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