[PATCH v4 03/10] sparc64: avoid huge kernel PUD mappings with small hardware pages

[Magnus Lindholm <linmag7@gmail.com>]

The kernel PUD refill path merges virtual address bits 32:28 into a
TTE, requiring hardware pages of at least 256MB. Sun4u instead selects
4MB pages, losing bits 27:22 when refilling a huge PUD. Sun4v also falls
back to 4MB pages when neither 256MB nor 2GB pages are selected.

Keep PMD mappings when the selected PUD encoding matches the small-page
entry. The PMD refill path preserves the bits needed by the 4MB TTE;
sun4v selections of 256MB or larger remain eligible for huge PUDs.

Fixes: 0dd5b7b09e13 ("sparc64: Fix physical memory management regressions with large max_phys_bits.")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/sparc/mm/init_64.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/sparc/mm/init_64.c b/arch/sparc/mm/init_64.c
index 103db4683b16..f3f7eba08469 100644
--- a/arch/sparc/mm/init_64.c
+++ b/arch/sparc/mm/init_64.c
@@ -1696,6 +1696,14 @@ static unsigned long __ref kernel_map_hugepud(unsigned long vstart,
 static bool kernel_can_map_hugepud(unsigned long vstart, unsigned long vend,
 				   bool guard)
 {
+	/*
+	 * The PUD miss path propagates VA bits 32:28, assuming at least
+	 * 256MB hardware pages. Retain PMD mappings when the selected PUD
+	 * encoding falls back to the small-page entry.
+	 */
+	if (kern_linear_pte_xor[2] == kern_linear_pte_xor[0])
+		return false;
+
 	if (guard && !(vstart & ~PUD_MASK) && (vend - vstart) >= PUD_SIZE)
 		return true;
 
-- 
2.43.0