From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joel Fernandes Subject: Re: possible deadlock in __do_page_fault Date: Mon, 28 Jan 2019 11:45:02 -0500 Message-ID: <20190128164502.GA260885@google.com> References: <201901230201.x0N214eq043832@www262.sakura.ne.jp> <20190123155751.GA168927@google.com> <201901240152.x0O1qUUU069046@www262.sakura.ne.jp> <20190124134646.GA53008@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org 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 List-Id: linux-mm.kvack.org On Sat, Jan 26, 2019 at 01:02:06AM +0900, Tetsuo Handa wrote: > On 2019/01/24 22:46, Joel Fernandes wrote: > > On Thu, Jan 24, 2019 at 10:52:30AM +0900, Tetsuo Handa wrote: > >> Joel Fernandes wrote: > >>>> 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? > > OK. Here is an updated patch. > Passed syzbot's best-effort testing using reproducers on all three reports. > > From f192176dbee54075d41249e9f22918c32cb4d4fc Mon Sep 17 00:00:00 2001 > From: Tetsuo Handa > Date: Fri, 25 Jan 2019 23:43:01 +0900 > Subject: [PATCH] staging: android: ashmem: Don't call fallocate() with ashmem_mutex held. > > syzbot is hitting lockdep warnings [1][2][3]. This patch tries to fix > the warning by eliminating ashmem_shrink_scan() => {shmem|vfs}_fallocate() > sequence. > > [1] https://syzkaller.appspot.com/bug?id=87c399f6fa6955006080b24142e2ce7680295ad4 > [2] https://syzkaller.appspot.com/bug?id=7ebea492de7521048355fc84210220e1038a7908 > [3] https://syzkaller.appspot.com/bug?id=e02419c12131c24e2a957ea050c2ab6dcbbc3270 > > Reported-by: syzbot > Reported-by: syzbot > Reported-by: syzbot > Signed-off-by: Tetsuo Handa > --- > drivers/staging/android/ashmem.c | 23 ++++++++++++++++++----- > 1 file changed, 18 insertions(+), 5 deletions(-) > > diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c > index 90a8a9f..d40c1d2 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. > * > @@ -438,7 +441,6 @@ 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 +450,27 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma) > 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); Let us replace mutex_lock with mutex_trylock, as done before the loop? Here is there is an opportunity to not block other ashmem operations. Otherwise LGTM. Also, CC stable. thanks, - Joel