[swsusp] Add big fat comment to calc_order().

kernel/power/swsusp.c

@@ -659,13 +659,38 @@ void swsusp_free(void)
 }
 
 
+/**
+ *	calc_order - Determine the order of allocation needed for pagedir_save.
+ *
+ *	This looks tricky, but is just subtle. Please fix it some time.
+ *	Since there are %nr_copy_pages worth of pages in the snapshot, we need
+ *	to allocate enough contiguous space to hold 
+ *		(%nr_copy_pages * sizeof(struct pbe)), 
+ *	which has the saved/orig locations of the page.. 
+ *
+ *	SUSPEND_PD_PAGES() tells us how many pages we need to hold those 
+ *	structures, then we call get_bitmask_order(), which will tell us the
+ *	last bit set in the number, starting with 1. (If we need 30 pages, that
+ *	is 0x0000001e in hex. The last bit is the 5th, which is the order we 
+ *	would use to allocate 32 contiguous pages).
+ *
+ *	Since we also need to save those pages, we add the number of pages that
+ *	we need to nr_copy_pages, and in case of an overflow, do the 
+ *	calculation again to update the number of pages needed. 
+ *
+ *	With this model, we will tend to waste a lot of memory if we just cross
+ *	an order boundary. Plus, the higher the order of allocation that we try
+ *	to do, the more likely we are to fail in a low-memory situtation 
+ *	(though	we're unlikely to get this far in such a case, since swsusp 
+ *	requires half of memory to be free anyway).
+ */
+
+
 static void calc_order(void)
 {
-	int diff;
-	int order;
+	int diff = 0;
+	int order = 0;
 
-	order = get_bitmask_order(SUSPEND_PD_PAGES(nr_copy_pages));
-	nr_copy_pages += 1 << order;
 	do {
 		diff = get_bitmask_order(SUSPEND_PD_PAGES(nr_copy_pages)) - order;
 		if (diff) {
@@ -687,7 +712,7 @@ static void calc_order(void)
 static int alloc_pagedir(void)
 {
 	calc_order();
-	pagedir_save = (suspend_pagedir_t *)__get_free_pages(GFP_ATOMIC | __GFP_COLD, 
+	pagedir_save = (suspend_pagedir_t *)__get_free_pages(GFP_ATOMIC | __GFP_COLD,
 							     pagedir_order);
 	if(!pagedir_save)
 		return -ENOMEM;