[PATCH v4 0/3] FUSE: Implement atomic lookup + open/create

[Dharmendra Singh <dharamhans87@gmail.com>]

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. These lookup calls possibly can be avoided
in some cases. Incoming three patches address this issue.


Fist patch handles the case where we are creating a file with O_CREAT.
Before we go for file creation, we do a lookup on the file which is most
likely non-existent. After this lookup is done, we again go into libfuse
to create file. Such lookups where file is most likely non-existent, can
be avoided.

Second patch handles the case where we open first time a file/dir
but do a lookup first on it. After lookup is performed we make another
call into libfuse to open the file. Now these two separate calls into
libfuse can be combined and performed as a single call into libfuse.

Third patch handles the case when we are opening an already existing file
(positive dentry). Before this open call, we re-validate the inode and
this re-validation does a lookup on the file and verify the inode.
This separate lookup also can be avoided (for non-dir) and combined
with open call into libfuse. After open returns we can revalidate the inode.
This optimisation is performed only when we do not have default permissions
enabled.

Here is the link to performance numbers
https://lore.kernel.org/linux-fsdevel/20220322121212.5087-1-dharamhans87@gmail.com/


Dharmendra Singh (3):
  FUSE: Implement atomic lookup + create
  Implement atomic lookup + open
  Avoid lookup in d_revalidate()

 fs/fuse/dir.c             | 211 +++++++++++++++++++++++++++++++++++---
 fs/fuse/file.c            |  30 +++++-
 fs/fuse/fuse_i.h          |  16 ++-
 fs/fuse/inode.c           |   4 +-
 fs/fuse/ioctl.c           |   2 +-
 include/uapi/linux/fuse.h |   5 +
 6 files changed, 246 insertions(+), 22 deletions(-)

---
v4: Addressed all comments and refactored the code into 3 separate patches
    respectively for Atomic create, Atomic open, optimizing lookup in
    d_revalidate().
---