nfs4.2: open UNCACHEABLE_FILE_DATA files with O_DIRECT

Honor the per-file UNCACHEABLE_FILE_DATA attribute by transparently
opening such regular files with O_DIRECT, so reads and writes bypass the
page cache as the attribute requires, without the application having to
request O_DIRECT itself.

This follows the model the specification describes: the attribute is
"similar in intent to O_DIRECT" and clients "retain flexibility in how
they satisfy the requirements" (draft-ietf-nfsv4-uncacheable-files
Section 4.4, "Relationship to Direct I/O"), and its Implementation
Status (Section 6) describes a prototype Linux client that "treats the
attribute as an indication to use O_DIRECT-like behavior for file
access".

Introduce an NFS_CONTEXT_O_DIRECT open-context flag: nfs4_atomic_open()
sets it when the resolved inode has uncacheable_file_data set (and the
open is not O_APPEND), and the open paths nfs_atomic_open() and
nfs4_file_open() apply O_DIRECT to the file when the flag is set.

The I/O mode is thus selected at open time and is not changed for an
already-open file: a later change to the attribute takes effect on the
next open.  The specification permits this -- a client that has already
opened a file MAY continue with its existing caching behavior and apply
the updated attribute to subsequent operations (Section 5).

The delegation interaction in Section 4.3 was considered: it permits read
caching to remain when another NFSv4.2 mechanism, such as a delegation,
already ensures a consistent view of the file.  That relaxation is
optional ("may remain appropriate") and read-only -- it does not relax
write-behind suppression (Section 4.1) or the WRITE durability invariant
(Section 4.2).  This implementation deliberately does not take it: an
uncacheable file is opened O_DIRECT regardless of any delegation held,
which is compliant (read caching is simply suppressed more aggressively
than the Section 4.3 minimum) and avoids decoupling read vs write caching
behind a single open flag.  Relaxing reads under a delegation is left as
a possible future optimization.

Section 6 observes the benefit holds "for applications that issue
well-formed I/O requests".  That alignment caveat does not constrain the
Linux NFS client's over-the-wire path: the client readily issues
misaligned I/O using O_DIRECT over SunRPC to the remote NFS server.  The
only place a fallback from O_DIRECT to buffered I/O for misaligned I/O
applies is NFS LOCALIO (fs/nfs/localio.c), which detects non-DIO-aligned
I/O and falls back internally; that path is unaffected by this change.

Link: https://datatracker.ietf.org/doc/draft-ietf-nfsv4-uncacheable-files/

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

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