[PATCH 0/3] Support negative dentry cache for FUSE and virtiofs

[Keiichi Watanabe <keiichiw@chromium.org>]

This patch series adds a new mount option called negative_dentry_timeout
for FUSE and virtio-fs filesystems. This option allows the kernel to cache
negative dentries, which are dentries that represent a non-existent file.
When this option is enabled, the kernel will skip FUSE_LOOKUP requests for
second and subsequent lookups to a non-existent file.

Unlike negative dentry caches on a normal filesystem such as ext4, the
kernel may need to refresh the cache for FUSE and virtio-fs filesystems.
This is because the kernel does not know when a FUSE server or a virtio-fs
device creates or deletes files. To address this, the new
negative_dentry_timeout option takes an expiration time for cache entries.
The appropriate timeout duration should be determined by considering how
often a FUSE server updates file paths and the amount of memory the kernel
can use for the cache.

As we evaluated the virtio-fs's performance on a guest Linux on crosvm
[1]'s virtiofs device[2], the `negative_dentry_timeout` option saved ~1
second per 10000 `stat` call against a non-existent path. The experiment
settings and results are as follows:

* Command to start VM with crosvm:
sudo crosvm run \
  --disable-sandbox \
  --cpus 1 \
  --mem 2048 \
  --rwroot debian.img \
  --shared-dir \
  /path/:my_virtiofs:type=fs:cache=always:timeout=3600 \
  -p "console=hvc0 init=/bin/bash" \
  vmlinux

* Mount command in the guest
Default:
$ mount -t virtiofs my_virtiofs ./workspace/
With negative dentry cache:
$ mount -t virtiofs -o negative_dentry_timeout=10 my_virtiofs ./workspace/

* Test commands
$ cd workspace
$ echo 3 > /proc/sys/vm/drop_caches
$ time for i in `seq 1 10000`; \
  do stat non-existent.txt 2> /dev/null; \
  done

* Results:
Default:
real 0m12.606s
user 0m3.624s
sys 0m7.756s

With `-o negative_dentry_timeout=10`:
real 0m11.276s
user 0m3.514s
sys 0m7.544s

[1]: https://crosvm.dev/book/
[2]: https://crosvm.dev/book/devices/fs.html


Keiichi Watanabe (3):
  docs: virtiofs: Fix descriptions about virtiofs mount option
  fuse: Add negative_dentry_timeout option
  virtiofs: Add negative_dentry_timeout option

 Documentation/filesystems/dax.rst      |  1 +
 Documentation/filesystems/fuse.rst     |  6 ++++++
 Documentation/filesystems/virtiofs.rst |  9 ++++++++-
 fs/fuse/dir.c                          |  3 ++-
 fs/fuse/fuse_i.h                       |  4 ++++
 fs/fuse/inode.c                        | 12 +++++++++++-
 fs/fuse/virtio_fs.c                    |  8 ++++++++
 7 files changed, 40 insertions(+), 3 deletions(-)

-- 
2.41.0.185.g7c58973941-goog