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Subject[PATCH v10 25/25] 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.vnet.ibm.com>
---
arch/powerpc/mm/fault.c | 33 +++++++++++++++++++++++++++++++--
1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index c01d627e687a..37191147026e 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -464,6 +464,26 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
if (is_exec)
flags |= FAULT_FLAG_INSTRUCTION;

+ if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) {
+ fault = handle_speculative_fault(mm, address, flags, &vma);
+ /*
+ * 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.
+ */
+ 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;
+ }
+ } else {
+ vma = NULL;
+ }
+
/* 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
@@ -494,7 +514,8 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
might_sleep();
}

- vma = find_vma(mm, address);
+ if (!vma || !can_reuse_spf_vma(vma, address))
+ vma = find_vma(mm, address);
if (unlikely(!vma))
return bad_area(regs, address);
if (likely(vma->vm_start <= address))
@@ -551,8 +572,15 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
*/
flags &= ~FAULT_FLAG_ALLOW_RETRY;
flags |= FAULT_FLAG_TRIED;
- if (!fatal_signal_pending(current))
+ if (!fatal_signal_pending(current)) {
+ /*
+ * Do not try to reuse this vma and fetch it
+ * again since we will release the mmap_sem.
+ */
+ if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
+ vma = NULL;
goto retry;
+ }
}

/*
@@ -564,6 +592,7 @@ 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
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