mm: introduce mmap_lock_speculate_{try_begin|retry}

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.

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/20241122174416.1367052-3-surenb@google.com

include/linux/mmap_lock.h

@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
+
 static inline void mm_lock_seqcount_init(struct mm_struct *mm)
 {
 	seqcount_init(&mm->mm_lock_seq);
@@ -87,11 +88,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
 	do_raw_write_seqcount_end(&mm->mm_lock_seq);
 }
 
-#else
+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	/*
+	 * Since mmap_lock is a sleeping lock, and waiting for it to become
+	 * unlocked is more or less equivalent with taking it ourselves, don't
+	 * bother with the speculative path if mmap_lock is already write-locked
+	 * and take the slow path, which takes the lock.
+	 */
+	return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+	return read_seqcount_retry(&mm->mm_lock_seq, seq);
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
 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 bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	return false;
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+	return true;
+}
+
+#endif /* CONFIG_PER_VMA_LOCK */
 
 static inline void mmap_init_lock(struct mm_struct *mm)
 {