[PATCH 5/6] alpha: invalidate the local context in flush_tlb_mm()

[Magnus Lindholm <linmag7@gmail.com>]

The SMP flush_tlb_mm() only touches the calling CPU when the mm is its
active_mm:

	if (mm == current->active_mm) {
		flush_tlb_current(mm);
		...
	}

	smp_call_function(ipi_flush_tlb_mm, mm, 1);

If it is not, nothing happens locally at all: smp_call_function() does not
call back into the caller. mm->context[cpu] is left valid, so this CPU
may later reuse the old ASN, and any translations it still holds for MM
stay usable.

Callers reach this regularly - counting the branch gave 2934 such calls
over a fork-heavy workload and 632 while otherwise idle.

Add the missing else. The UP implementation in asm/tlbflush.h already has
exactly this shape:

	if (mm == current->active_mm)
		flush_tlb_current(mm);
	else
		flush_tlb_other(mm);

Unlike flush_tlb_page(), the active_mm test itself is valid here:
flush_tlb_current() calls __load_new_mm_context(), which allocates and
loads a fresh context rather than relying on a targeted tbi() to operate
on whatever ASN happened to be loaded beforehand.

Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/kernel/smp.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index d167b3ba1303..f501a91001cc 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -649,6 +649,13 @@ flush_tlb_mm(struct mm_struct *mm)
 			preempt_enable();
 			return;
 		}
+	} else {
+		/*
+		 * smp_call_function() below does not call back into this
+		 * CPU, so this is the only chance to retire what it holds
+		 * for MM.  The UP implementation already does this.
+		 */
+		flush_tlb_other(mm);
 	}
 
 	smp_call_function(ipi_flush_tlb_mm, mm, 1);
-- 
2.53.0