[PATCH] mm: Delete non-atomic mm counter implementation

[Matt Fleming]

From: Matt Fleming <matt.fleming@xxxxxxxxxxxxxxx>

The problem with having two different types of counters is that
developers adding new code need to keep in mind whether it's safe to
use both the atomic and non-atomic implementations. For example, when
adding new callers of the *_mm_counter() functions a developer needs
to ensure that those paths are always executed with page_table_lock
held, in case we're using the non-atomic implementation of mm
counters.

Hugh Dickins introduced the atomic mm counters in commit f412ac08c986
("[PATCH] mm: fix rss and mmlist locking"). When asked why he left the
non-atomic counters around he said,

  | The only reason was to avoid adding costly atomic operations into a
  | configuration that had no need for them there: the page_table_lock
  | sufficed.
  |
  | Certainly it would be simpler just to delete the non-atomic variant.
  |
  | And I think it's fair to say that any configuration on which we're
  | measuring performance to that degree (rather than "does it boot fast?"
  | type measurements), would already be going the split ptlocks route.

Removing the non-atomic counters eases the maintenance burden because
developers no longer have to mindful of the two implementations when
using *_mm_counter().

Note that all architectures provide a means of atomically updating
atomic_long_t variables, even if they have to revert to the generic
spinlock implementation because they don't support 64-bit atomic
instructions (see lib/atomic64.c).

Signed-off-by: Matt Fleming <matt.fleming@xxxxxxxxxxxxxxx>
---

Dave, you might want to take this into your pagetable counters series
so that you only need to worry about atomic mm counters.

 include/linux/mm.h       |   44 +++++++-------------------------------------
 include/linux/mm_types.h |    9 +++------
 2 files changed, 10 insertions(+), 43 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd87a78..ee64af2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1034,65 +1034,35 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 /*
  * per-process(per-mm_struct) statistics.
  */
-#if defined(SPLIT_RSS_COUNTING)
-/*
- * The mm counters are not protected by its page_table_lock,
- * so must be incremented atomically.
- */
 static inline void set_mm_counter(struct mm_struct *mm, int member, long value)
 {
 	atomic_long_set(&mm->rss_stat.count[member], value);
 }
 
+#if defined(SPLIT_RSS_COUNTING)
 unsigned long get_mm_counter(struct mm_struct *mm, int member);
-
-static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
-{
-	atomic_long_add(value, &mm->rss_stat.count[member]);
-}
-
-static inline void inc_mm_counter(struct mm_struct *mm, int member)
-{
-	atomic_long_inc(&mm->rss_stat.count[member]);
-}
-
-static inline void dec_mm_counter(struct mm_struct *mm, int member)
-{
-	atomic_long_dec(&mm->rss_stat.count[member]);
-}
-
-#else  /* !USE_SPLIT_PTLOCKS */
-/*
- * The mm counters are protected by its page_table_lock,
- * so can be incremented directly.
- */
-static inline void set_mm_counter(struct mm_struct *mm, int member, long value)
-{
-	mm->rss_stat.count[member] = value;
-}
-
+#else
 static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
 {
-	return mm->rss_stat.count[member];
+	return atomic_long_read(&mm->rss_stat.count[member]);
 }
+#endif
 
 static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
 {
-	mm->rss_stat.count[member] += value;
+	atomic_long_add(value, &mm->rss_stat.count[member]);
 }
 
 static inline void inc_mm_counter(struct mm_struct *mm, int member)
 {
-	mm->rss_stat.count[member]++;
+	atomic_long_inc(&mm->rss_stat.count[member]);
 }
 
 static inline void dec_mm_counter(struct mm_struct *mm, int member)
 {
-	mm->rss_stat.count[member]--;
+	atomic_long_dec(&mm->rss_stat.count[member]);
 }
 
-#endif /* !USE_SPLIT_PTLOCKS */
-
 static inline unsigned long get_mm_rss(struct mm_struct *mm)
 {
 	return get_mm_counter(mm, MM_FILEPAGES) +
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index ca01ab2..b8ca318 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -205,19 +205,16 @@ enum {
 
 #if USE_SPLIT_PTLOCKS && defined(CONFIG_MMU)
 #define SPLIT_RSS_COUNTING
-struct mm_rss_stat {
-	atomic_long_t count[NR_MM_COUNTERS];
-};
 /* per-thread cached information, */
 struct task_rss_stat {
 	int events;	/* for synchronization threshold */
 	int count[NR_MM_COUNTERS];
 };
-#else  /* !USE_SPLIT_PTLOCKS */
+#endif /* USE_SPLIT_PTLOCKS */
+
 struct mm_rss_stat {
-	unsigned long count[NR_MM_COUNTERS];
+	atomic_long_t count[NR_MM_COUNTERS];
 };
-#endif /* !USE_SPLIT_PTLOCKS */
 
 struct mm_struct {
 	struct vm_area_struct * mmap;		/* list of VMAs */
-- 
1.7.4.4

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