Re: iov_iter_pipe warning.

[Darrick J. Wong]

On Mon, Aug 28, 2017 at 04:31:30PM -0400, Dave Jones wrote:
> On Mon, Aug 07, 2017 at 04:18:18PM -0400, Dave Jones wrote:
>  > On Fri, Apr 28, 2017 at 06:20:25PM +0100, Al Viro wrote:
>  >  > On Fri, Apr 28, 2017 at 12:50:24PM -0400, Dave Jones wrote:
>  >  > > currently running v4.11-rc8-75-gf83246089ca0
>  >  > > 
>  >  > > sunrpc bit is for the other unrelated problem I'm chasing.
>  >  > > 
>  >  > > note also, I saw the backtrace without the fs/splice.c changes.
>  >  > 
>  >  > 	Interesting...  Could you add this and see if that triggers?
>  >  > 
>  >  > diff --git a/fs/splice.c b/fs/splice.c
>  >  > index 540c4a44756c..12a12d9c313f 100644
>  >  > --- a/fs/splice.c
>  >  > +++ b/fs/splice.c
>  >  > @@ -306,6 +306,9 @@ ssize_t generic_file_splice_read(struct file *in, loff_t *ppos,
>  >  >  	kiocb.ki_pos = *ppos;
>  >  >  	ret = call_read_iter(in, &kiocb, &to);
>  >  >  	if (ret > 0) {
>  >  > +		if (WARN_ON(iov_iter_count(&to) != len - ret))
>  >  > +			printk(KERN_ERR "ops %p: was %zd, left %zd, returned %d\n",
>  >  > +				in->f_op, len, iov_iter_count(&to), ret);
>  >  >  		*ppos = kiocb.ki_pos;
>  >  >  		file_accessed(in);
>  >  >  	} else if (ret < 0) {
>  > 
>  > Hey Al,
>  >  Due to a git stash screw up on my part, I've had this leftover WARN_ON
>  > in my tree for the last couple months. (That screw-up might turn out to be
>  > serendipitous if this is a real bug..)
>  > 
>  > Today I decided to change things up and beat up on xfs for a change, and
>  > was able to trigger this again.
>  > 
>  > Is this check no longer valid, or am I triggering the same bug we were chased
>  > down in nfs, but now in xfs ?  (None of the other detritus from that debugging
>  > back in April made it, just those three lines above).
> 
> Revisiting this. I went back and dug out some of the other debug diffs [1]
> from that old thread.
> 
> I can easily trigger this spew on xfs.
> 
> 
> WARNING: CPU: 1 PID: 2251 at fs/splice.c:292 test_it+0xd4/0x1d0
> CPU: 1 PID: 2251 Comm: trinity-c42 Not tainted 4.13.0-rc7-think+ #1 
> task: ffff880459173a40 task.stack: ffff88044f7d0000
> RIP: 0010:test_it+0xd4/0x1d0
> RSP: 0018:ffff88044f7d7878 EFLAGS: 00010283
> RAX: 0000000000000000 RBX: ffff88044f44b968 RCX: ffffffff81511ea0
> RDX: 0000000000000003 RSI: dffffc0000000000 RDI: ffff88044f44ba68
> RBP: ffff88044f7d78c8 R08: ffff88046b218ec0 R09: 0000000000000000
> R10: ffff88044f7d7518 R11: 0000000000000000 R12: 0000000000001000
> R13: 0000000000000001 R14: 00000000ffffffff R15: 0000000000000001
> FS:  00007fdbc09b2700(0000) GS:ffff88046b200000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000000 CR3: 0000000459e1d000 CR4: 00000000001406e0
> Call Trace:
>  generic_file_splice_read+0x414/0x4e0
>  ? opipe_prep.part.14+0x180/0x180
>  ? lockdep_init_map+0xb2/0x2b0
>  ? rw_verify_area+0x65/0x150
>  do_splice_to+0xab/0xc0
>  splice_direct_to_actor+0x1f5/0x540
>  ? generic_pipe_buf_nosteal+0x10/0x10
>  ? do_splice_to+0xc0/0xc0
>  ? rw_verify_area+0x9d/0x150
>  do_splice_direct+0x1b9/0x230
>  ? splice_direct_to_actor+0x540/0x540
>  ? __sb_start_write+0x164/0x1c0
>  ? do_sendfile+0x7b3/0x840
>  do_sendfile+0x428/0x840
>  ? do_compat_pwritev64+0xb0/0xb0
>  ? __might_sleep+0x72/0xe0
>  ? kasan_check_write+0x14/0x20
>  SyS_sendfile64+0xa4/0x120
>  ? SyS_sendfile+0x150/0x150
>  ? mark_held_locks+0x23/0xb0
>  ? do_syscall_64+0xc0/0x3e0
>  ? SyS_sendfile+0x150/0x150
>  do_syscall_64+0x1bc/0x3e0
>  ? syscall_return_slowpath+0x240/0x240
>  ? mark_held_locks+0x23/0xb0
>  ? return_from_SYSCALL_64+0x2d/0x7a
>  ? trace_hardirqs_on_caller+0x182/0x260
>  ? trace_hardirqs_on_thunk+0x1a/0x1c
>  entry_SYSCALL64_slow_path+0x25/0x25
> RIP: 0033:0x7fdbc02dd219
> RSP: 002b:00007ffc5024fa48 EFLAGS: 00000246
>  ORIG_RAX: 0000000000000028
> RAX: ffffffffffffffda RBX: 0000000000000028 RCX: 00007fdbc02dd219
> RDX: 00007fdbbe348000 RSI: 0000000000000011 RDI: 0000000000000015
> RBP: 00007ffc5024faf0 R08: 000000000000006d R09: 0094e82f2c730a50
> R10: 0000000000001000 R11: 0000000000000246 R12: 0000000000000002
> R13: 00007fdbc0885058 R14: 00007fdbc09b2698 R15: 00007fdbc0885000
> ---[ end trace a5847ef0f7be7e20 ]---
> asked to read 4096, claims to have read 1
> actual size of data in pipe 4096 
> [0:4096]
> f_op: ffffffffa058c920, f_flags: 49154, pos: 0/1, size: 0
> 
> 
> I'm still trying to narrow down an exact reproducer, but it seems having
> trinity do a combination of sendfile & writev, with pipes and regular
> files as fd's is the best repro.
> 
> Is this a real problem, or am I chasing ghosts ?  That it doesn't happen
> on ext4 or btrfs is making me wonder...

<shrug> I haven't heard of any problems w/ directio xfs lately, but OTOH
I think it's the only filesystem that uses iomap_dio_rw, which would
explain why ext4/btrfs don't have this problem.

Granted that's idle speculation; is there a reproducer/xfstest for this?

--D

> 
> 	Dave
> 
> 
> [1] https://lkml.org/lkml/2017/4/11/921
> 
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