0UL,0))))) {
+
+	if (p == swap_info + MAX_SWAPFILES) {
 		spin_unlock(&swap_lock);
 		goto out;
 	}
-	if (type >= nr_swapfiles)
-		nr_swapfiles = type+1;
+	if (p >= swap_info + nr_swapfiles)
+		nr_swapfiles++;
+
 	INIT_LIST_HEAD(&p->extent_list);
 	p->flags = SWP_USED;
 	p->swap_file = NULL;
@@ -1445,10 +1434,9 @@ asmlinkage long sys_swapon(const char __
 	inode = mapping->host;
 
 	error = -EBUSY;
-	for (i = 0; i < nr_swapfiles; i++) {
-		struct swap_info_struct *q = &swap_info[i];
+	for (q = swap_info; q < swap_info + nr_swapfiles; q++) {
 
-		if (i == type || !q->swap_file)
+		if (q == p || !q->swap_file)
 			continue;
 		if (mapping == q->swap_file->f_mapping)
 			goto bad_swap;




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