Re: [RFC PATCH 0/2] NFSD: add a setting to disable splice reads

[Jeff Layton <jlayton@kernel.org>]

On Sat, 2025-03-08 at 15:14 -0500, cel@kernel.org wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> The usual policy for kernel-user space APIs is that once a public
> API appears, it is difficult to change or remove, in order to
> maintain backwards compatibility with user space software.
> 
> This series introduces /sys/kernel/debug/nfsd/ where we can
> hopefully place ephemeral and undocumented settings for testing NFSD
> features that are to be added or deprecated without the worry of
> having to support an administrative API forever without change,
> amen.
> 
> As a first consumer of this user-kernel API, the series adds a
> simple disable-splice-read setting, which can force all NFS READ
> operations to use vfs_iter_read() rather than page splicing to fill
> data content for an NFS reply.
> 
> The splice read path is the default on most file systems, so it gets
> most of the test experience. The purpose of this new setting is to
> enable test runners to force the use of the iov iter path. We are
> also interested in comparing the performance of the splice and iter
> paths, as a prelude to potentially removing page splicing. This new
> setting makes it easy to benchmark either read mode without having
> to rebuild the kernel.
> 
> We have an eye on a few other consumers, such as uncached I/O and
> increasing the maximum r/wsize, for which /sys/kernel/debug/nfsd
> might be suitable while their performance impact is studied before
> a concrete administrative interface is agreed upon.
> 
> Opinions and code review are welcome, as always.
> 
> Chuck Lever (2):
>   NFSD: Add /sys/kernel/debug/nfsd
>   NFSD: Add experimental setting to disable the use of splice read
> 
>  fs/nfsd/Makefile  |  1 +
>  fs/nfsd/debugfs.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
>  fs/nfsd/nfsctl.c  |  4 ++++
>  fs/nfsd/nfsd.h    | 10 ++++++++++
>  fs/nfsd/vfs.c     |  4 ++++
>  5 files changed, 66 insertions(+)
>  create mode 100644 fs/nfsd/debugfs.c
> 

Looks good to me:

Reviewed-by: Jeff Layton <jlayton@kernel.org>