[PATCH v12 29/31] powerpc/mm: add speculative page fault

[Laurent Dufour]

This patch enable the speculative page fault on the PowerPC
architecture.

This will try a speculative page fault without holding the mmap_sem,
if it returns with VM_FAULT_RETRY, the mmap_sem is acquired and the
traditional page fault processing is done.

The speculative path is only tried for multithreaded process as there is no
risk of contention on the mmap_sem otherwise.

Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
---
 arch/powerpc/mm/fault.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index ec74305fa330..5d48016073cb 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -491,6 +491,21 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 	if (is_exec)
 		flags |= FAULT_FLAG_INSTRUCTION;
 
+	/*
+	 * Try speculative page fault before grabbing the mmap_sem.
+	 * The Page fault is done if VM_FAULT_RETRY is not returned.
+	 * But if the memory protection keys are active, we don't know if the
+	 * fault is due to key mistmatch or due to a classic protection check.
+	 * To differentiate that, we will need the VMA we no more have, so
+	 * let's retry with the mmap_sem held.
+	 */
+	fault = handle_speculative_fault(mm, address, flags);
+	if (fault != VM_FAULT_RETRY && (IS_ENABLED(CONFIG_PPC_MEM_KEYS) &&
+					fault != VM_FAULT_SIGSEGV)) {
+		perf_sw_event(PERF_COUNT_SW_SPF, 1, regs, address);
+		goto done;
+	}
+
 	/* When running in the kernel we expect faults to occur only to
 	 * addresses in user space.  All other faults represent errors in the
 	 * kernel and should generate an OOPS.  Unfortunately, in the case of an
@@ -600,6 +615,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 
 	up_read(&current->mm->mmap_sem);
 
+done:
 	if (unlikely(fault & VM_FAULT_ERROR))
 		return mm_fault_error(regs, address, fault);
 
-- 
2.21.0