[PATCH v2] arch/ia64: Restore arch-specific pgd_offset_k implementation

[Jessica Clarke <jrtc27@jrtc27.com>]

IA-64 is special and treats pgd_offset_k differently from pgd_offset by
not including the region number, and init_mm's PGD is such that it only
points to the kernel's region's PGD. This was broken in 974b9b2c68 which
unified the two and therefore included the region number, causing it to
go way out of bounds of the kernel's PGD, which made the kernel hang
during early boot. Thus, permit pgd_offset_k to be overridden like the
other macros and override it on IA-64 with the old implementation. Also
update the comment to clarify that this is not just an optimisation but
a required implementation detail.

Fixes: 974b9b2c68 ("mm: consolidate pte_index() and pte_offset_*() definitions")
Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
---
Changes since v1:
 * Fixed typo in commit message
 * Slightly reworded commit message to sound less weird
 * Included Adrian's Tested-by

 arch/ia64/include/asm/pgtable.h | 8 ++++++++
 include/linux/pgtable.h         | 2 ++
 2 files changed, 10 insertions(+)

diff --git a/arch/ia64/include/asm/pgtable.h b/arch/ia64/include/asm/pgtable.h
index 10850897a91c..2ac2199d99ce 100644
--- a/arch/ia64/include/asm/pgtable.h
+++ b/arch/ia64/include/asm/pgtable.h
@@ -366,6 +366,14 @@ pgd_index (unsigned long address)
 }
 #define pgd_index pgd_index
 
+/*
+ * In the kernel's mapped region we know everything is in region number 5, so
+ * as an optimisation its PGD already points to the area for that region, but
+ * that means not adding the region here is required, not just an optimisation.
+ */
+#define pgd_offset_k(addr) \
+	(init_mm.pgd + (((addr) >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1)))
+
 /* Look up a pgd entry in the gate area.  On IA-64, the gate-area
    resides in the kernel-mapped segment, hence we use pgd_offset_k()
    here.  */
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 53e97da1e8e2..73c64fe098ba 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -117,7 +117,9 @@ static inline pgd_t *pgd_offset_pgd(pgd_t *pgd, unsigned long address)
  * a shortcut which implies the use of the kernel's pgd, instead
  * of a process's
  */
+#ifndef pgd_offset_k
 #define pgd_offset_k(address)		pgd_offset(&init_mm, (address))
+#endif
 
 /*
  * In many cases it is known that a virtual address is mapped at PMD or PTE
-- 
2.23.0