[PATCH v4 02/10] NFSD: mark the direct middle of a split WRITE IOCB_DONTCACHE as well

[Mike Snitzer <snitzer@kernel.org>]

nfsd_write_dio_iters_init() marks the DIO-aligned middle of a misaligned
WRITE IOCB_DIRECT, and nfsd_direct_write() issues it that way.  A file
system may service that request with buffered I/O instead:
xfs_file_write_iter() retries through xfs_file_buffered_write() with the
same kiocb when xfs_file_dio_write() returns -ENOTBLK, which
iomap_dio_rw() does when it cannot invalidate page cache overlapping the
range.

Since the previous commit, the middle of a split WRITE shares no page
with the buffered ends of its neighbours, so a streaming misaligned
WRITE workload no longer causes this by itself.  It still happens
whenever something dirties a page in the middle's range between its
writeback and its invalidation without taking the lock the file system
holds across a direct write: a local process writing the exported file
through mmap, which XFS's IOLOCK does not keep out, or, on a file system
that does not hold such a lock for the length of a direct write, an
overlapping WRITE that is issued buffered as a whole (too small to
split, or its payload misaligned in memory) or a local buffered writer.
All of the middle, nearly the whole WRITE, is then re-issued as a cached
buffered write that nothing ever drops.

Set IOCB_DONTCACHE on the middle segment alongside IOCB_DIRECT when
the file system supports FOP_DONTCACHE.  The direct path ignores the
flag; a buffered fallback now drops its pages once written back.  Say
so in nfsd-io-modes.rst.

Fixes: 06c5c97293e3 ("NFSD: Implement NFSD_IO_DIRECT for NFS WRITE")
Assisted-by: LLM
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 Documentation/filesystems/nfs/nfsd-io-modes.rst | 4 ++++
 fs/nfsd/vfs.c                                   | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/Documentation/filesystems/nfs/nfsd-io-modes.rst b/Documentation/filesystems/nfs/nfsd-io-modes.rst
index 1916b2eee58ad..f9b63f1a33990 100644
--- a/Documentation/filesystems/nfs/nfsd-io-modes.rst
+++ b/Documentation/filesystems/nfs/nfsd-io-modes.rst
@@ -132,6 +132,10 @@ Misaligned WRITE:
     one; a start or end that is already DIO-aligned gets no buffered
     segment at all. If the filesystem supports FOP_DONTCACHE, the
     misaligned segments are marked DONTCACHE.
+    So is the O_DIRECT segment, where the flag has no effect unless the
+    filesystem services it with buffered IO instead, as XFS does when it
+    cannot invalidate page cache that overlaps the segment. The
+    iomap_dio_invalidate_fail trace event reports such a fallback.
 
 Tracing:
     The nfsd_read_direct trace event shows how NFSD expands any
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index ed5903c3becac..bd3dab36e2962 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1375,6 +1375,9 @@ nfsd_write_dio_iters_init(struct nfsd_file *nf, struct bio_vec *bvec,
 	if (iov_iter_bvec_offset(&segments[nsegs].iter) & (mem_align - 1))
 		goto no_dio;
 	segments[nsegs].flags |= IOCB_DIRECT;
+	/* In case the file system falls back to buffered I/O (-ENOTBLK). */
+	if (nf->nf_file->f_op->fop_flags & FOP_DONTCACHE)
+		segments[nsegs].flags |= IOCB_DONTCACHE;
 	nsegs++;
 
 	if (suffix)
-- 
2.52.0