x86/crashdump/32: Simplify copy_oldmem_page()

Replace kmap_atomic_pfn() with kmap_local_pfn() which is preemptible and
can take page faults.

Remove the indirection of the dump page and the related cruft which is not
longer required.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201118204007.670851839@linutronix.de

arch/x86/kernel/crash_dump_32.c

@@ -13,8 +13,6 @@
 
 #include <linux/uaccess.h>
 
-static void *kdump_buf_page;
-
 static inline bool is_crashed_pfn_valid(unsigned long pfn)
 {
 #ifndef CONFIG_X86_PAE
@@ -41,15 +39,11 @@ static inline bool is_crashed_pfn_valid(unsigned long pfn)
  * @userbuf: if set, @buf is in user address space, use copy_to_user(),
  *	otherwise @buf is in kernel address space, use memcpy().
  *
- * Copy a page from "oldmem". For this page, there is no pte mapped
- * in the current kernel. We stitch up a pte, similar to kmap_atomic.
- *
- * Calling copy_to_user() in atomic context is not desirable. Hence first
- * copying the data to a pre-allocated kernel page and then copying to user
- * space in non-atomic context.
+ * Copy a page from "oldmem". For this page, there might be no pte mapped
+ * in the current kernel.
  */
-ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
-                               size_t csize, unsigned long offset, int userbuf)
+ssize_t copy_oldmem_page(unsigned long pfn, char *buf, size_t csize,
+			 unsigned long offset, int userbuf)
 {
 	void  *vaddr;
 
@@ -59,38 +53,16 @@ ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
 	if (!is_crashed_pfn_valid(pfn))
 		return -EFAULT;
 
-	vaddr = kmap_atomic_pfn(pfn);
+	vaddr = kmap_local_pfn(pfn);
 
 	if (!userbuf) {
-		memcpy(buf, (vaddr + offset), csize);
-		kunmap_atomic(vaddr);
+		memcpy(buf, vaddr + offset, csize);
 	} else {
-		if (!kdump_buf_page) {
-			printk(KERN_WARNING "Kdump: Kdump buffer page not"
-				" allocated\n");
-			kunmap_atomic(vaddr);
-			return -EFAULT;
-		}
-		copy_page(kdump_buf_page, vaddr);
-		kunmap_atomic(vaddr);
-		if (copy_to_user(buf, (kdump_buf_page + offset), csize))
-			return -EFAULT;
+		if (copy_to_user(buf, vaddr + offset, csize))
+			csize = -EFAULT;
 	}
 
-	return csize;
-}
+	kunmap_local(vaddr);
 
-static int __init kdump_buf_page_init(void)
-{
-	int ret = 0;
-
-	kdump_buf_page = kmalloc(PAGE_SIZE, GFP_KERNEL);
-	if (!kdump_buf_page) {
-		printk(KERN_WARNING "Kdump: Failed to allocate kdump buffer"
-			 " page\n");
-		ret = -ENOMEM;
-	}
-
-	return ret;
+	return csize;
 }
-arch_initcall(kdump_buf_page_init);