Re: [RFC PATCH v2 6/8] NFSD: add io_cache_read controls to debugfs interface

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jul 08, 2025 at 12:06:17PM -0400, Mike Snitzer wrote:
> 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).
> - not cached (NFSD_IO_DIRECT=2)
> 
> io_cache_read is 0 by default.  It may be set by writing to:
>   /sys/kernel/debug/nfsd/io_cache_read
> 
> If NFSD_IO_DONTCACHE is specified using 1, FOP_DONTCACHE must be
> advertised as supported by the underlying filesystem (e.g. XFS),
> otherwise all IO flagged with RWF_DONTCACHE will fail with
> -EOPNOTSUPP.
> 
> If NFSD_IO_DIRECT is specified using 2, the IO must be aligned
> relative to the underlying block device's logical_block_size. Also the
> memory buffer used to store the read must be aligned relative to the
> underlying block device's dma_alignment.

Does this also need some kind of check that direct I/O is supported
at all by the file system / fops instance?