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Date: Tue, 22 Jan 2019 19:02:35 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Andrew Morton <akpm@...ux-foundation.org>,
Joel Fernandes <joel@...lfernandes.org>
Cc: Todd Kjos <tkjos@...gle.com>, Joel Fernandes <joelaf@...gle.com>,
syzbot+a76129f18c89f3e2ddd4@...kaller.appspotmail.com,
ak@...ux.intel.com, Johannes Weiner <hannes@...xchg.org>,
jack@...e.cz, jrdr.linux@...il.com,
LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
mawilcox@...rosoft.com, mgorman@...hsingularity.net,
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 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.
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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