[PATCH RFC v3 11/13] mm: introduce mmap_lock_speculation_{start|end}

Andrii Nakryiko
2024-08-13T04:30:16Z

From: 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>
---
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 485424979254..d5e3f907eea4 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -876,6 +876,9 @@ 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 cc760491f201..76ebafb956a6 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;
--
2.43.5