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

[Mike Snitzer <snitzer@kernel.org>]

On Thu, Jul 10, 2025 at 12:47:43AM -0700, Christoph Hellwig wrote:
> 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?

Long-term: definitely.  Near-term: not going to add it because it'd
have to be checked on a per-IO basis (because this debugfs-based
interface is a global setting so we don't have a specific export to
check at the time the request to use O_DIRECT is made via debugfs).

Once we graduate to having a per-export control interface in NFSD we
can then verify export's underlying filesystem supports O_DIRECT.

Hope this makes sense, thanks.

Mike