linux-kernel - Re: iov_iter

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Date:   Mon, 28 Aug 2017 16:31:30 -0400
From:   Dave Jones <davej@...emonkey.org.uk>
To:     Al Viro <viro@...IV.linux.org.uk>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Cc:     linux-xfs@...r.kernel.org
Subject: Re: iov_iter_pipe warning.

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 Is this check no longer valid, or am I triggering the same bug we were chased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...

	Dave


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