[PATCH] powerpc/kdump: Fix /dev/oldmem interface

["Sachin P. Sant" <sachinp@in.ibm.com>]

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This patch fixes the /dev/oldmem interface for kdump on ppc64.
The patch originally came from Michael Ellerman hence have
retained the signed-off line. I just rediffed/tested against
latest git. Michael has ack'ed this patch.

Ben this is not a must for 2.6.27 but would be good if it's 
included.

Thanks
-Sachin

Signed-off-by : Michael Ellerman <michael@ellerman.id.au>
Acked-by : Michael Ellerman <michael@ellerman.id.au>

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Fix /dev/oldmem for kdump

A change to __ioremap() broke reading /dev/oldmem because we're no
longer able to ioremap pfn 0 (d177c207ba16b1db31283e2d1fee7ad4a863584b).

We actually don't need to ioremap for anything that's part of the linear
mapping, so just read it directly.

Also make sure we're only reading one page or less at a time.

Signed-off-by : Michael Ellerman <michael@ellerman.id.au>
---

diff -Naurp 1/arch/powerpc/kernel/crash_dump.c 2/arch/powerpc/kernel/crash_dump.c
--- 1/arch/powerpc/kernel/crash_dump.c	2008-07-31 11:55:39.000000000 +0530
+++ 2/arch/powerpc/kernel/crash_dump.c	2008-07-31 12:09:49.000000000 +0530
@@ -86,6 +86,19 @@ static int __init parse_savemaxmem(char 
 }
 __setup("savemaxmem=", parse_savemaxmem);
 
+
+static size_t copy_oldmem_vaddr(void *vaddr, char *buf, size_t csize,
+                               unsigned long offset, int userbuf)
+{
+	if (userbuf) {
+		if (copy_to_user((char __user *)buf, (vaddr + offset), csize))
+			return -EFAULT;
+	} else
+		memcpy(buf, (vaddr + offset), csize);
+
+	return csize;
+}
+
 /**
  * copy_oldmem_page - copy one page from "oldmem"
  * @pfn: page frame number to be copied
@@ -107,16 +120,16 @@ ssize_t copy_oldmem_page(unsigned long p
 	if (!csize)
 		return 0;
 
-	vaddr = __ioremap(pfn << PAGE_SHIFT, PAGE_SIZE, 0);
+	csize = min(csize, PAGE_SIZE);
 
-	if (userbuf) {
-		if (copy_to_user((char __user *)buf, (vaddr + offset), csize)) {
-			iounmap(vaddr);
-			return -EFAULT;
-		}
-	} else
-		memcpy(buf, (vaddr + offset), csize);
+	if (pfn < max_pfn) {
+		vaddr = __va(pfn << PAGE_SHIFT);
+		csize = copy_oldmem_vaddr(vaddr, buf, csize, offset, userbuf);
+	} else {
+		vaddr = __ioremap(pfn << PAGE_SHIFT, PAGE_SIZE, 0);
+		csize = copy_oldmem_vaddr(vaddr, buf, csize, offset, userbuf);
+		iounmap(vaddr);
+	}
 
-	iounmap(vaddr);
 	return csize;
 }