[RFC PATCH 09/37] mm: add per-mm mmap sequence counter for speculative page fault handling.

[Michel Lespinasse <michel@lespinasse.org>]

The counter's write side is hooked into the existing mmap locking API:
mmap_write_lock() increments the counter to the next (odd) value, and
mmap_write_unlock() increments it again to the next (even) value.

The counter's speculative read side is supposed to be used as follows:

seq = mmap_seq_read_start(mm);
if (seq & 1)
	goto fail;
.... speculative handling here ....
if (!mmap_seq_read_check(mm, seq)
	goto fail;

This API guarantees that, if none of the "fail" tests abort
speculative execution, the speculative code section did not run
concurrently with any mmap writer.

This is very similar to a seqlock, but both the writer and speculative
readers are allowed to block. In the fail case, the speculative reader
does not spin on the sequence counter; instead it should fall back to
a different mechanism such as grabbing the mmap lock read side.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
---
 include/linux/mm_types.h  |  4 +++
 include/linux/mmap_lock.h | 58 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6613b26a8894..70882e628908 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -461,6 +461,10 @@ struct mm_struct {
 					     * counters
 					     */
 		struct rw_semaphore mmap_lock;
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+		unsigned long mmap_seq;
+#endif
+
 
 		struct list_head mmlist; /* List of maybe swapped mm's.	These
 					  * are globally strung together off
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 8ff276a7560e..8f4eca2d0f43 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -8,8 +8,16 @@
 #include <linux/tracepoint-defs.h>
 #include <linux/types.h>
 
-#define MMAP_LOCK_INITIALIZER(name) \
-	.mmap_lock = __RWSEM_INITIALIZER((name).mmap_lock),
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+#define MMAP_LOCK_SEQ_INITIALIZER(name) \
+	.mmap_seq = 0,
+#else
+#define MMAP_LOCK_SEQ_INITIALIZER(name)
+#endif
+
+#define MMAP_LOCK_INITIALIZER(name)				\
+	.mmap_lock = __RWSEM_INITIALIZER((name).mmap_lock),	\
+	MMAP_LOCK_SEQ_INITIALIZER(name)
 
 DECLARE_TRACEPOINT(mmap_lock_start_locking);
 DECLARE_TRACEPOINT(mmap_lock_acquire_returned);
@@ -63,13 +71,52 @@ static inline void __mmap_lock_trace_released(struct mm_struct *mm, bool write)
 static inline void mmap_init_lock(struct mm_struct *mm)
 {
 	init_rwsem(&mm->mmap_lock);
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+	mm->mmap_seq = 0;
+#endif
 }
 
+static inline void __mmap_seq_write_lock(struct mm_struct *mm)
+{
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+	VM_BUG_ON_MM(mm->mmap_seq & 1, mm);
+	mm->mmap_seq++;
+	smp_wmb();
+#endif
+}
+
+static inline void __mmap_seq_write_unlock(struct mm_struct *mm)
+{
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+	smp_wmb();
+	mm->mmap_seq++;
+	VM_BUG_ON_MM(mm->mmap_seq & 1, mm);
+#endif
+}
+
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+static inline unsigned long mmap_seq_read_start(struct mm_struct *mm)
+{
+	unsigned long seq;
+
+	seq = READ_ONCE(mm->mmap_seq);
+	smp_rmb();
+	return seq;
+}
+
+static inline bool mmap_seq_read_check(struct mm_struct *mm, unsigned long seq)
+{
+	smp_rmb();
+	return seq == READ_ONCE(mm->mmap_seq);
+}
+#endif
+
 static inline void mmap_write_lock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write(&mm->mmap_lock);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
+	__mmap_seq_write_lock(mm);
 }
 
 static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass)
@@ -77,6 +124,7 @@ static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass)
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write_nested(&mm->mmap_lock, subclass);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
+	__mmap_seq_write_lock(mm);
 }
 
 static inline int mmap_write_lock_killable(struct mm_struct *mm)
@@ -86,6 +134,8 @@ static inline int mmap_write_lock_killable(struct mm_struct *mm)
 	__mmap_lock_trace_start_locking(mm, true);
 	error = down_write_killable(&mm->mmap_lock);
 	__mmap_lock_trace_acquire_returned(mm, true, !error);
+	if (likely(!error))
+		__mmap_seq_write_lock(mm);
 	return error;
 }
 
@@ -96,17 +146,21 @@ static inline bool mmap_write_trylock(struct mm_struct *mm)
 	__mmap_lock_trace_start_locking(mm, true);
 	ok = down_write_trylock(&mm->mmap_lock) != 0;
 	__mmap_lock_trace_acquire_returned(mm, true, ok);
+	if (likely(ok))
+		__mmap_seq_write_lock(mm);
 	return ok;
 }
 
 static inline void mmap_write_unlock(struct mm_struct *mm)
 {
+	__mmap_seq_write_unlock(mm);
 	up_write(&mm->mmap_lock);
 	__mmap_lock_trace_released(mm, true);
 }
 
 static inline void mmap_write_downgrade(struct mm_struct *mm)
 {
+	__mmap_seq_write_unlock(mm);
 	downgrade_write(&mm->mmap_lock);
 	__mmap_lock_trace_acquire_returned(mm, false, true);
 }
-- 
2.20.1