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
