In FUSE, as of now, uncached lookups are expensive over the wire. E.g additional latencies and stressing (meta data) servers from thousands of clients. With atomic-open lookup before open can be avoided. Here is the link to performance numbers https://lore.kernel.org/linux-fsdevel/20220322121212.5087-1-dharamhans87@xxxxxxxxx/ Here is the libfuse pull request https://github.com/libfuse/libfuse/pull/813 The patches are passing passthrough_hp xfstests (libfuse part applied), although we had to introduce umount retries into xfstests, as recent kernels/xfstests fail umount in some tests with EBUSY - independent of atomic open. (Although outstanding for v7) I'm especially interested in Al's and Christians opinion about the atomic open dentry revalidation in v7. If the vfs changes are acceptable, would it be possible to also look at the other patches and their vfs/dcache interaction? I __hope__ I got it right and I hope the vfs can be accepted. v7: - Indentation and style fixes for _fuse_atomic_open. - Remodel atomic open to avoid races with parallel lookup, similar to NFS commit c94c09535c4debcc439f55b5b6d9ebe57bd4665a and what is done in _nfs4_open_and_get_state() A WARN_ONCE() and fallback is added to ensure operation is on negative dentries only. - Error handling is done via the fallback fuse_create_open() to reduce complexity and code duplication. - Remove entry cache invalidation on ENOENT in the atomic-open patch, as atomic-open so far operates on negative dentries only. - Remove fuse_advise_use_readdirplus() in _fuse_atomic_open (Thanks Miklos) - Add forgotten free_ext_value() (Thanks Miklos). - Declare struct fuse_inode per condition as the value needs to be retrieved anyway per condition. - Added atomic open-revalidation and required vfs changes - Added myself (Bernd) as Co-developed-by to Dharmendras patches, as I did substantial modifications. - More as reminder for myself, so far these tests below are done manually or with custom scripts, I think we need xfstests for these. With updated libfuse /scratch/dest is mounted by: passthrough_hp -o allow_other --foreground --debug-fuse /scratch/source /scratch/dest 1) Test atomic open (file create) and negative dentry open rm -f /scratch/source/file # ensure file does not exist mount /scratch/dest # overlay of /scratch source echo a > /scratch/dest/file # non-existing file umount and mount /scratch/test (clear cache) cat /scratch/dest/file rm -f /scratch/dest/file 2) Test dir open mkdir /scratch/source/dir mount /scratch/dest # overlay of /scratch source cat /scratch/source/dir rmdir /scratch/source/dir 3) Test revalidate without file change mount /scratch/dest echo "a" > /scratch/dest/file echo "b" >> /scratch/dest/file echo "c" >> /scratch/dest/file cat /scratch/dest/file rm -f /scratch/dest/file 4) Test revalidate by underlying file change mount /scratch/dest echo "a" > /scratch/dest/file cat /scratch/dest/file rm -f /scratch/source/file # unknown to dest mount str="b" echo "${str}" > /scratch/source/file reval=$(cat /scratch/dest/file) if [ "$str" != "reval" ]; then echo "String mismatch after revalidation" exit 1 fi rm -f /scratch/dest/file 5) Test revalidate by underlying file change, but with O_CREATE included. Tests dentry creation by the atomic revalidate mount /scratch/dest echo "a" >> /scratch/dest/file rm -f /scratch/source/file echo "b" > /scratch/source/file # revalidate includes O_CREATE echo "c" >> /scratch/dest/file 6) Repeat above tests, but with additional "--nocache" passthrough_hp option v6: Addressed Miklos comments and rewrote atomic open into its own function. Also dropped for now is the revalidate optimization, we have the code/patch, but it needs more testing. Also easier to first agree on atomic open and then to land the next optimization. v5: Addressed comments v3: Addressed comments v4: Addressed all comments and refactored the code into 3 separate patches respectively for Atomic create, Atomic open, optimizing lookup in d_revalidate(). v3: handle review comments v2: fixed a memory leak in <fuse_atomic_open_common> Bernd Schubert (5): fuse: rename fuse_create_open fuse: introduce atomic open fuse: Revalidate positive entries in fuse_atomic_open fuse: revalidate Set DCACHE_ATOMIC_OPEN for cached dentries fuse: Avoid code duplication in atomic open Miklos Szeredi (1): [RFC] Allow atomic_open() on positive dentry fs/fuse/dir.c | 398 +++++++++++++++++++++++++++++++++++++- fs/fuse/fuse_i.h | 6 + fs/namei.c | 17 +- include/linux/dcache.h | 6 + include/linux/namei.h | 1 + include/uapi/linux/fuse.h | 3 + 6 files changed, 422 insertions(+), 9 deletions(-) Cc: Miklos Szeredi <miklos@xxxxxxxxxx> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx> Cc: Christian Brauner <brauner@xxxxxxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Dharmendra Singh <dsingh@xxxxxxx> Cc: Horst Birthelmer <hbirthelmer@xxxxxxx> Cc: linux-fsdevel@xxxxxxxxxxxxxxx -- 2.34.1
