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Subject[PATCH v12 29/31] powerpc/mm: add speculative page fault
Date
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@linux.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 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
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