powerpc/crashdump : Fix page frame number check in copy_oldmem_page

In copy_oldmem_page, the current check using max_pfn and min_low_pfn to
decide if the page is backed or not, is not valid when the memory layout is
not continuous.

This happens when running as a QEMU/KVM guest, where RTAS is mapped higher
in the memory. In that case max_pfn points to the end of RTAS, and a hole
between the end of the kdump kernel and RTAS is not backed by PTEs. As a
consequence, the kdump kernel is crashing in copy_oldmem_page when accessing
in a direct way the pages in that hole.

This fix relies on the memblock's service memblock_is_region_memory to
check if the read page is part or not of the directly accessible memory.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Tested-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/arch/powerpc/kernel/crash_dump.c b/arch/powerpc/kernel/crash_dump.c
index 11c1d069d920..7a13f378ca2c 100644
--- a/arch/powerpc/kernel/crash_dump.c
+++ b/arch/powerpc/kernel/crash_dump.c
@@ -98,17 +98,19 @@ ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
                         size_t csize, unsigned long offset, int userbuf)
 {
         void  *vaddr;
+        phys_addr_t paddr;
 
         if (!csize)
                 return 0;
 
         csize = min_t(size_t, csize, PAGE_SIZE);
+        paddr = pfn << PAGE_SHIFT;
 
-        if ((min_low_pfn < pfn) && (pfn < max_pfn)) {
-                vaddr = __va(pfn << PAGE_SHIFT);
+        if (memblock_is_region_memory(paddr, csize)) {
+                vaddr = __va(paddr);
                 csize = copy_oldmem_vaddr(vaddr, buf, csize, offset, userbuf);
         } else {
-                vaddr = __ioremap(pfn << PAGE_SHIFT, PAGE_SIZE, 0);
+                vaddr = __ioremap(paddr, PAGE_SIZE, 0);
                 csize = copy_oldmem_vaddr(vaddr, buf, csize, offset, userbuf);
                 iounmap(vaddr);
         }