mm: convert mm_lock_seq to a proper seqcount

Convert mm_lock_seq to be seqcount_t and change all mmap_write_lock
variants to increment it, in-line with the usual seqcount usage pattern.
This lets us check whether the mmap_lock is write-locked by checking
mm_lock_seq.sequence counter (odd=locked, even=unlocked). This will be
used when implementing mmap_lock speculation functions.
As a result vm_lock_seq is also change to be unsigned to match the type
of mm_lock_seq.sequence.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Link: https://lkml.kernel.org/r/20241122174416.1367052-2-surenb@google.com

include/linux/mm.h

@@ -710,7 +710,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	 * we don't rely on for anything - the mm_lock_seq read against which we
 	 * need ordering is below.
 	 */
-	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq))
+	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
 		return false;
 
 	if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0))
@@ -727,7 +727,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	 * after it has been unlocked.
 	 * This pairs with RELEASE semantics in vma_end_write_all().
 	 */
-	if (unlikely(vma->vm_lock_seq == smp_load_acquire(&vma->vm_mm->mm_lock_seq))) {
+	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
 		up_read(&vma->vm_lock->lock);
 		return false;
 	}
@@ -742,15 +742,15 @@ static inline void vma_end_read(struct vm_area_struct *vma)
 }
 
 /* WARNING! Can only be used if mmap_lock is expected to be write-locked */
-static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
+static bool __is_vma_write_locked(struct vm_area_struct *vma, unsigned int *mm_lock_seq)
 {
 	mmap_assert_write_locked(vma->vm_mm);
 
 	/*
 	 * current task is holding mmap_write_lock, both vma->vm_lock_seq and
 	 * mm->mm_lock_seq can't be concurrently modified.
 	 */
-	*mm_lock_seq = vma->vm_mm->mm_lock_seq;
+	*mm_lock_seq = vma->vm_mm->mm_lock_seq.sequence;
 	return (vma->vm_lock_seq == *mm_lock_seq);
 }
 
@@ -761,7 +761,7 @@ static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
  */
 static inline void vma_start_write(struct vm_area_struct *vma)
 {
-	int mm_lock_seq;
+	unsigned int mm_lock_seq;
 
 	if (__is_vma_write_locked(vma, &mm_lock_seq))
 		return;
@@ -779,7 +779,7 @@ static inline void vma_start_write(struct vm_area_struct *vma)
 
 static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 {
-	int mm_lock_seq;
+	unsigned int mm_lock_seq;
 
 	VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
 }

include/linux/mm_types.h

@@ -697,7 +697,7 @@ struct vm_area_struct {
 	 * counter reuse can only lead to occasional unnecessary use of the
 	 * slowpath.
 	 */
-	int vm_lock_seq;
+	unsigned int vm_lock_seq;
 	/* Unstable RCU readers are allowed to read this. */
 	struct vma_lock *vm_lock;
 #endif
@@ -891,6 +891,9 @@ 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.
@@ -899,7 +902,7 @@ struct mm_struct {
 		 * Can be read with ACQUIRE semantics if not holding write
 		 * mmap_lock.
 		 */
-		int mm_lock_seq;
+		seqcount_t mm_lock_seq;
 #endif
 
 

include/linux/mmap_lock.h

@@ -71,46 +71,47 @@ 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 mm_lock_seqcount_init(struct mm_struct *mm)
 {
-	mmap_assert_write_locked(mm);
-	/*
-	 * Nobody can concurrently modify mm->mm_lock_seq due to exclusive
-	 * mmap_lock being held.
-	 * We need RELEASE semantics here to ensure that preceding stores into
-	 * the VMA take effect before we unlock it with this store.
-	 * Pairs with ACQUIRE semantics in vma_start_read().
-	 */
-	smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1);
+	seqcount_init(&mm->mm_lock_seq);
+}
+
+static inline void mm_lock_seqcount_begin(struct mm_struct *mm)
+{
+	do_raw_write_seqcount_begin(&mm->mm_lock_seq);
+}
+
+static inline void mm_lock_seqcount_end(struct mm_struct *mm)
+{
+	ASSERT_EXCLUSIVE_WRITER(mm->mm_lock_seq);
+	do_raw_write_seqcount_end(&mm->mm_lock_seq);
 }
+
 #else
-static inline void vma_end_write_all(struct mm_struct *mm) {}
+static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
+static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
+static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
 #endif
 
 static inline void mmap_init_lock(struct mm_struct *mm)
 {
 	init_rwsem(&mm->mmap_lock);
+	mm_lock_seqcount_init(mm);
 }
 
 static inline void mmap_write_lock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write(&mm->mmap_lock);
+	mm_lock_seqcount_begin(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
 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);
+	mm_lock_seqcount_begin(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
@@ -120,10 +121,26 @@ 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)
+		mm_lock_seqcount_begin(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, ret == 0);
 	return ret;
 }
 
+/*
+ * 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)
+{
+	mmap_assert_write_locked(mm);
+	mm_lock_seqcount_end(mm);
+}
+
 static inline void mmap_write_unlock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_released(mm, true);

kernel/fork.c

@@ -448,7 +448,7 @@ static bool vma_lock_alloc(struct vm_area_struct *vma)
 		return false;
 
 	init_rwsem(&vma->vm_lock->lock);
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return true;
 }
@@ -1267,9 +1267,6 @@ 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;

mm/init-mm.c

@@ -40,7 +40,7 @@ struct mm_struct init_mm = {
 	.arg_lock	=  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
 	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
 #ifdef CONFIG_PER_VMA_LOCK
-	.mm_lock_seq	= 0,
+	.mm_lock_seq	= SEQCNT_ZERO(init_mm.mm_lock_seq),
 #endif
 	.user_ns	= &init_user_ns,
 	.cpu_bitmap	= CPU_BITS_NONE,

tools/testing/vma/vma.c

@@ -89,7 +89,7 @@ static struct vm_area_struct *alloc_and_link_vma(struct mm_struct *mm,
 	 * begun. Linking to the tree will have caused this to be incremented,
 	 * which means we will get a false positive otherwise.
 	 */
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return vma;
 }
@@ -214,7 +214,7 @@ static bool vma_write_started(struct vm_area_struct *vma)
 	int seq = vma->vm_lock_seq;
 
 	/* We reset after each check. */
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	/* The vma_start_write() stub simply increments this value. */
 	return seq > -1;

tools/testing/vma/vma_internal.h

@@ -241,7 +241,7 @@ struct vm_area_struct {
 	 * counter reuse can only lead to occasional unnecessary use of the
 	 * slowpath.
 	 */
-	int vm_lock_seq;
+	unsigned int vm_lock_seq;
 	struct vma_lock *vm_lock;
 #endif
 
@@ -416,7 +416,7 @@ static inline bool vma_lock_alloc(struct vm_area_struct *vma)
 		return false;
 
 	init_rwsem(&vma->vm_lock->lock);
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return true;
 }