[PATCH] x86/mm: Fix boot with some memory above MAXMEM

Kirill A. Shutemov
2020-05-11T19:17:42Z

A 5-level paging capable machine can have memory above 46-bit in the
physical address space. This memory is only addressable in the 5-level
paging mode: we don't have enough virtual address space to create direct
mapping for such memory in the 4-level paging mode.

Currently, we fail boot completely: NULL pointer dereference in
subsection_map_init().

Skip creating a memblock for such memory instead and notify user that
some memory is not addressable.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Cc: stable@vger.kernel.org # v4.14
---

Tested with a hacked QEMU: https://gist.github.com/kiryl/d45eb54110944ff95e544972d8bdac1d

---
arch/x86/kernel/e820.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index c5399e80c59c..d320d37d0f95 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -1280,8 +1280,8 @@ void __init e820__memory_setup(void)

void __init e820__memblock_setup(void)
{
+ u64 size, end, not_addressable = 0;
int i;
- u64 end;

/*
* The bootstrap memblock region count maximum is 128 entries
@@ -1307,7 +1307,22 @@ void __init e820__memblock_setup(void)
if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
continue;

- memblock_add(entry->addr, entry->size);
+ if (entry->addr >= MAXMEM) {
+ not_addressable += entry->size;
+ continue;
+ }
+
+ end = min_t(u64, end, MAXMEM - 1);
+ size = end - entry->addr;
+ not_addressable += entry->size - size;
+ memblock_add(entry->addr, size);
+ }
+
+ if (not_addressable) {
+ pr_err("%lldGB of physical memory is not addressable in the paging mode\n",
+ not_addressable >> 30);
+ if (!pgtable_l5_enabled())
+ pr_err("Consider enabling 5-level paging\n");
}

/* Throw away partial pages: */
--
2.26.2