Re: possible deadlock in __do_page_fault

[Tetsuo Handa]

Joel Fernandes wrote:
> > Why do we need to call fallocate() synchronously with ashmem_mutex held?
> > Why can't we call fallocate() asynchronously from WQ_MEM_RECLAIM workqueue
> > context so that we can call fallocate() with ashmem_mutex not held?
> > 
> > I don't know how ashmem works, but as far as I can guess, offloading is
> > possible as long as other operations which depend on the completion of
> > fallocate() operation (e.g. read()/mmap(), querying/changing pinned status)
> > wait for completion of asynchronous fallocate() operation (like a draft
> > patch shown below is doing).
> 
> This adds a bit of complexity, I am worried if it will introduce more
> bugs especially because ashmem is going away in the long term, in favor of
> memfd - and if its worth adding more complexity / maintenance burden to it.

I don't care migrating to memfd. I care when bugs are fixed.

> 
> I am wondering if we can do this synchronously, without using a workqueue.
> All you would need is a temporary list of areas to punch. In
> ashmem_shrink_scan, you would create this list under mutex and then once you
> release the mutex, you can go through this list and do the fallocate followed
> by the wake up of waiters on the wait queue, right? If you can do it this
> way, then it would be better IMO.

Are you sure that none of locks held before doing GFP_KERNEL allocation
interferes lock dependency used by fallocate() ? If yes, we can do without a
workqueue context (like a draft patch shown below). Since I don't understand
what locks are potentially involved, I offloaded to a clean workqueue context.

Anyway, I need your checks regarding whether this approach is waiting for
completion at all locations which need to wait for completion.

---
 drivers/staging/android/ashmem.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 90a8a9f1ac7d..6a267563cb66 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -75,6 +75,9 @@ struct ashmem_range {
 /* LRU list of unpinned pages, protected by ashmem_mutex */
 static LIST_HEAD(ashmem_lru_list);
 
+static atomic_t ashmem_shrink_inflight = ATOMIC_INIT(0);
+static DECLARE_WAIT_QUEUE_HEAD(ashmem_shrink_wait);
+
 /*
  * long lru_count - The count of pages on our LRU list.
  *
@@ -292,6 +295,7 @@ static ssize_t ashmem_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	int ret = 0;
 
 	mutex_lock(&ashmem_mutex);
+	wait_event(ashmem_shrink_wait, !atomic_read(&ashmem_shrink_inflight));
 
 	/* If size is not set, or set to 0, always return EOF. */
 	if (asma->size == 0)
@@ -359,6 +363,7 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
 	int ret = 0;
 
 	mutex_lock(&ashmem_mutex);
+	wait_event(ashmem_shrink_wait, !atomic_read(&ashmem_shrink_inflight));
 
 	/* user needs to SET_SIZE before mapping */
 	if (!asma->size) {
@@ -438,7 +443,6 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
 static unsigned long
 ashmem_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 {
-	struct ashmem_range *range, *next;
 	unsigned long freed = 0;
 
 	/* We might recurse into filesystem code, so bail out if necessary */
@@ -448,17 +452,27 @@ ashmem_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 	if (!mutex_trylock(&ashmem_mutex))
 		return -1;
 
-	list_for_each_entry_safe(range, next, &ashmem_lru_list, lru) {
+	while (!list_empty(&ashmem_lru_list)) {
+		struct ashmem_range *range =
+			list_first_entry(&ashmem_lru_list, typeof(*range), lru);
 		loff_t start = range->pgstart * PAGE_SIZE;
 		loff_t end = (range->pgend + 1) * PAGE_SIZE;
+		struct file *f = range->asma->file;
 
-		range->asma->file->f_op->fallocate(range->asma->file,
-				FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
-				start, end - start);
+		get_file(f);
+		atomic_inc(&ashmem_shrink_inflight);
 		range->purged = ASHMEM_WAS_PURGED;
 		lru_del(range);
 
 		freed += range_size(range);
+		mutex_unlock(&ashmem_mutex);
+		f->f_op->fallocate(f,
+				   FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+				   start, end - start);
+		fput(f);
+		if (atomic_dec_and_test(&ashmem_shrink_inflight))
+			wake_up_all(&ashmem_shrink_wait);
+		mutex_lock(&ashmem_mutex);
 		if (--sc->nr_to_scan <= 0)
 			break;
 	}
@@ -713,6 +727,7 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
 		return -EFAULT;
 
 	mutex_lock(&ashmem_mutex);
+	wait_event(ashmem_shrink_wait, !atomic_read(&ashmem_shrink_inflight));
 
 	if (!asma->file)
 		goto out_unlock;
-- 
2.17.1