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Message-ID: <CACT4Y+ZTjCGd9XYUCUoqv+AqXrPwX4OqWMC0jFgjNxZRFkNYXw@mail.gmail.com>
Date: Tue, 22 Jan 2019 11:12:04 +0100
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Joel Fernandes <joel@...lfernandes.org>,
Todd Kjos <tkjos@...gle.com>,
Joel Fernandes <joelaf@...gle.com>,
syzbot+a76129f18c89f3e2ddd4@...kaller.appspotmail.com,
Andi Kleen <ak@...ux.intel.com>,
Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
Souptick Joarder <jrdr.linux@...il.com>,
LKML <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
Matthew Wilcox <mawilcox@...rosoft.com>,
Mel Gorman <mgorman@...hsingularity.net>,
syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
Arve Hjønnevåg <arve@...roid.com>,
Todd Kjos <tkjos@...roid.com>,
Martijn Coenen <maco@...roid.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: possible deadlock in __do_page_fault
On Tue, Jan 22, 2019 at 11:02 AM Tetsuo Handa
<penguin-kernel@...ove.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@...lfernandes.org> wrote:
> >
> >> On Thu, Sep 20, 2018 at 5:12 PM Todd Kjos <tkjos@...gle.com> wrote:
> >>>
> >>> +Joel Fernandes
> >>>
> >>> On Thu, Sep 20, 2018 at 2:11 PM Andrew Morton <akpm@...ux-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
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