Random IO errors on nfs clients running linux > 4.20

[Alberto Gonzalez Iniesta <alberto.gonzalez@udima.es>]

Hello NFS maintainers,

I'm sorry for reporting this (a little bit) late, but it took us (Miguel
in Cc:) some time to track this issue to an exact kernel update.

We're running a +200 clients NFS server with Ubuntu 16.04 and 18.04
clients. The server runs Debian 8.11 (jessie) with Linux 3.16.0 and
nfs-kernel-server 1:1.2.8-9+deb8u1. It has been working some years now
without issues.

But since we started moving clients from Ubuntu 16.04 to Ubuntu 18.04
some of them started experiencing failures while working on NFS mounts.
The failures are arbitrary and sometimes it may take more than 20 minutes
to come out (which made finding out which kernel version introduced
this a pain). We are almost sure that some directories are more prone to
suffer from this than others (maybe related to path length/chars?).

The error is also not very "verbose", from an strace:

execve("/bin/ls", ["ls", "-lR", "Becas y ayudas/"], 0x7ffccb7f5b20 /* 16 vars */) = 0
[lots of uninteresting output]
openat(AT_FDCWD, "Becas y ayudas/", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
fstat(3, {st_mode=S_IFDIR|0775, st_size=4096, ...}) = 0
fstat(3, {st_mode=S_IFDIR|0775, st_size=4096, ...}) = 0
fstat(1, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
ioctl(1, TCGETS, 0x7ffd8b725c80)        = -1 ENOTTY (Inappropriate ioctl for device)
getdents(3, /* 35 entries */, 32768)    = 1936
[lots of lstats)
lstat("Becas y ayudas/Convocatorias", {st_mode=S_IFDIR|0775, st_size=4096, ...}) = 0
getdents(3, 0x561af78de890, 32768)      = -1 EIO (Input/output error)

(I can send you the full output if you need it)

We can run the previous "ls -lR" 20 times and get no error, or get
this "ls: leyendo el directorio 'Becas y ayudas/': Error de entrada/salida"
(ls: reading directorio 'Becas y ayudas/': Input/Output Error") every
now and then.

The error happens (obviously?) with ls, rsync and the users's GUI tools.

There's nothing in dmesg (or elsewhere).
These are the kernels with tried:
4.18.0-25   -> Can't reproduce
4.19.0      -> Can't reproduce
4.20.17     -> Happening (hard to reproduce)
5.0.0-15    -> Happening (hard to reproduce)
5.3.0-45    -> Happening (more frequently)
5.6.0-rc7   -> Reproduced a couple of times after boot, then nothing

We did long (as in daylong) testing trying to reproduce this with all
those kernel versions, so we are pretty sure 4.18 and 4.19 don't
experience this and our Ubuntu 16.04 clients don't have any issue.

I know we aren't providing much info but we are really looking forward
to doing all the testing required (we already spent lots of time in it).

Thanks for your work.

Regards,

Alberto

-- 
Alberto González Iniesta             | Universidad a Distancia
alberto.gonzalez@udima.es            | de Madrid