From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lj1-f197.google.com (mail-lj1-f197.google.com [209.85.208.197]) by kanga.kvack.org (Postfix) with ESMTP id 20CBC6B788E for ; Thu, 6 Sep 2018 08:02:17 -0400 (EDT) Received: by mail-lj1-f197.google.com with SMTP id m21-v6so2222061ljh.2 for ; Thu, 06 Sep 2018 05:02:17 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id o71-v6sor1884312lff.55.2018.09.06.05.02.15 for (Google Transport Security); Thu, 06 Sep 2018 05:02:15 -0700 (PDT) MIME-Version: 1.0 References: <0000000000004f6b5805751a8189@google.com> <20180905085545.GD24902@quack2.suse.cz> <20180906081253.GB19319@quack2.suse.cz> In-Reply-To: <20180906081253.GB19319@quack2.suse.cz> From: Souptick Joarder Date: Thu, 6 Sep 2018 17:32:02 +0530 Message-ID: Subject: Re: linux-next test error Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: syzbot+87a05ae4accd500f5242@syzkaller.appspotmail.com, ak@linux.intel.com, Andrew Morton , linux-kernel@vger.kernel.org, Linux-MM , mawilcox@microsoft.com, mgorman@techsingularity.net, syzkaller-bugs@googlegroups.com, tim.c.chen@linux.intel.com, zwisler@kernel.org On Thu, Sep 6, 2018 at 1:42 PM Jan Kara wrote: > > On Thu 06-09-18 00:37:06, Souptick Joarder wrote: > > On Wed, Sep 5, 2018 at 2:25 PM Jan Kara wrote: > > > > > > On Wed 05-09-18 00:13:02, syzbot wrote: > > > > Hello, > > > > > > > > syzbot found the following crash on: > > > > > > > > HEAD commit: 387ac6229ecf Add linux-next specific files for 20180905 > > > > git tree: linux-next > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=149c67a6400000 > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=ad5163873ecfbc32 > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=87a05ae4accd500f5242 > > > > compiler: gcc (GCC) 8.0.1 20180413 (experimental) > > > > > > > > Unfortunately, I don't have any reproducer for this crash yet. > > > > > > > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > > > > Reported-by: syzbot+87a05ae4accd500f5242@syzkaller.appspotmail.com > > > > > > > > INFO: task hung in do_page_mkwriteINFO: task syz-fuzzer:4876 blocked for > > > > more than 140 seconds. > > > > Not tainted 4.19.0-rc2-next-20180905+ #56 > > > > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > > > > syz-fuzzer D21704 4876 4871 0x00000000 > > > > Call Trace: > > > > context_switch kernel/sched/core.c:2825 [inline] > > > > __schedule+0x87c/0x1df0 kernel/sched/core.c:3473 > > > > schedule+0xfb/0x450 kernel/sched/core.c:3517 > > > > io_schedule+0x1c/0x70 kernel/sched/core.c:5140 > > > > wait_on_page_bit_common mm/filemap.c:1100 [inline] > > > > __lock_page+0x5b7/0x7a0 mm/filemap.c:1273 > > > > lock_page include/linux/pagemap.h:483 [inline] > > > > do_page_mkwrite+0x429/0x520 mm/memory.c:2391 > > > > > > Waiting for page lock after ->page_mkwrite callback. Which means > > > ->page_mkwrite did not return VM_FAULT_LOCKED but 0. Looking into > > > linux-next... indeed "fs: convert return type int to vm_fault_t" has busted > > > block_page_mkwrite(). It has to return VM_FAULT_LOCKED and not 0 now. > > > Souptick, can I ask you to run 'fstests' for at least common filesystems > > > like ext4, xfs, btrfs when you change generic filesystem code please? That > > > would catch a bug like this immediately. Thanks. > > > > Looking into existing code block_page_mkwrite() returns 0, not VM_FAULT_LOCKED > > in true path and this patch doesn't change any existing behaviour of > > block_page_mkwrite() > > except adding one new input parameter to return err value to caller function. > > Yeah, you are right and this confused me. In your version > block_page_mkwrite() returns block_page_mkwrite_return(err1) in case of > error but 0 in case of success and the caller - ext4_page_mkwrite() - then > uses block_page_mkwrite_return() again if block_page_mkwrite() returned 0. > So I agree the code path I pointed out won't result in returning 0 instead > of VM_FAULT_LOCKED but the calling convention is really very confusing. > > > -int ext4_page_mkwrite(struct vm_fault *vmf) > > +vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf) > > > > + err = 0; > > + ret = block_page_mkwrite(vma, vmf, get_block, &err); > > if (!ret && ext4_should_journal_data(inode)) { > > if (ext4_walk_page_buffers(handle, page_buffers(page), 0, > > PAGE_SIZE, NULL, do_journal_get_write_access)) { > > unlock_page(page); > > - ret = VM_FAULT_SIGBUS; > > > > I think, this part has created problem where page_mkwrite() > > end up with returning 0. > > So this branch is definitely wrong but I somewhat doubt it's the one we've > taken - this can happen only in case of IO error. Looking into the patch, this is only part of code where page_mkwrite() end up with returning 0.