From: Rachita Kothiyal <rachita@in.ibm.com>

This patch allocates a page to copy the previous kernel's memory before we
copy it onto a user buffer using copy_to_user(), thereby taking care of the
scenario of a possible page fault in an atomic context.

Signed-off-by: Rachita Kothiyal <rachita@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 arch/i386/kernel/crash_dump.c |   39 +++++++++++++++++++++++++++++++++------
 1 files changed, 33 insertions(+), 6 deletions(-)

diff -puN arch/i386/kernel/crash_dump.c~kdump-read-previous-kernels-memory-fix arch/i386/kernel/crash_dump.c
--- devel/arch/i386/kernel/crash_dump.c~kdump-read-previous-kernels-memory-fix	2005-11-26 18:28:14.000000000 -0800
+++ devel-akpm/arch/i386/kernel/crash_dump.c	2005-11-26 18:28:14.000000000 -0800
@@ -11,6 +11,8 @@
 
 #include <asm/uaccess.h>
 
+static void *kdump_buf_page;
+
 /**
  * copy_oldmem_page - copy one page from "oldmem"
  * @pfn: page frame number to be copied
@@ -23,6 +25,10 @@
  *
  * 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.
  */
 ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
                                size_t csize, unsigned long offset, int userbuf)
@@ -34,14 +40,35 @@ ssize_t copy_oldmem_page(unsigned long p
 
 	vaddr = kmap_atomic_pfn(pfn, KM_PTE0);
 
-	if (userbuf) {
-		if (copy_to_user(buf, (vaddr + offset), csize)) {
-			kunmap_atomic(vaddr, KM_PTE0);
+	if (!userbuf) {
+		memcpy(buf, (vaddr + offset), csize);
+		kunmap_atomic(vaddr, KM_PTE0);
+	} else {
+		if (!kdump_buf_page) {
+			printk(KERN_WARNING "Kdump: Kdump buffer page not"
+				" allocated\n");
 			return -EFAULT;
 		}
-	} else
-	memcpy(buf, (vaddr + offset), csize);
+		copy_page(kdump_buf_page, vaddr);
+		kunmap_atomic(vaddr, KM_PTE0);
+		if (copy_to_user(buf, (kdump_buf_page + offset), csize))
+			return -EFAULT;
+	}
 
-	kunmap_atomic(vaddr, KM_PTE0);
 	return csize;
 }
+
+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;
+}
+arch_initcall(kdump_buf_page_init);
_