Message-ID: <20151107210203.GW29330@redhat.com>
Date:	Sat, 7 Nov 2015 21:02:03 +0000
From:	"Richard W.M. Jones" <rjones@...hat.com>
To:	Jason Pepas <jasonpepas@...il.com>
Cc:	libguestfs@...hat.com, linux-ext4@...r.kernel.org
Subject: Re: [Libguestfs] mkfs.ext2 succeeds despite nbd write errors?

[Adding linux-ext4 mailing list.  The original bug report is here:
https://www.redhat.com/archives/libguestfs/2015-November/msg00078.html ]

On Sat, Nov 07, 2015 at 01:22:45PM -0600, Jason Pepas wrote:
> On Sat, Nov 7, 2015 at 5:03 AM, Richard W.M. Jones <rjones@...hat.com> wrote:
> > How about 'strace mkfs.ext2 ..' and see if any system calls are
> > returning errors.  That would show you whether nbd-client is throwing
> > errors away, or whether mkfs is getting the errors and ignoring them
> > (seems pretty unlikely, but you never know).
> >
> > After that, it'd be down to tracing where the errors end up in the
> > kernel.
> 
> Thanks for the tip!
> 
> The results are interesting.  It looks like all of mkfs's pwrite()
> calls succeed, but its final fsync() calls do actually fail:
> 
> 
> root@...ian:~# strace mkfs.ext2 /dev/nbd0 2>&1 | tee strace.out
> 
> root@...ian:~# cat strace.out | grep pwrite
> pwrite(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 32768, 8187379712) = 32768
> pwrite(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 32768, 8187412480) = 32768
> pwrite(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 32768, 8187445248) = 32768
> ...
> 
> root@...ian:~# cat strace.out | grep fsync
> fsync(3)                                = -1 EIO (Input/output error)
> fsync(3)                                = -1 EIO (Input/output error)
> 
> 
> The fsync() calls happen just before mkfs exists success:
> 
> 
> root@...ian:~# cat strace.out | tail
> pwrite(3, "\1\2\0\0\2\2\0\0\3\2\0\0\367{\365\37\2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
> 4096, 6576672768) = 4096
> fsync(3)                                = -1 EIO (Input/output error)
> pwrite(3, "\0\0\10\0\0\0
> \0\231\231\1\0qm\37\0\365\377\7\0\0\0\0\0\2\0\0\0\2\0\0\0"..., 1024,
> 1024) = 1024
> fsync(3)                                = -1 EIO (Input/output error)
> close(3)                                = 0
> write(1, "done\n\n", 6done
> 
> )                 = 6
> exit_group(0)                           = ?
> +++ exited with 0 +++
> root@...ian:~#
> 
> 
> I did manage to find two calls to fsync in the e2fsprogs source which
> are not return-value-checked:
> 
> https://github.com/tytso/e2fsprogs/blob/956b0f18a5ddb6815a9dff4f10a1e3125cdca9ba/misc/filefrag.c#L303
> https://github.com/tytso/e2fsprogs/blob/956b0f18a5ddb6815a9dff4f10a1e3125cdca9ba/lib/ext2fs/unix_io.c#L915

That second one looks very suspicious to me.  I don't think that it's
ever right for mke2fs to ignore the return value from an fsync call,
so assuming mke2fs calls that function it's surely a bug.

> I'll see about submitting a patch there.
> 
> I'm not sure where to start with hunting down why mkfs's pwrite()
> calls aren't failing.  I'd look to the kernel source for that?

It looks like it's really an e2fsprogs problem, not a kernel problem.
That's pretty surprising - I wasn't expecting it.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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