MIPS: crash_dump.c: 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.

Remove unused or redundant header files.
Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/kernel/crash_dump.c

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/highmem.h>
-#include <linux/memblock.h>
 #include <linux/crash_dump.h>
-#include <linux/uaccess.h>
-#include <linux/slab.h>
-
-static void *kdump_buf_page;
 
 /**
  * copy_oldmem_page - copy one page from "oldmem"
@@ -19,10 +14,6 @@ static void *kdump_buf_page;
  *
  * Copy a page from "oldmem". For this page, there is no pte mapped
  * in the current kernel.
- *
- * 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.
  */
 ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
 			 size_t csize, unsigned long offset, int userbuf)
@@ -32,36 +23,16 @@ ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
 	if (!csize)
 		return 0;
 
-	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) {
-			pr_warn("Kdump: Kdump buffer page not allocated\n");
-
-			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;
-}
-
-static int __init kdump_buf_page_init(void)
-{
-	int ret = 0;
+	kunmap_local(vaddr);
 
-	kdump_buf_page = kmalloc(PAGE_SIZE, GFP_KERNEL);
-	if (!kdump_buf_page) {
-		pr_warn("Kdump: Failed to allocate kdump buffer page\n");
-		ret = -ENOMEM;
-	}
-
-	return ret;
+	return csize;
 }
-arch_initcall(kdump_buf_page_init);