[PATCH 3/6] alpha: fix the local TLB invalidate in flush_tlb_page()

[Magnus Lindholm <linmag7@gmail.com>]

flush_tlb_page() invalidates the calling CPU itself before asking the
others, and gates that on current->active_mm. For a non-executable vma
that means a targeted tbi(2, addr), which acts on the context currently
loaded, so as in the IPI handler it reaches nothing when only active_mm
names the mm, and nothing forces the old ASN to be retired afterwards.

Test current->mm instead. When no thread of the mm is current, clear
mm->context[cpu] so a fresh ASN is taken at the next switch. That also
covers the case where the mm is not this CPU's active_mm at all: the CPU
may still hold translations for it, and smp_call_function() does not call
back into the caller.

Reached in practice by folio_mkclean() from the writeback flusher
kworker, which has no mm of its own: about half the calls during
writeback of a shared mapping, and none at all on anonymous memory.

flush_tlb_mm() does not need the same active_mm to current->mm change,
because its active_mm path loads a new context rather than issuing a
targeted tbi(). It does have the separate caller-CPU omission when the
target mm is not active_mm; that is fixed in the following patch.

Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/kernel/smp.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index 0dfe29b59039..d167b3ba1303 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -696,7 +696,15 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 
 	preempt_disable();
 
-	if (mm == current->active_mm) {
+	/*
+	 * As in ipi_flush_tlb_page(): the targeted tbi() reaches MM's
+	 * translations only when a thread of MM is current, so test
+	 * current->mm.  Otherwise - lazily borrowing MM, or not running it
+	 * at all - clear mm->context[cpu] so a fresh ASN is taken at the
+	 * next switch.  smp_call_function() below does not call back into
+	 * this CPU, so this is the only chance to retire what it holds.
+	 */
+	if (mm == current->mm) {
 		flush_tlb_current_page(mm, vma, addr);
 		if (atomic_read(&mm->mm_users) <= 1) {
 			int cpu, this_cpu = smp_processor_id();
@@ -709,6 +717,8 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 			preempt_enable();
 			return;
 		}
+	} else {
+		flush_tlb_other(mm);
 	}
 
 	data.vma = vma;
-- 
2.53.0