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

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" <redacted>
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;
 }

+/*

+		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.