[RFC 08/20] mm: store completed TLB generation

[Nadav Amit <nadav.amit@xxxxxxxxx>]

From: Nadav Amit <namit@xxxxxxxxxx>

To detect deferred TLB flushes in fine granularity, we need to keep
track on the completed TLB flush generation for each mm.

Add logic to track for each mm the tlb_gen_completed, which tracks the
completed TLB generation. It is the arch responsibility to call
mark_mm_tlb_gen_done() whenever a TLB flush is completed.

Start the generation numbers from 1 instead of 0. This would allow later
to detect whether flushes of a certain generation were completed.

Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
---
 arch/x86/mm/tlb.c         | 10 ++++++++++
 include/asm-generic/tlb.h | 33 +++++++++++++++++++++++++++++++++
 include/linux/mm_types.h  | 15 ++++++++++++++-
 3 files changed, 57 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 7ab21430be41..d17b5575531e 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -14,6 +14,7 @@
 #include <asm/nospec-branch.h>
 #include <asm/cache.h>
 #include <asm/apic.h>
+#include <asm/tlb.h>
 
 #include "mm_internal.h"
 
@@ -915,6 +916,9 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 	if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids)
 		flush_tlb_others(mm_cpumask(mm), info);
 
+	/* Update the completed generation */
+	mark_mm_tlb_gen_done(mm, new_tlb_gen);
+
 	put_flush_tlb_info();
 	put_cpu();
 }
@@ -1147,6 +1151,12 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
 
 	cpumask_clear(&batch->cpumask);
 
+	/*
+	 * We cannot call mark_mm_tlb_gen_done() since we do not know which
+	 * mm's should be flushed. This may lead to some unwarranted TLB
+	 * flushes, but not to correction problems.
+	 */
+
 	put_cpu();
 }
 
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index 517c89398c83..427bfcc6cdec 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -513,6 +513,39 @@ static inline void tlb_end_vma(struct mmu_gather *tlb, struct vm_area_struct *vm
 }
 #endif
 
+#ifdef CONFIG_ARCH_HAS_TLB_GENERATIONS
+
+/*
+ * Helper function to update a generation to have a new value, as long as new
+ * value is greater or equal to gen.
+ */
+static inline void tlb_update_generation(atomic64_t *gen, u64 new_gen)
+{
+	u64 cur_gen = atomic64_read(gen);
+
+	while (cur_gen < new_gen) {
+		u64 old_gen = atomic64_cmpxchg(gen, cur_gen, new_gen);
+
+		/* Check if we succeeded in the cmpxchg */
+		if (likely(cur_gen == old_gen))
+			break;
+
+		cur_gen = old_gen;
+	};
+}
+
+
+static inline void mark_mm_tlb_gen_done(struct mm_struct *mm, u64 gen)
+{
+	/*
+	 * Update the completed generation to the new generation if the new
+	 * generation is greater than the previous one.
+	 */
+	tlb_update_generation(&mm->tlb_gen_completed, gen);
+}
+
+#endif /* CONFIG_ARCH_HAS_TLB_GENERATIONS */
+
 /*
  * tlb_flush_{pte|pmd|pud|p4d}_range() adjust the tlb->start and tlb->end,
  * and set corresponding cleared_*.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 2035ac319c2b..8a5eb4bfac59 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -571,6 +571,13 @@ struct mm_struct {
 		 * This is not used on Xen PV.
 		 */
 		atomic64_t tlb_gen;
+
+		/*
+		 * TLB generation which is guarnateed to be flushed, including
+		 * all the PTE changes that were performed before tlb_gen was
+		 * incremented.
+		 */
+		atomic64_t tlb_gen_completed;
 #endif
 	} __randomize_layout;
 
@@ -690,7 +697,13 @@ static inline bool mm_tlb_flush_nested(struct mm_struct *mm)
 #ifdef CONFIG_ARCH_HAS_TLB_GENERATIONS
 static inline void init_mm_tlb_gen(struct mm_struct *mm)
 {
-	atomic64_set(&mm->tlb_gen, 0);
+	/*
+	 * Start from generation of 1, so default generation 0 will be
+	 * considered as flushed and would not be regarded as an outstanding
+	 * deferred invalidation.
+	 */
+	atomic64_set(&mm->tlb_gen, 1);
+	atomic64_set(&mm->tlb_gen_completed, 1);
 }
 
 static inline u64 inc_mm_tlb_gen(struct mm_struct *mm)
-- 
2.25.1