mm/vma: introduce helper struct + thread through exclusive lock fns

It is confusing to have __vma_start_exclude_readers() return 0, 1 or an
error (but only when waiting for readers in TASK_KILLABLE state), and
having the return value be stored in a stack variable called 'locked'
is further confusion.

More generally, we are doing a lot of rather finnicky things during the
acquisition of a state in which readers are excluded and moving out of
this state, including tracking whether we are detached or not or
whether an error occurred.

We are implementing logic in __vma_start_exclude_readers() that
effectively acts as if 'if one caller calls us do X, if another then do
Y', which is very confusing from a control flow perspective.

Introducing the shared helper object state helps us avoid this, as we
can now handle the 'an error arose but we're detached' condition
correctly in both callers - a warning if not detaching, and treating
the situation as if no error arose in the case of a VMA detaching.

This also acts to help document what's going on and allows us to add
some more logical debug asserts.

Also update vma_mark_detached() to add a guard clause for the likely
'already detached' state (given we hold the mmap write lock), and add a
comment about ephemeral VMA read lock reference count increments to
clarify why we are entering/exiting an exclusive locked state here.

Finally, separate vma_mark_detached() into its fast-path component and
make it inline, then place the slow path for excluding readers in
mmap_lock.c.

No functional change intended.

[akpm@linux-foundation.org: fix function naming in comments, add comment per Vlastimil per Lorenzo]
Link: https://lkml.kernel.org/r/7d3084d596c84da10dd374130a5055deba6439c0.1769198904.git.lorenzo.stoakes@oracle.com
Link: https://lkml.kernel.org/r/7d3084d596c84da10dd374130a5055deba6439c0.1769198904.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Waiman Long <longman@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3e608d22cab0dbd0ce5ecf6dcd91f814720b0f13..8731606d8d366fdd62d0716a5bf7da7e7caeaecc 100644 (file)
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1011,15 +1011,15 @@ struct vm_area_struct {
         * decrementing it again.
         *
         * VM_REFCNT_EXCLUDE_READERS_FLAG - Detached, pending
-        * __vma_exit_locked() completion which will decrement the reference
-        * count to zero. IMPORTANT - at this stage no further readers can
-        * increment the reference count. It can only be reduced.
+        * __vma_end_exclude_readers() completion which will decrement the
+        * reference count to zero. IMPORTANT - at this stage no further readers
+        * can increment the reference count. It can only be reduced.
         *
         * VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - A thread is either write-locking
-        * an attached VMA and has yet to invoke __vma_exit_locked(), OR a
-        * thread is detaching a VMA and is waiting on a single spurious reader
-        * in order to decrement the reference count. IMPORTANT - as above, no
-        * further readers can increment the reference count.
+        * an attached VMA and has yet to invoke __vma_end_exclude_readers(),
+        * OR a thread is detaching a VMA and is waiting on a single spurious
+        * reader in order to decrement the reference count. IMPORTANT - as
+        * above, no further readers can increment the reference count.
         *
         * > VM_REFCNT_EXCLUDE_READERS_FLAG + 1 - A thread is either
         * write-locking or detaching a VMA is waiting on readers to

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index d6df6aad3e24ec9513627e4c5acc214cf25f652b..678f90080fa6036508f88ae3945fcd07d6635cc7 100644 (file)
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -358,7 +358,28 @@ static inline void vma_mark_attached(struct vm_area_struct *vma)
        refcount_set_release(&vma->vm_refcnt, 1);
 }
 
-void vma_mark_detached(struct vm_area_struct *vma);
+void __vma_exclude_readers_for_detach(struct vm_area_struct *vma);
+
+static inline void vma_mark_detached(struct vm_area_struct *vma)
+{
+       vma_assert_write_locked(vma);
+       vma_assert_attached(vma);
+
+       /*
+        * The VMA still being attached (refcnt > 0) - is unlikely, because the
+        * vma has been already write-locked and readers can increment vm_refcnt
+        * only temporarily before they check vm_lock_seq, realize the vma is
+        * locked and drop back the vm_refcnt. That is a narrow window for
+        * observing a raised vm_refcnt.
+        *
+        * See the comment describing the vm_area_struct->vm_refcnt field for
+        * details of possible refcnt values.
+        */
+       if (likely(!__vma_refcount_put_return(vma)))
+               return;
+
+       __vma_exclude_readers_for_detach(vma);
+}
 
 struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
                                          unsigned long address);

diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index 72f15f606093fa6850a68cf8d3e140398adc6015..490793ac88edda2575d2a5c399cc13fbd43af257 100644 (file)
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c
@@ -46,20 +46,38 @@ EXPORT_SYMBOL(__mmap_lock_do_trace_released);
 #ifdef CONFIG_MMU
 #ifdef CONFIG_PER_VMA_LOCK
 
+/* State shared across __vma_[start, end]_exclude_readers. */
+struct vma_exclude_readers_state {
+       /* Input parameters. */
+       struct vm_area_struct *vma;
+       int state; /* TASK_KILLABLE or TASK_UNINTERRUPTIBLE. */
+       bool detaching;
+
+       /* Output parameters. */
+       bool detached;
+       bool exclusive; /* Are we exclusively locked? */
+};
+
 /*
  * Now that all readers have been evicted, mark the VMA as being out of the
  * 'exclude readers' state.
- *
- * Returns true if the VMA is now detached, otherwise false.
  */
-static bool __must_check __vma_end_exclude_readers(struct vm_area_struct *vma)
+static void __vma_end_exclude_readers(struct vma_exclude_readers_state *ves)
 {
-       bool detached;
+       struct vm_area_struct *vma = ves->vma;
 
-       detached = refcount_sub_and_test(VM_REFCNT_EXCLUDE_READERS_FLAG,
-                                        &vma->vm_refcnt);
+       VM_WARN_ON_ONCE(ves->detached);
+
+       ves->detached = refcount_sub_and_test(VM_REFCNT_EXCLUDE_READERS_FLAG,
+                                             &vma->vm_refcnt);
        __vma_lockdep_release_exclusive(vma);
-       return detached;
+}
+
+static unsigned int get_target_refcnt(struct vma_exclude_readers_state *ves)
+{
+       const unsigned int tgt = ves->detaching ? 0 : 1;
+
+       return tgt | VM_REFCNT_EXCLUDE_READERS_FLAG;
 }
 
 /*
@@ -69,32 +87,29 @@ static bool __must_check __vma_end_exclude_readers(struct vm_area_struct *vma)
  * Note that this function pairs with vma_refcount_put() which will wake up this
  * thread when it detects that the last reader has released its lock.
  *
- * The state parameter ought to be set to TASK_UNINTERRUPTIBLE in cases where we
- * wish the thread to sleep uninterruptibly or TASK_KILLABLE if a fatal signal
- * is permitted to kill it.
+ * The ves->state parameter ought to be set to TASK_UNINTERRUPTIBLE in cases
+ * where we wish the thread to sleep uninterruptibly or TASK_KILLABLE if a fatal
+ * signal is permitted to kill it.
  *
- * The function will return 0 immediately if the VMA is detached, or wait for
- * readers and return 1 once they have all exited, leaving the VMA exclusively
- * locked.
+ * The function sets the ves->exclusive parameter to true if readers were
+ * excluded, or false if the VMA was detached or an error arose on wait.
  *
- * If the function returns 1, the caller is required to invoke
- * __vma_end_exclude_readers() once the exclusive state is no longer required.
+ * If the function indicates an exclusive lock was acquired via ves->exclusive
+ * the caller is required to invoke __vma_end_exclude_readers() once the
+ * exclusive state is no longer required.
  *
- * If state is set to something other than TASK_UNINTERRUPTIBLE, the function
- * may also return -EINTR to indicate a fatal signal was received while waiting.
+ * If ves->state is set to something other than TASK_UNINTERRUPTIBLE, the
+ * function may also return -EINTR to indicate a fatal signal was received while
+ * waiting.  Otherwise, the function returns 0.
  */
-static int __vma_start_exclude_readers(struct vm_area_struct *vma,
-               bool detaching, int state)
+static int __vma_start_exclude_readers(struct vma_exclude_readers_state *ves)
 {
-       int err;
-       unsigned int tgt_refcnt = VM_REFCNT_EXCLUDE_READERS_FLAG;
+       struct vm_area_struct *vma = ves->vma;
+       unsigned int tgt_refcnt = get_target_refcnt(ves);
+       int err = 0;
 
        mmap_assert_write_locked(vma->vm_mm);
 
-       /* Additional refcnt if the vma is attached. */
-       if (!detaching)
-               tgt_refcnt++;
-
        /*
         * If vma is detached then only vma_mark_attached() can raise the
         * vm_refcnt. mmap_write_lock prevents racing with vma_mark_attached().
@@ -102,37 +117,39 @@ static int __vma_start_exclude_readers(struct vm_area_struct *vma,
         * See the comment describing the vm_area_struct->vm_refcnt field for
         * details of possible refcnt values.
         */
-       if (!refcount_add_not_zero(VM_REFCNT_EXCLUDE_READERS_FLAG, &vma->vm_refcnt))
+       if (!refcount_add_not_zero(VM_REFCNT_EXCLUDE_READERS_FLAG, &vma->vm_refcnt)) {
+               ves->detached = true;
                return 0;
+       }
 
        __vma_lockdep_acquire_exclusive(vma);
        err = rcuwait_wait_event(&vma->vm_mm->vma_writer_wait,
                   refcount_read(&vma->vm_refcnt) == tgt_refcnt,
-                  state);
+                  ves->state);
        if (err) {
-               if (__vma_end_exclude_readers(vma)) {
-                       /*
-                        * The wait failed, but the last reader went away
-                        * as well. Tell the caller the VMA is detached.
-                        */
-                       WARN_ON_ONCE(!detaching);
-                       err = 0;
-               }
+               __vma_end_exclude_readers(ves);
                return err;
        }
-       __vma_lockdep_stat_mark_acquired(vma);
 
-       return 1;
+       __vma_lockdep_stat_mark_acquired(vma);
+       ves->exclusive = true;
+       return 0;
 }
 
 int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
                int state)
 {
-       int locked;
+       int err;
+       struct vma_exclude_readers_state ves = {
+               .vma = vma,
+               .state = state,
+       };
 
-       locked = __vma_start_exclude_readers(vma, false, state);
-       if (locked < 0)
-               return locked;
+       err = __vma_start_exclude_readers(&ves);
+       if (err) {
+               WARN_ON_ONCE(ves.detached);
+               return err;
+       }
 
        /*
         * We should use WRITE_ONCE() here because we can have concurrent reads
@@ -142,45 +159,42 @@ int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
         */
        WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq);
 
-       if (locked) {
-               bool detached = __vma_end_exclude_readers(vma);
-
-               /* The VMA should remain attached. */
-               WARN_ON_ONCE(detached);
+       if (ves.exclusive) {
+               __vma_end_exclude_readers(&ves);
+               /* VMA should remain attached. */
+               WARN_ON_ONCE(ves.detached);
        }
 
        return 0;
 }
 EXPORT_SYMBOL_GPL(__vma_start_write);
 
-void vma_mark_detached(struct vm_area_struct *vma)
+void __vma_exclude_readers_for_detach(struct vm_area_struct *vma)
 {
-       vma_assert_write_locked(vma);
-       vma_assert_attached(vma);
+       struct vma_exclude_readers_state ves = {
+               .vma = vma,
+               .state = TASK_UNINTERRUPTIBLE,
+               .detaching = true,
+       };
+       int err;
 
        /*
-        * This condition - that the VMA is still attached (refcnt > 0) - is
-        * unlikely, because the vma has been already write-locked and readers
-        * can increment vm_refcnt only temporarily before they check
-        * vm_lock_seq, realize the vma is locked and drop back the
-        * vm_refcnt. That is a narrow window for observing a raised vm_refcnt.
-        *
-        * See the comment describing the vm_area_struct->vm_refcnt field for
-        * details of possible refcnt values.
+        * Wait until the VMA is detached with no readers. Since we hold the VMA
+        * write lock, the only read locks that might be present are those from
+        * threads trying to acquire the read lock and incrementing the
+        * reference count before realising the write lock is held and
+        * decrementing it.
         */
-       if (unlikely(__vma_refcount_put_return(vma))) {
-               /* Wait until vma is detached with no readers. */
-               if (__vma_start_exclude_readers(vma, true, TASK_UNINTERRUPTIBLE)) {
-                       bool detached;
-
-                       /*
-                        * Once this is complete, no readers can increment the
-                        * reference count, and the VMA is marked detached.
-                        */
-                       detached = __vma_end_exclude_readers(vma);
-                       WARN_ON_ONCE(!detached);
-               }
+       err = __vma_start_exclude_readers(&ves);
+       if (!err && ves.exclusive) {
+               /*
+                * Once this is complete, no readers can increment the
+                * reference count, and the VMA is marked detached.
+                */
+               __vma_end_exclude_readers(&ves);
        }
+       /* If an error arose but we were detached anyway, we don't care. */
+       WARN_ON_ONCE(!ves.detached);
 }
 
 /*