[PATCH v3 3/5] mm: Add RCU-based VMA lookup helper that waits for writers

[Suren Baghdasaryan <surenb@google.com>]

From: Dave Hansen <dave.hansen@linux.intel.com>

== Background ==

There are basically two parallel ways to look up a VMA: the
traditional way, which is protected by mmap_read_lock, and the RCU-based
per-VMA lock way which is based on RCU and refcounts.

== Problem ==

The mmap_lock one is more straightforward to use but it has a big
disadvantage in that it can not be mixed with page faults since those
can take mmap_lock for read, which can deadlock when mixed with nested
page faults and parallel writers.
For example:

	mmap_read_lock(mm);
	// Another thread does mmap_write_lock().
	// New mmap_lock readers are blocked.
	vma = vma_lookup(mm, address);
	// This deadlocks on mmap_read_lock() if it faults:
	copy_from_user(address);
	mmap_read_unlock(mm);

The per-VMA lock can be mixed with faults, but they can fail and need to
be able to fall back to the traditional way.

== Solution ==

Add a variant of the RCU-based lookup that waits for writers. This is
basically the same as the existing RCU-based lookup, but on a failure to
lock it temporarily takes mmap_lock for read and waits for writers
to finish before locking the VMA, dropping the mmap_lock and returning
the locked VMA. This has some advantages:

 1. Callers do not need to have a fallback path for when they
    collide with writers.
 2. It can be used in contexts where page faults can happen because
    it can take the mmap_lock for read but never *holds* it.
 3. Its fast path does not require taking mmap_lock for read.

Basically, when applied correctly, this approach results in faster
*and* simpler code.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: linux-mm@kvack.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Todd Kjos <tkjos@android.com>
Cc: Christian Brauner <christian@brauner.io>
Cc: Carlos Llamas <cmllamas@google.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: David Ahern <dsahern@kernel.org>
Cc: netdev@vger.kernel.org
---
 include/linux/mmap_lock.h | 15 +++++++++++----
 mm/mmap_lock.c            | 29 +++++++++++++++++++++++++++++
 mm/userfaultfd.c          |  6 ++++--
 3 files changed, 44 insertions(+), 6 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index eb32b482434e..fdd8f5cf5722 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -228,10 +228,12 @@ static inline void vma_refcount_put(struct vm_area_struct *vma)
 }
 
 /*
- * Use only while holding mmap read lock which guarantees that locking will not
- * fail (nobody can concurrently write-lock the vma). vma_start_read() should
+ * Use only while holding mmap read lock which guarantees that vma lock is not
+ * contended (nobody can concurrently write-lock the vma). vma_start_read() should
  * not be used in such cases because it might fail due to mm_lock_seq overflow.
  * This functionality is used to obtain vma read lock and drop the mmap read lock.
+ * VMA can't be detached while we are holding mmap lock, therefore in practice this
+ * function can fail only when there are so many readers that vm_refcnt overflows.
  */
 static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
 {
@@ -247,16 +249,21 @@ static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int
 }
 
 /*
- * Use only while holding mmap read lock which guarantees that locking will not
- * fail (nobody can concurrently write-lock the vma). vma_start_read() should
+ * Use only while holding mmap read lock which guarantees that vma lock is not
+ * contended (nobody can concurrently write-lock the vma). vma_start_read() should
  * not be used in such cases because it might fail due to mm_lock_seq overflow.
  * This functionality is used to obtain vma read lock and drop the mmap read lock.
+ * VMA can't be detached while we are holding mmap lock, therefore in practice this
+ * function can fail only when there are so many readers that vm_refcnt overflows.
  */
 static inline bool vma_start_read_locked(struct vm_area_struct *vma)
 {
 	return vma_start_read_locked_nested(vma, 0);
 }
 
+struct vm_area_struct *vma_start_read_unlocked(struct mm_struct *mm,
+					       unsigned long address);
+
 static inline void vma_end_read(struct vm_area_struct *vma)
 {
 	vma_refcount_put(vma);
diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index e20d01e8d38f..6ff05e68e61b 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -338,6 +338,35 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	return NULL;
 }
 
+/*
+ * Find the VMA covering 'address' and lock it for reading. Waits for writers to
+ * finish if the VMA is being modified. Returns NULL if there is no VMA covering
+ * 'address'.
+ *
+ * Use only in code paths where no mmap_lock and no VMA lock is held.
+ *
+ * The fast path does not take mmap_lock.
+ */
+struct vm_area_struct *vma_start_read_unlocked(struct mm_struct *mm,
+					       unsigned long address)
+{
+	struct vm_area_struct *vma;
+
+	/* Fast path: return stable VMA covering 'address': */
+	vma = lock_vma_under_rcu(mm, address);
+	if (vma)
+		return vma;
+
+	/* Slow path: preclude VMA writers by temporarily getting mmap read lock. */
+	mmap_read_lock(mm);
+	vma = vma_lookup(mm, address);
+	if (vma && !vma_start_read_locked(vma))
+		vma = NULL;
+	mmap_read_unlock(mm);
+
+	return vma;
+}
+
 static struct vm_area_struct *lock_next_vma_under_mmap_lock(struct mm_struct *mm,
 							    struct vma_iterator *vmi,
 							    unsigned long from_addr)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index edd90892f8cc..c3a0c38a3dc3 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -129,8 +129,10 @@ struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm,
  *
  * Should be called without holding mmap_lock.
  *
- * Return: A locked vma containing @address, -ENOENT if no vma is found, or
- * -ENOMEM if anon_vma couldn't be allocated.
+ * Return: A locked vma containing @address, -ENOENT if no vma is found,
+ * -ENOMEM if anon_vma couldn't be allocated, or -EAGAIN if vma refcount
+ * overflow happened due to high number of readers and the caller should
+ * retry later.
  */
 static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
 				       unsigned long address)
-- 
2.55.0.508.g3f0d502094-goog