[PATCH 2/3] slab_arena: Provide slab_arena_madvise_create to madvice slabs

[Cyrill Gorcunov]

The purpose of this helper is to be able to eliminate
some of slab arenas from dumping, especially when huge
amount of memory involves.

For this sake one should call slab_arena_madvise_create
right after area's creation. Strictly speaking this helper
may be called in any time but then some of already created
slabs which are not inside preallocated area won't be
madvised.

Part of #3509
---
https://github.com/tarantool/tarantool/issues/3509

 small/slab_arena.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++
 small/slab_arena.h | 27 +++++++++++++++++++++++++
 2 files changed, 76 insertions(+)

diff --git a/small/slab_arena.c b/small/slab_arena.c
index 26e2620..12f6198 100644
--- a/small/slab_arena.c
+++ b/small/slab_arena.c
@@ -149,6 +149,9 @@ slab_arena_create(struct slab_arena *arena, struct quota *quota,
 	arena->used = 0;
 
 	arena->flags = flags;
+#ifdef TARANTOOL_SMALL_HAS_MADVISE
+	pm_atomic_store(&arena->advice, MADV_NORMAL);
+#endif
 
 	if (arena->prealloc) {
 		arena->arena = mmap_checked(arena->prealloc,
@@ -205,6 +208,12 @@ slab_map(struct slab_arena *arena)
 		__sync_sub_and_fetch(&arena->used, arena->slab_size);
 		quota_release(arena->quota, arena->slab_size);
 	}
+
+#ifdef TARANTOOL_SMALL_HAS_MADVISE
+	if (ptr && pm_atomic_load(&arena->advice) != MADV_NORMAL)
+		madvise(ptr, arena->slab_size, arena->advice);
+#endif
+
 	VALGRIND_MAKE_MEM_UNDEFINED(ptr, arena->slab_size);
 	return ptr;
 }
@@ -226,3 +235,43 @@ slab_arena_mprotect(struct slab_arena *arena)
 	if (arena->arena)
 		mprotect(arena->arena, arena->prealloc, PROT_READ);
 }
+
+#ifdef TARANTOOL_SMALL_HAS_MADVISE
+/**
+ * slab_arena_madvise_create - Initialize madvise permanent strategy
+ * @arena:	slab arena to madvise
+ * @advice:	strategy (MADV_DONTDUMP, ...)
+ *
+ * This function setups @slab_arena strategy so that new memory
+ * allocation with mmap() call gets madvise() call with @advice.
+ *
+ * Note that we don't allow to change strategy once choosen because
+ * otherwise we need to walk over all possible slab cache entry.
+ *
+ * This function should be called right after the slab arena creation,
+ * otherwise if called later some of the non-preallocated slabs won't be
+ * madvise'd.
+ */
+int
+slab_arena_madvise_create(struct slab_arena *arena, int advice)
+{
+	int expected = MADV_NORMAL;
+
+	if (!pm_atomic_compare_exchange_strong(&arena->advice,
+					       &expected, advice)) {
+		return -EBUSY;
+	}
+
+	if (arena->arena)
+		return madvise(arena->arena, arena->prealloc, advice);
+	return 0;
+}
+#else
+int
+slab_arena_madvise_create(struct slab_arena *arena, int advice)
+{
+	(void)arena;
+	(void)advice;
+	return -EINVAL;
+}
+#endif /* TARANTOOL_SMALL_HAS_MADVISE */
diff --git a/small/slab_arena.h b/small/slab_arena.h
index 364252f..4d86545 100644
--- a/small/slab_arena.h
+++ b/small/slab_arena.h
@@ -31,8 +31,10 @@
  * SUCH DAMAGE.
  */
 #include "lf_lifo.h"
+#include "config.h"
 #include <sys/mman.h>
 #include <limits.h>
+#include <errno.h>
 
 #if defined(__cplusplus)
 extern "C" {
@@ -97,6 +99,12 @@ struct slab_arena {
 	 * mmap() flags: MAP_SHARED or MAP_PRIVATE
 	 */
 	int flags;
+#ifdef TARANTOOL_SMALL_HAS_MADVISE
+	/**
+	 * madvise() flag
+	 */
+	int advice;
+#endif
 };
 
 /** Initialize an arena.  */
@@ -120,6 +128,25 @@ slab_unmap(struct slab_arena *arena, void *ptr);
 void
 slab_arena_mprotect(struct slab_arena *arena);
 
+/** madvise() all slabs. */
+int
+slab_arena_madvise_create(struct slab_arena *arena, int advice);
+
+/** handy helpers for slab_arena_madvise_create. */
+#ifdef TARANTOOL_SMALL_HAS_MADVISE
+static inline int
+slab_arena_madvise_dontdump(struct slab_arena *arena)
+{
+	return slab_arena_madvise_create(arena, MADV_DONTDUMP);
+}
+#else
+static inline int
+slab_arena_madvise_dontdump(struct slab_arena *arena)
+{
+	return -EINVAL;
+}
+#endif
+
 /**
  * Align a size - round up to nearest divisible by the given alignment.
  * Alignment must be a power of 2
-- 
2.20.1