[RFC 1/1] mm: introduce mmap_lock_speculation_{start|end}

[Suren Baghdasaryan <surenb@google.com>]

Add helper functions to speculatively perform operations without
read-locking mmap_lock, expecting that mmap_lock will not be
write-locked and mm is not modified from under us.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
---
Discussion [1] follow-up. If proves to be useful can be included in that
patchset. Based on mm-unstable.

[1] https://lore.kernel.org/all/20240730134605.GO33588@noisy.programming.kicks-ass.net/

 include/linux/mm_types.h  |  3 +++
 include/linux/mmap_lock.h | 53 +++++++++++++++++++++++++++++++--------
 kernel/fork.c             |  3 ---
 3 files changed, 46 insertions(+), 13 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 003619fab20e..a426e6ced604 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -887,6 +887,9 @@ struct mm_struct {
 		 * Roughly speaking, incrementing the sequence number is
 		 * equivalent to releasing locks on VMAs; reading the sequence
 		 * number can be part of taking a read lock on a VMA.
+		 * Incremented every time mmap_lock is write-locked/unlocked.
+		 * Initialized to 0, therefore odd values indicate mmap_lock
+		 * is write-locked and even values that it's released.
 		 *
 		 * Can be modified under write mmap_lock using RELEASE
 		 * semantics.
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index de9dc20b01ba..5410ce741d75 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -71,15 +71,12 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
-/*
- * Drop all currently-held per-VMA locks.
- * This is called from the mmap_lock implementation directly before releasing
- * a write-locked mmap_lock (or downgrading it to read-locked).
- * This should normally NOT be called manually from other places.
- * If you want to call this manually anyway, keep in mind that this will release
- * *all* VMA write locks, including ones from further up the stack.
- */
-static inline void vma_end_write_all(struct mm_struct *mm)
+static inline void init_mm_lock_seq(struct mm_struct *mm)
+{
+	mm->mm_lock_seq = 0;
+}
+
+static inline void inc_mm_lock_seq(struct mm_struct *mm)
 {
 	mmap_assert_write_locked(mm);
 	/*
@@ -91,19 +88,52 @@ static inline void vma_end_write_all(struct mm_struct *mm)
 	 */
 	smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1);
 }
+
+static inline bool mmap_lock_speculation_start(struct mm_struct *mm, int *seq)
+{
+	/* Pairs with RELEASE semantics in inc_mm_lock_seq(). */
+	*seq = smp_load_acquire(&mm->mm_lock_seq);
+	/* Allow speculation if mmap_lock is not write-locked */
+	return (*seq & 1) == 0;
+}
+
+static inline bool mmap_lock_speculation_end(struct mm_struct *mm, int seq)
+{
+	/* Pairs with RELEASE semantics in inc_mm_lock_seq(). */
+	return seq == smp_load_acquire(&mm->mm_lock_seq);
+}
+
 #else
-static inline void vma_end_write_all(struct mm_struct *mm) {}
+static inline void init_mm_lock_seq(struct mm_struct *mm) {}
+static inline void inc_mm_lock_seq(struct mm_struct *mm) {}
+static inline bool mmap_lock_speculation_start(struct mm_struct *mm, int *seq) { return false; }
+static inline bool mmap_lock_speculation_end(struct mm_struct *mm, int seq) { return false; }
 #endif
 
+/*
+ * Drop all currently-held per-VMA locks.
+ * This is called from the mmap_lock implementation directly before releasing
+ * a write-locked mmap_lock (or downgrading it to read-locked).
+ * This should normally NOT be called manually from other places.
+ * If you want to call this manually anyway, keep in mind that this will release
+ * *all* VMA write locks, including ones from further up the stack.
+ */
+static inline void vma_end_write_all(struct mm_struct *mm)
+{
+	inc_mm_lock_seq(mm);
+}
+
 static inline void mmap_init_lock(struct mm_struct *mm)
 {
 	init_rwsem(&mm->mmap_lock);
+	init_mm_lock_seq(mm);
 }
 
 static inline void mmap_write_lock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write(&mm->mmap_lock);
+	inc_mm_lock_seq(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
@@ -111,6 +141,7 @@ 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);
+	inc_mm_lock_seq(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
@@ -120,6 +151,8 @@ static inline int mmap_write_lock_killable(struct mm_struct *mm)
 
 	__mmap_lock_trace_start_locking(mm, true);
 	ret = down_write_killable(&mm->mmap_lock);
+	if (!ret)
+		inc_mm_lock_seq(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, ret == 0);
 	return ret;
 }
diff --git a/kernel/fork.c b/kernel/fork.c
index 3d590e51ce84..73e37af8a24d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1259,9 +1259,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	seqcount_init(&mm->write_protect_seq);
 	mmap_init_lock(mm);
 	INIT_LIST_HEAD(&mm->mmlist);
-#ifdef CONFIG_PER_VMA_LOCK
-	mm->mm_lock_seq = 0;
-#endif
 	mm_pgtables_bytes_init(mm);
 	mm->map_count = 0;
 	mm->locked_vm = 0;

base-commit: 98808d08fc0f78ee638e0c0a88020fbbaf581ec6
-- 
2.46.0.rc2.264.g509ed76dc8-goog