[PATCH v3 3/7] 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 and is only guaranteed to invalidate the intended translations
when the target mm's context is the loaded one. A lazy active_mm does
not establish that, so as in ipi_flush_tlb_page() the target mm's stale
translations can survive, and nothing forces the old ASN to be retired
afterwards.

Test current->mm instead. The calling CPU then has no local invalidate
at all when the mm is only lazily borrowed; the next patch adds it.

Which callers reach here with a foreign mm depends on the configuration.
folio_mkclean(), from the writeback flusher kworker that has no mm of its
own, accounted for about half the calls during writeback of a shared
mapping and none at all on anonymous memory. Those counts were measured
with CONFIG_COMPACTION=n: with COMPACTION=y, asm/pgtable.h overrides
ptep_clear_flush() to call migrate_flush_tlb_page(), which rendezvouses
with every CPU and handles the context itself, so folio_mkclean() does
not reach this function.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/kernel/smp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index 1ad448105201..7856d23b3384 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -684,7 +684,8 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 
 	preempt_disable();
 
-	if (mm == current->active_mm) {
+	/* As in ipi_flush_tlb_page(): a targeted tbi() needs MM current.  */
+	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();
-- 
2.43.0