Revert "mm/vmalloc: interchage the implementation of vmalloc_to_{pfn,page}"

Revert commit ece86e222db4, which was intended as a small performance
improvement.

Despite the claim that the patch doesn't introduce any functional
changes in fact it does.

The "no page" path behaves different now.  Originally, vmalloc_to_page
might return NULL under some conditions, with new implementation it
returns pfn_to_page(0) which is not the same as NULL.

Simple test shows the difference.

test.c

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/vmalloc.h>
#include <linux/mm.h>

int __init myi(void)
{
	struct page *p;
	void *v;

	v = vmalloc(PAGE_SIZE);
	/* trigger the "no page" path in vmalloc_to_page*/
	vfree(v);

	p = vmalloc_to_page(v);

	pr_err("expected val = NULL, returned val = %p", p);

	return -EBUSY;
}

void __exit mye(void)
{

}
module_init(myi)
module_exit(mye)

Before interchange:
expected val = NULL, returned val =   (null)

After interchange:
expected val = NULL, returned val = c7ebe000

Signed-off-by: Vladimir Murzin <murzin.v@gmail.com>
Cc: Jianyu Zhan <nasa4836@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmalloc.c

@@ -220,12 +220,12 @@ int is_vmalloc_or_module_addr(const void *x)
 }
 
 /*
- * Walk a vmap address to the physical pfn it maps to.
+ * Walk a vmap address to the struct page it maps.
  */
-unsigned long vmalloc_to_pfn(const void *vmalloc_addr)
+struct page *vmalloc_to_page(const void *vmalloc_addr)
 {
 	unsigned long addr = (unsigned long) vmalloc_addr;
-	unsigned long pfn = 0;
+	struct page *page = NULL;
 	pgd_t *pgd = pgd_offset_k(addr);
 
 	/*
@@ -244,23 +244,23 @@ unsigned long vmalloc_to_pfn(const void *vmalloc_addr)
 				ptep = pte_offset_map(pmd, addr);
 				pte = *ptep;
 				if (pte_present(pte))
-					pfn = pte_pfn(pte);
+					page = pte_page(pte);
 				pte_unmap(ptep);
 			}
 		}
 	}
-	return pfn;
+	return page;
 }
-EXPORT_SYMBOL(vmalloc_to_pfn);
+EXPORT_SYMBOL(vmalloc_to_page);
 
 /*
- * Map a vmalloc()-space virtual address to the struct page.
+ * Map a vmalloc()-space virtual address to the physical page frame number.
  */
-struct page *vmalloc_to_page(const void *vmalloc_addr)
+unsigned long vmalloc_to_pfn(const void *vmalloc_addr)
 {
-	return pfn_to_page(vmalloc_to_pfn(vmalloc_addr));
+	return page_to_pfn(vmalloc_to_page(vmalloc_addr));
 }
-EXPORT_SYMBOL(vmalloc_to_page);
+EXPORT_SYMBOL(vmalloc_to_pfn);
 
 
 /*** Global kva allocator ***/