Re: possible deadlock in __do_page_fault

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, Jan 22, 2019 at 11:02 AM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> On 2018/09/22 8:21, Andrew Morton wrote:
> > On Thu, 20 Sep 2018 19:33:15 -0400 Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> >> On Thu, Sep 20, 2018 at 5:12 PM Todd Kjos <tkjos@google.com> wrote:
> >>>
> >>> +Joel Fernandes
> >>>
> >>> On Thu, Sep 20, 2018 at 2:11 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> >>>>
> >>>>
> >>>> Thanks.  Let's cc the ashmem folks.
> >>>>
> >>
> >> This should be fixed by https://patchwork.kernel.org/patch/10572477/
> >>
> >> It has Neil Brown's Reviewed-by but looks like didn't yet appear in
> >> anyone's tree, could Greg take this patch?
> >
> > All is well.  That went into mainline yesterday, with a cc:stable.
> >
>
> This problem was not fixed at all.

There are at least 2 other open deadlocks involving ashmem:

https://syzkaller.appspot.com/bug?extid=148c2885d71194f18d28
https://syzkaller.appspot.com/bug?extid=4b8b031b89e6b96c4b2e

Does this fix any of these too?


> 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).
>
> ---
>  drivers/staging/android/ashmem.c | 50 ++++++++++++++++++++++++++++----
>  1 file changed, 45 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
> index 90a8a9f1ac7d..1a890c43a10a 100644
> --- a/drivers/staging/android/ashmem.c
> +++ b/drivers/staging/android/ashmem.c
> @@ -75,6 +75,17 @@ struct ashmem_range {
>  /* LRU list of unpinned pages, protected by ashmem_mutex */
>  static LIST_HEAD(ashmem_lru_list);
>
> +static struct workqueue_struct *ashmem_wq;
> +static atomic_t ashmem_shrink_inflight = ATOMIC_INIT(0);
> +static DECLARE_WAIT_QUEUE_HEAD(ashmem_shrink_wait);
> +
> +struct ashmem_shrink_work {
> +       struct work_struct work;
> +       struct file *file;
> +       loff_t start;
> +       loff_t end;
> +};
> +
>  /*
>   * long lru_count - The count of pages on our LRU list.
>   *
> @@ -292,6 +303,7 @@ 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 +371,7 @@ 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) {
> @@ -421,6 +434,19 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
>         return ret;
>  }
>
> +static void ashmem_shrink_worker(struct work_struct *work)
> +{
> +       struct ashmem_shrink_work *w = container_of(work, typeof(*w), work);
> +
> +       w->file->f_op->fallocate(w->file,
> +                                FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> +                                w->start, w->end - w->start);
> +       fput(w->file);
> +       kfree(w);
> +       if (atomic_dec_and_test(&ashmem_shrink_inflight))
> +               wake_up_all(&ashmem_shrink_wait);
> +}
> +
>  /*
>   * ashmem_shrink - our cache shrinker, called from mm/vmscan.c
>   *
> @@ -449,12 +475,18 @@ ashmem_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>                 return -1;
>
>         list_for_each_entry_safe(range, next, &ashmem_lru_list, lru) {
> -               loff_t start = range->pgstart * PAGE_SIZE;
> -               loff_t end = (range->pgend + 1) * PAGE_SIZE;
> +               struct ashmem_shrink_work *w = kzalloc(sizeof(*w), GFP_ATOMIC);
> +
> +               if (!w)
> +                       break;
> +               INIT_WORK(&w->work, ashmem_shrink_worker);
> +               w->file = range->asma->file;
> +               get_file(w->file);
> +               w->start = range->pgstart * PAGE_SIZE;
> +               w->end = (range->pgend + 1) * PAGE_SIZE;
> +               atomic_inc(&ashmem_shrink_inflight);
> +               queue_work(ashmem_wq, &w->work);
>
> -               range->asma->file->f_op->fallocate(range->asma->file,
> -                               FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> -                               start, end - start);
>                 range->purged = ASHMEM_WAS_PURGED;
>                 lru_del(range);
>
> @@ -713,6 +745,7 @@ 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;
> @@ -883,8 +916,15 @@ static int __init ashmem_init(void)
>                 goto out_free2;
>         }
>
> +       ashmem_wq = alloc_workqueue("ashmem_wq", WQ_MEM_RECLAIM, 0);
> +       if (!ashmem_wq) {
> +               pr_err("failed to create workqueue\n");
> +               goto out_demisc;
> +       }
> +
>         ret = register_shrinker(&ashmem_shrinker);
>         if (ret) {
> +               destroy_workqueue(ashmem_wq);
>                 pr_err("failed to register shrinker!\n");
>                 goto out_demisc;
>         }
> --
> 2.17.1