Re: [PATCH v1 3/3] NFSD: Implement NFSD_IO_DIRECT for NFS READ

[Mike Snitzer <snitzer@kernel.org>]

On Tue, Sep 09, 2025 at 03:05:25PM -0400, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> Add an experimental option that forces NFS READ operations to use
> direct I/O instead of reading through the NFS server's page cache.
> 
> There are already other layers of caching:
>  - The page cache on NFS clients
>  - The block device underlying the exported file system
> 
> The server's page cache, in many cases, is unlikely to provide
> additional benefit. Some benchmarks have demonstrated that the
> server's page cache is actively detrimental for workloads whose
> working set is larger than the server's available physical memory.
> 
> For instance, on small NFS servers, cached NFS file content can
> squeeze out local memory consumers. For large sequential workloads,
> an enormous amount of data flows into and out of the page cache
> and is consumed by NFS clients exactly once -- caching that data
> is expensive to do and totally valueless.
> 
> For now this is a hidden option that can be enabled on test
> systems for benchmarking. In the longer term, this option might
> be enabled persistently or per-export. When the exported file
> system does not support direct I/O, NFSD falls back to using
> either DONTCACHE or buffered I/O to fulfill NFS READ requests.
> 
> Suggested-by: Mike Snitzer <snitzer@kernel.org>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Reviewed-by: Mike Snitzer <snitzer@kernel.org>