Re: Files leak from nfsd in 4.7.1-rc1 (and more?)

Jeff Layton
2016-06-08T00:03:50Z

On Tue, 2016-06-07 at 19:39 -0400, Oleg Drokin wrote:
> On Jun 7, 2016, at 4:04 PM, Jeff Layton wrote:
>
> >
> > On Tue, 2016-06-07 at 13:30 -0400, Oleg Drokin wrote:
> > >
> > > On Jun 7, 2016, at 1:10 PM, Jeff Layton wrote:
> > >
> > > >
> > > >
> > > > On Tue, 2016-06-07 at 11:37 -0400, Oleg Drokin wrote:
> > > > >
> > > > >
> > > > > Hello!
> > > > >
> > > > >    I've been trying to better understand this problem I was having where sometimes
> > > > >    a formerly NFS-exported mountpoint becomes unmountable (after nfsd stop).
> > > > >
> > > > >    I finally traced it to a leaked filedescriptor that was allocated from
> > > > >    nfsd4_open()->nfsd4_process_open2()->nfs4_get_vfs_file()->nfsd_open().
> > > > >
> > > > >    Also together with it we see leaked credentials allocated along the same path from
> > > > >    fh_verify() and groups allocated from svcauth_unix_accept()->groups_alloc() that
> > > > >    are presumably used by the credentials.
> > > > >
> > > > >    Unfortunately I was not able to make total sense out of the state handling in nfsd,
> > > > >    but it's clear that one of the file descriptors inside struct nfs4_file is
> > > > >    lost. I added a patch like this (always a good idea, so surprised it was not
> > > > >    there already):
> > > > > @@ -271,6 +274,9 @@ static void nfsd4_free_file_rcu(struct rcu_head *rcu)
> > > > >  {
> > > > >         struct nfs4_file *fp = container_of(rcu, struct nfs4_file, fi_rcu);
> > > > >
> > > > > +       WARN_ON(fp->fi_fds[0]);
> > > > > +       WARN_ON(fp->fi_fds[1]);
> > > > > +       WARN_ON(fp->fi_fds[2]);
> > > > >         kmem_cache_free(file_slab, fp);
> > > > >  }
> > > > >
> > > > >    And when the problem is hit, I am also triggering (Always this one which is fd[1])
> > > > > [ 3588.143002] ------------[ cut here ]------------
> > > > > [ 3588.143662] WARNING: CPU: 5 PID: 9 at /home/green/bk/linux/fs/nfsd/nfs4state.c:278 nfsd4_free_file_rcu+0x65/0x80 [nfsd]
> > > > > [ 3588.144947] Modules linked in: loop rpcsec_gss_krb5 joydev acpi_cpufreq tpm_tis i2c_piix4 tpm virtio_console pcspkr nfsd ttm drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops drm floppy serio_raw virtio_blk
> > > > > [ 3588.147135] CPU: 5 PID: 9 Comm: rcuos/0 Not tainted 4.7.0-rc1-vm-nfs+ #120
> > > > > [ 3588.153826] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> > > > > [ 3588.153830]  0000000000000286 00000000e2d5ccdf ffff88011965bd50 ffffffff814a11a5
> > > > > [ 3588.153832]  0000000000000000 0000000000000000 ffff88011965bd90 ffffffff8108806b
> > > > > [ 3588.153834]  0000011600000000 ffff8800c476a0b8 ffff8800c476a048 ffffffffc0110fc0
> > > > > [ 3588.153834] Call Trace:
> > > > > [ 3588.153839]  [] dump_stack+0x86/0xc1
> > > > > [ 3588.153841]  [] __warn+0xcb/0xf0
> > > > > [ 3588.153852]  [] ? trace_raw_output_fh_want_write+0x60/0x60 [nfsd]
> > > > > [ 3588.153853]  [] warn_slowpath_null+0x1d/0x20
> > > > > [ 3588.153859]  [] nfsd4_free_file_rcu+0x65/0x80 [nfsd]
> > > > > [ 3588.153861]  [] rcu_nocb_kthread+0x335/0x510
> > > > > [ 3588.153862]  [] ? rcu_nocb_kthread+0x27f/0x510
> > > > > [ 3588.153863]  [] ? rcu_cpu_notify+0x3e0/0x3e0
> > > > > [ 3588.153866]  [] kthread+0x101/0x120
> > > > > [ 3588.153868]  [] ? trace_hardirqs_on_caller+0xf4/0x1b0
> > > > > [ 3588.153871]  [] ret_from_fork+0x1f/0x40
> > > > > [ 3588.153872]  [] ? kthread_create_on_node+0x250/0x250
> > > > >
> > > > >
> > > > >   release_all_access() seems to be doing correct job of all that cleaning, so
> > > > >   there must be some other path that I do not quite see.
> > > > >
> > > > >   Hopefully you are more familiar with the code and can see the problem right away ;)
> > > > Hmm...well I'm not seeing it right offhand, and haven't been able to
> > > > reproduce the problem so far after a couple of attempts by hand. What
> > > > sort of workload are you running before you see that warning pop?
> > > Well, the workload is a crazed twisty conflicting operations set in a very limited
> > > namespace (called racer.sh,
> > > http://git.whamcloud.com/fs/lustrerelease.git/tree/refs/heads/master:/lustre/tests/racer )
> > >
> > > It does all sorts of crazy unimaginable stuff that is hard to predict, and typically I
> > > run it on several mountpoints all pointing to the same spot
> > > (now 3-ways: one instance on direct mount point in ext4, one on nfs mount mounted as
> > > mount -o nfs and one on mount -o nfs4).
> > > It must be hitting some rare race or a race + operations combination that leads to
> > > the leak.
> > >
> > > To reproduce I run it like this until it stops (and it usually stops in
> > > 2 or 3 iterations, so it takes some time):
> > > $ cat ~/stress.sh
> > > #!/bin/bash
> > >
> > > set -x
> > >
> > > cd /home/green/git/lustre-release/lustre/tests/racer
> > > dd if=/dev/zero of=/tmp/loop bs=1024k count=1024
> > > mkfs.ext4 /tmp/loop
> > > service rpcbind start
> > > mount none /var/lib/nfs -t tmpfs
> > > touch /var/lib/nfs/etab
> > > service nfs-mountd start
> > >
> > > while :; do
> > >
> > > mount /tmp/loop /mnt/lustre -o loop || exit 1
> > > mkdir /mnt/lustre/racer
> > > service nfs-server start || exit 2
> > > mount localhost:/mnt/lustre /mnt/nfs -t nfs -o nolock || exit 3
> > > mount localhost:/ /mnt/nfs2 -t nfs4 || exit 4
> > > DURATION=3600 sh racer.sh /mnt/nfs/racer &
> > > DURATION=3600 sh racer.sh /mnt/nfs2/racer &
> > > DURATION=3600 sh racer.sh /mnt/lustre/racer &
> > > wait %1 %2 %3
> > > umount /mnt/nfs || exit 5
> > > umount /mnt/nfs2 || exit 6
> > > service nfs-server stop || exit 7
> > > umount /mnt/lustre || exit 8
> > > e2fsck -n -f /tmp/loop || exit 9
> > > done
> > >
> > > Please ignore all the Lustre references, the test is generic, it's just I am running
> > > off nfs-readonly-root setup, so want to reuse existing mountpoint locations,
> > > there's no Lustre in picture here).
> > > With all the other problems from before fixed, now it typically stops either for
> > > this leak discussed here, or for this other problem in nfs4 I described here:
> > > http://marc.info/?l=linux-nfs&m=146518324823160&w=2
> > Thanks. So it seems likely we're looking at a race of some sort then...
> >
> > So, the way this is supposed to work is that the stateids each hold a
> > reference to the nfs4_file. They also take an fi_access reference for
> > the open modes that they represent. This is how we know when it's OK to
> > close one of the files in the fd[] slots. It might be interesting to
> > also print out the fi_access counters if the slot hasn't been zeroed
> > out if you feel up to it.
> Ok, I have primed that and will let you know next time it triggers.
>

Thanks. Maybe that will give us some idea of where to look...

> >
> > I think though that the open upgrade accounting may be wrong.
> > __nfs4_file_get_access does an unconditional atomic_inc, so if the
> > client does a open for write and then opens again for read/write, then
> > we may end up with too many write fi_access references taken.
>
> The read-write access is all fine I think, because when dropping read or write
> it also checks if the readwrite is filled?
> Also whoever did that atomic_inc for get_access would eventually do put,
> and that should free the file, no?
> I was mostly thinking there's an error exit path somewhere that forgets to do
> the put.
> This also would explain the other pieces leaked that are not directly attached
> into the nfs4_file.
>
> >
> > That said, this code is quite subtle. I'd need to look over it in more
> > detail before I offer up any fixes. I'd also appreciate it if anyone
> > else wants to sanity check my analysis there.
> >

Yeah, I think you're right. It's fine since r/w opens have a distinct
slot, even though the refcounting just tracks the number of read and
write references. So yeah, the leak probably is in an error path
someplace, or maybe a race someplace.

--
Jeff Layton <jlayton@poochiereds.net>