From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt1-f197.google.com (mail-qt1-f197.google.com [209.85.160.197]) by kanga.kvack.org (Postfix) with ESMTP id E02C28E001A for ; Wed, 23 Jan 2019 10:57:55 -0500 (EST) Received: by mail-qt1-f197.google.com with SMTP id w18so2829879qts.8 for ; Wed, 23 Jan 2019 07:57:55 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id q20sor58155257qke.121.2019.01.23.07.57.54 for (Google Transport Security); Wed, 23 Jan 2019 07:57:54 -0800 (PST) Date: Wed, 23 Jan 2019 10:57:51 -0500 From: Joel Fernandes Subject: Re: possible deadlock in __do_page_fault Message-ID: <20190123155751.GA168927@google.com> References: <4b0a5f8c-2be2-db38-a70d-8d497cb67665@I-love.SAKURA.ne.jp> <20190122153220.GA191275@google.com> <201901230201.x0N214eq043832@www262.sakura.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201901230201.x0N214eq043832@www262.sakura.ne.jp> Sender: owner-linux-mm@kvack.org List-ID: To: Tetsuo Handa Cc: Andrew Morton , Todd Kjos , syzbot+a76129f18c89f3e2ddd4@syzkaller.appspotmail.com, ak@linux.intel.com, Johannes Weiner , jack@suse.cz, jrdr.linux@gmail.com, LKML , linux-mm@kvack.org, mawilcox@microsoft.com, mgorman@techsingularity.net, syzkaller-bugs@googlegroups.com, Arve =?iso-8859-1?B?SGr4bm5lduVn?= , Todd Kjos , Martijn Coenen , Greg Kroah-Hartman On Wed, Jan 23, 2019 at 11:01:04AM +0900, Tetsuo Handa wrote: > 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. That's fair. I'm not a fan of bugs either. I was just making a point that - we want to fix things while not introducing unwanted complexity and cause more bugs. That said, thanks for the patch and trying to fix it. > > 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. fallocate acquires inode locks. So there is a lock dependency between - memory reclaim (fake lock) - inode locks. This dependency is there whether we have your patch or not. I am not aware of any other locks that are held other than these. But you could also just use lockdep to dump all held locks at that point to confirm. > Anyway, I need your checks regarding whether this approach is waiting for > completion at all locations which need to wait for completion. I think you are waiting in unwanted locations. The only location you need to wait in is ashmem_pin_unpin. So, to my eyes all that is needed to fix this bug is: 1. Delete the range from the ashmem_lru_list 2. Release the ashmem_mutex 3. fallocate the range. 4. Do the completion so that any waiting pin/unpin can proceed. Could you clarify why you feel you need to wait for completion at those other locations? Note that once a range is unpinned, it is open sesame and userspace cannot really expect consistent data from such range till it is pinned again. Thanks! - Joel > --- > 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