Re: NFSD: Add io_cache_{read,write} controls to debugfs

[Mike Snitzer <snitzer@kernel.org>]

On Fri, Sep 05, 2025 at 10:55:09AM -0400, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>

Aside from attribution, which I suspect you didn't intend to switch
these changes from being attributed to me, this looks good (one small
nit below).

> Add 'io_cache_read' to NFSD's debugfs interface so that any data
> read by NFSD will either be:
> - cached using page cache (NFSD_IO_BUFFERED=0)
> - cached but removed from the page cache upon completion
>   (NFSD_IO_DONTCACHE=1).
> 
> io_cache_read may be set by writing to:
>   /sys/kernel/debug/nfsd/io_cache_read
> 
> Add 'io_cache_write' to NFSD's debugfs interface so that any data
> written by NFSD will either be:
> - cached using page cache (NFSD_IO_BUFFERED=0)
> - cached but removed from the page cache upon completion
>   (NFSD_IO_DONTCACHE=1).
> 
> io_cache_write may be set by writing to:
>   /sys/kernel/debug/nfsd/io_cache_write
> 
> The default value for both settings is NFSD_IO_BUFFERED, which is
> NFSD's existing behavior for both read and write. Changes to these
> settings take immediate effect for all exports and NFS versions.
> 
> If NFSD_IO_DONTCACHE is specified, all exported filesystems must
> implement FOP_DONTCACHE, otherwise IO flagged with RWF_DONTCACHE
> will fail with -EOPNOTSUPP.
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/debugfs.c | 93 +++++++++++++++++++++++++++++++++++++++++++++++
>  fs/nfsd/nfsd.h    |  9 +++++
>  fs/nfsd/vfs.c     | 19 ++++++++++
>  3 files changed, 121 insertions(+)
> 
> Changes from Mike's v9:
> - Squashed the "io controls" patches together
> - Removed NFSD_IO_DIRECT for the moment
> - Addressed a few more checkpatch.pl nits
> 
> This gives a cleaner platform on which to build the direct I/O code
> paths, and does not expose partially implemented I/O modes to users.
>
> diff --git a/fs/nfsd/debugfs.c b/fs/nfsd/debugfs.c
> index 84b0c8b559dc..2b1bb716b608 100644
> --- a/fs/nfsd/debugfs.c
> +++ b/fs/nfsd/debugfs.c
> @@ -44,4 +131,10 @@ void nfsd_debugfs_init(void)
>  
>  	debugfs_create_file("disable-splice-read", S_IWUSR | S_IRUGO,
>  			    nfsd_top_dir, NULL, &nfsd_dsr_fops);
> +
> +	debugfs_create_file("io_cache_read", 0644, nfsd_top_dir, NULL,
> +			    &nfsd_io_cache_read_fops);
> +
> +	debugfs_create_file("io_cache_write", 0644, nfsd_top_dir, NULL,
> +			    &nfsd_io_cache_write_fops);
>  }

Relative to checkpatch warnings, this ^ code is what I'm aware of.
For consistency I stuck with "S_IWUSR | S_IRUG", whereas you honored
checkpatch's suggestion to use 0644.

Maybe update disable-splice-read to also use 0644 too? But your call!

Thanks,
Mike