[PATCH v11 25/26] 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 <lduf...@linux.vnet.ibm.com>
---
 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 ef268d5d9db7..d7b5742ffb2b 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -465,6 +465,21 @@ 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
@@ -565,6 +580,7 @@ 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);
 
-- 
2.7.4