x86/mm: Rework lazy TLB to track the actual loaded mm

Lazy TLB state is currently managed in a rather baroque manner.
AFAICT, there are three possible states:

 - Non-lazy.  This means that we're running a user thread or a
   kernel thread that has called use_mm().  current->mm ==
   current->active_mm == cpu_tlbstate.active_mm and
   cpu_tlbstate.state == TLBSTATE_OK.

 - Lazy with user mm.  We're running a kernel thread without an mm
   and we're borrowing an mm_struct.  We have current->mm == NULL,
   current->active_mm == cpu_tlbstate.active_mm, cpu_tlbstate.state
   != TLBSTATE_OK (i.e. TLBSTATE_LAZY or 0).  The current cpu is set
   in mm_cpumask(current->active_mm).  CR3 points to
   current->active_mm->pgd.  The TLB is up to date.

 - Lazy with init_mm.  This happens when we call leave_mm().  We
   have current->mm == NULL, current->active_mm ==
   cpu_tlbstate.active_mm, but that mm is only relelvant insofar as
   the scheduler is tracking it for refcounting.  cpu_tlbstate.state
   != TLBSTATE_OK.  The current cpu is clear in
   mm_cpumask(current->active_mm).  CR3 points to swapper_pg_dir,
   i.e. init_mm->pgd.

This patch simplifies the situation.  Other than perf, x86 stops
caring about current->active_mm at all.  We have
cpu_tlbstate.loaded_mm pointing to the mm that CR3 references.  The
TLB is always up to date for that mm.  leave_mm() just switches us
to init_mm.  There are no longer any special cases for mm_cpumask,
and switch_mm() switches mms without worrying about laziness.

After this patch, cpu_tlbstate.state serves only to tell the TLB
flush code whether it may switch to init_mm instead of doing a
normal flush.

This makes fairly extensive changes to xen_exit_mmap(), which used
to look a bit like black magic.

Perf is unchanged.  With or without this change, perf may behave a bit
erratically if it tries to read user memory in kernel thread context.
We should build on this patch to teach perf to never look at user
memory when cpu_tlbstate.loaded_mm != current->mm.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index dbb5a9f0fed8..388c2463fde6 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -66,7 +66,13 @@ static inline void invpcid_flush_all_nonglobals(void)
 #endif
 
 struct tlb_state {
-	struct mm_struct *active_mm;
+	/*
+	 * cpu_tlbstate.loaded_mm should match CR3 whenever interrupts
+	 * are on.  This means that it may not match current->active_mm,
+	 * which will contain the previous user mm when we're in lazy TLB
+	 * mode even if we've already switched back to swapper_pg_dir.
+	 */
+	struct mm_struct *loaded_mm;
 	int state;
 
 	/*
@@ -256,7 +262,9 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
 static inline void reset_lazy_tlbstate(void)
 {
 	this_cpu_write(cpu_tlbstate.state, 0);
-	this_cpu_write(cpu_tlbstate.active_mm, &init_mm);
+	this_cpu_write(cpu_tlbstate.loaded_mm, &init_mm);
+
+	WARN_ON(read_cr3() != __pa_symbol(swapper_pg_dir));
 }
 
 static inline void arch_tlbbatch_add_mm(struct arch_tlbflush_unmap_batch *batch,