On Thu 06-09-18 00:37:06, Souptick Joarder wrote: > On Wed, Sep 5, 2018 at 2:25 PM Jan Kara <jack@xxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > 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. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
