[PATCH v3] powerpc/mm: Only read faulting instruction when necessary in do_page_fault()

[Christophe Leroy <christophe.leroy@c-s.fr>]

Commit a7a9dcd882a67 ("powerpc: Avoid taking a data miss on every
userspace instruction miss") has shown that limiting the read of
faulting instruction to likely cases improves performance.

This patch goes further into this direction by limiting the read
of the faulting instruction to the only cases where it is definitly
needed.

On an MPC885, with the same benchmark app as in the commit referred
above, we see a reduction of 4000 dTLB misses (approx 3%):

Before the patch:
 Performance counter stats for './fault 500' (10 runs):

         720495838      cpu-cycles                                                    ( +-  0.04% )
            141769      dTLB-load-misses                                              ( +-  0.02% )
             52722      iTLB-load-misses                                              ( +-  0.01% )
             19611      faults                                                        ( +-  0.02% )

       5.750535176 seconds time elapsed                                          ( +-  0.16% )

With the patch:
 Performance counter stats for './fault 500' (10 runs):

         717669123      cpu-cycles                                                    ( +-  0.02% )
            137344      dTLB-load-misses                                              ( +-  0.03% )
             52731      iTLB-load-misses                                              ( +-  0.01% )
             19614      faults                                                        ( +-  0.03% )

       5.728423115 seconds time elapsed                                          ( +-  0.14% )

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
 v3: Do a first try with pagefault disabled before releasing the semaphore

 v2: Changes 'if (cond1) if (cond2)' by 'if (cond1 && cond2)'

 In case the instruction we read has value 0, store_update_sp() will
 return false, so it will bail out.

 This patch applies after the serie "powerpc/mm: some cleanup of do_page_fault()"

 arch/powerpc/mm/fault.c | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 400f2d0d42f8..96121adeb247 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -280,14 +280,6 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
 
-	/*
-	 * We want to do this outside mmap_sem, because reading code around nip
-	 * can result in fault, which will cause a deadlock when called with
-	 * mmap_sem held
-	 */
-	if (is_write && is_user)
-		__get_user(inst, (unsigned int __user *)regs->nip);
-
 	if (is_user)
 		flags |= FAULT_FLAG_USER;
 
@@ -356,8 +348,29 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 		 * between the last mapped region and the stack will
 		 * expand the stack rather than segfaulting.
 		 */
-		if (address + 2048 < uregs->gpr[1] && !store_updates_sp(inst))
-			goto bad_area;
+		if (address + 2048 < uregs->gpr[1] && !inst) {
+			unsigned int __user *nip =
+					(unsigned int __user *)regs->nip;
+			/*
+			 * We want to do this outside mmap_sem, because reading
+			 * code around nip can result in fault, which will cause
+			 * a deadlock when called with mmap_sem held.
+			 * However, we do a first try with pagefault disabled as
+			 * a fault here is very unlikely.
+			 */
+			pagefault_disable();
+			if (__get_user_inatomic(inst, nip)) {
+				pagefault_enable();
+				up_read(&mm->mmap_sem);
+				__get_user(inst, nip);
+				if (!store_updates_sp(inst))
+					goto bad_area_nosemaphore;
+				goto retry;
+			}
+			pagefault_enable();
+			if (!store_updates_sp(inst))
+				goto bad_area;
+		}
 	}
 	if (expand_stack(vma, address))
 		goto bad_area;
-- 
2.12.0