[RFC 0/2] tmpfs: add the option to disable swap

[Luis Chamberlain <mcgrof@kernel.org>]

Many folks suggest using tmpfs is not great because it can use swap.
That's not a good reason to *not* use tmpfs, what's just missing is just
the option to let you disable it. And so this does that, to enable that
and also let users experiment with it.

Reconfiguring is not supported, so what you set up first is what you can keep
with regards to swap options. Enabling support to disable on the fly and
add swap on the fly *is* possible, but I'm not sure we need that yet.

Matthew -- the first patch just makes it easier to read the second, but it does
beg a few questions in it with regards to semantics about the folio and
the inode after splitting a huge page so review for that would be
greatly appreciated.

To test I've used kdevops [0] 8 vpcu 4 GiB libvirt guest on linux-next.

I'm doing this work as part of future experimentation with tmpfs and the
page cache, but given a common complaint found about tmpfs is the
innability to work without the page cache I figured this might be useful
to others. It would also prove useful if folks ask for patches for future
experimentation we are doing.

To see if you hit swap:

mkswap /dev/nvme2n1
swapon /dev/nvme2n1
free -h

With swap - what we see today
=============================
mount -t tmpfs            -o size=5G           tmpfs /data-tmpfs/
dd if=/dev/urandom of=/data-tmpfs/5g-rand2 bs=1G count=5
free -h
               total        used        free      shared  buff/cache   available
Mem:           3.7Gi       2.6Gi       1.2Gi       2.2Gi       2.2Gi       1.2Gi
Swap:           99Gi       2.8Gi        97Gi


Without swap
=============

free -h
               total        used        free      shared  buff/cache   available
Mem:           3.7Gi       387Mi       3.4Gi       2.1Mi        57Mi       3.3Gi
Swap:           99Gi          0B        99Gi
mount -t tmpfs            -o size=5G -o noswap tmpfs /data-tmpfs/
dd if=/dev/urandom of=/data-tmpfs/5g-rand2 bs=1G count=5
free -h
               total        used        free      shared  buff/cache   available
Mem:           3.7Gi       2.6Gi       1.2Gi       2.3Gi       2.3Gi       1.1Gi
Swap:           99Gi        21Mi        99Gi


The mix and match remount testing
=================================

# Cannot disable swap after it was first enabled:
mount -t tmpfs            -o size=5G           tmpfs /data-tmpfs/
mount -t tmpfs -o remount -o size=5G -o noswap tmpfs /data-tmpfs/
mount: /data-tmpfs: mount point not mounted or bad option.
       dmesg(1) may have more information after failed mount system call.
dmesg -c
tmpfs: Cannot disable swap on remount

# Remount with the same noswap option is OK:
mount -t tmpfs            -o size=5G -o noswap tmpfs /data-tmpfs/
mount -t tmpfs -o remount -o size=5G -o noswap tmpfs /data-tmpfs/
dmesg -c

# Trying to enable swap with a remount after it first disabled:
mount -t tmpfs            -o size=5G -o noswap tmpfs /data-tmpfs/
mount -t tmpfs -o remount -o size=5G           tmpfs /data-tmpfs/
mount: /data-tmpfs: mount point not mounted or bad option.
       dmesg(1) may have more information after failed mount system call.
dmesg -c
tmpfs: Cannot enable swap on remount if it was disabled on first mount

[0] https://github.com/linux-kdevops/kdevops

Luis Chamberlain (2):
  shmem: set shmem_writepage() variables early
  shmem: add support to ignore swap

 include/linux/shmem_fs.h |  1 +
 mm/shmem.c               | 33 ++++++++++++++++++++++++++++++---
 2 files changed, 31 insertions(+), 3 deletions(-)

-- 
2.39.0