[LSF/MM/BPF TOPIC] Implementing the NFS v4.2 WRITE_SAME operation: VFS or NFS ioctl() ?

[Anna Schumaker <anna.schumaker@oracle.com>]

I've seen a few requests for implementing the NFS v4.2 WRITE_SAME [1] operation over the last few months [2][3] to accelerate writing patterns of data on the server, so it's been in the back of my mind for a future project. I'll need to write some code somewhere so NFS & NFSD can handle this request. I could keep any implementation internal to NFS / NFSD, but I'd like to find out if local filesystems would find this sort of feature useful and if I should put it in the VFS instead.

I was thinking I could keep it simple, and model a function call based on write(3) / pwrite(3) to write some pattern N times starting at either the file's current offset or at a user-provide offset. Something like:
    write_pattern(int filedes, const void *pattern, size_t nbytes, size_t count);
    pwrite_pattern(int filedes, const void *pattern, size_t nbytes, size_t count, offset_t offset);

I could then construct a WRITE_SAME call in the NFS client using this information. This seems "good enough" to me for what people have asked for, at least as a client-side interface. It wouldn't really help the server, which would still need to do several writes in a loop to be spec-compliant with writing the pattern to an offset inside the "application data block" [4] structure.

But maybe I'm simplifying this too much, and others would find the additional application data block fields useful? Or should I keep it all inside NFS, and call it with an ioctl instead of putting it into the VFS?

Thoughts?
Anna

[1]: https://datatracker.ietf.org/doc/html/rfc7862#section-15.12
[2]: https://lore.kernel.org/linux-nfs/CAAvCNcByQhbxh9aq_z7GfHx+_=S8zVcr9-04zzdRVLpLbhxxSg@mail.gmail.com/
[3]: https://lore.kernel.org/linux-nfs/CALWcw=Gg33HWRLCrj9QLXMPME=pnuZx_tE4+Pw8gwutQM4M=vw@mail.gmail.com/
[4]: https://datatracker.ietf.org/doc/html/rfc7862#section-8.1