[PATCH v3] arch/ia64: Restore arch-specific pgd_offset_k implementation

[Mike Rapoport <rppt@kernel.org>]

From: Jessica Clarke <jrtc27@jrtc27.com>

IA-64 is special and treats pgd_offset_k() differently from pgd_offset() by
using different formulas to calculate index into kernel and user PGD
tables. The index into user PGDs takes into account the region number
and the index into the kernel (init_mm) PGD always presumes predefined
kernel region number. Commit 974b9b2c68 ("mm: consolidate pte_index()
and pte_offset_*() definitions") made IA-64 to use generic
pgd_offset_k() which wrongly used pgd_index() for user page tables. As
the result, the index into kernel PGD was going out of bounds and the
kernel hang during early boot.

Allow overrides of pgd_offset_k() and use an override on IA-64 with the
old implementation that will correctly index kernel PGD.

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>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---
Changes since v2:
* Rephrase commit message and comment about pgd_offset_k()

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 | 9 +++++++++
 include/linux/pgtable.h         | 2 ++
 2 files changed, 11 insertions(+)

diff --git a/arch/ia64/include/asm/pgtable.h b/arch/ia64/include/asm/pgtable.h
index 10850897a91c..6ef501cd0166 100644
--- a/arch/ia64/include/asm/pgtable.h
+++ b/arch/ia64/include/asm/pgtable.h
@@ -366,6 +366,15 @@ 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.
+ * However, this also means that we cannot use pgd_index() and we never
+ * should add the region here.
+ */
+#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 a124c21e3204..e8cbc2e795d5 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
-- 
2.26.2