[PATCH v2 1/9] NFSD: mark the direct middle of a split WRITE IOCB_DONTCACHE as well

[Mike Snitzer <snitzer@kernel.org>]

nfsd_write_dio_iters_init() issues the DIO-aligned middle of a
misaligned WRITE with IOCB_DIRECT.  A file system is free to service
that request with buffered I/O instead: xfs_file_write_iter() retries
via xfs_file_buffered_write() with the same kiocb whenever
xfs_file_dio_write() returns -ENOTBLK, which iomap_dio_rw() produces
when the invalidation it runs before issuing the direct I/O cannot drop
page cache overlapping the range.

That happens routinely in a streaming misaligned WRITE workload with
more than one request in flight.  Request N's direct middle ends inside
the same page that request N+1's buffered prefix starts in.  If N+1's
prefix dirties that page between N's filemap_write_and_wait_range() and
N's invalidate_inode_pages2_range(), the invalidation returns -EBUSY,
iomap returns -ENOTBLK, and XFS re-issues N's entire middle, tens of
kilobytes, as a normal cached buffered write.  Nothing ever drops those
pages.  On a device with dma_alignment=3 (XFS then reports
dio_mem_align=4, which XDR-aligned RPC payloads always satisfy) this is
the only remaining path by which a stream of small misaligned WRITEs in
DIRECT mode fills the page cache.

Set IOCB_DONTCACHE on the middle segment alongside IOCB_DIRECT when the
file system supports FOP_DONTCACHE.  The direct path ignores the flag,
and generic_write_sync() only issues a harmless flusher kick; but if
the file system falls back to buffered I/O the write is now DONTCACHE
rather than cached.  The nfsd_write_direct trace event is unchanged
since it keys on IOCB_DIRECT; the fallback itself remains observable
via the iomap_dio_invalidate_fail event.

Fixes: 06c5c97293e3 ("NFSD: Implement NFSD_IO_DIRECT for NFS WRITE")
Assisted-by: Claude:claude-fable-5-1
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 Documentation/filesystems/nfs/nfsd-io-modes.rst | 10 ++++++++++
 fs/nfsd/vfs.c                                   |  9 +++++++++
 2 files changed, 19 insertions(+)

diff --git a/Documentation/filesystems/nfs/nfsd-io-modes.rst b/Documentation/filesystems/nfs/nfsd-io-modes.rst
index 0fd6e82478fe6..dc50c930f9762 100644
--- a/Documentation/filesystems/nfs/nfsd-io-modes.rst
+++ b/Documentation/filesystems/nfs/nfsd-io-modes.rst
@@ -130,6 +130,16 @@ Misaligned WRITE:
     segments because using normal buffered IO offers significant RMW
     performance benefit when handling streaming misaligned WRITEs.
 
+    The O_DIRECT middle segment also carries the DONTCACHE flag. It has
+    no effect while the IO really is O_DIRECT, but a filesystem may
+    decide on its own to service the segment with buffered IO instead
+    (XFS does so when it cannot invalidate page cache that overlaps the
+    segment, which can happen when another WRITE's buffered start or
+    end segment dirties the shared boundary page at the same time).
+    The flag makes that fallback DONTCACHE buffered IO rather than
+    normal buffered IO. Such fallbacks are visible through the
+    iomap_dio_invalidate_fail trace event; see Tracing below.
+
 Tracing:
     The nfsd_read_direct trace event shows how NFSD expands any
     misaligned READ to the next DIO-aligned block (on either end of the
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index f9131827d391e..5b963f0e3b3f2 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1341,6 +1341,15 @@ 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;
+	/*
+	 * Also mark the direct middle DONTCACHE: the file system may fall
+	 * back to buffered I/O on its own (e.g. XFS on -ENOTBLK when it
+	 * cannot invalidate page cache that a concurrent buffered prefix or
+	 * suffix of an adjacent WRITE just dirtied), and it reuses this kiocb
+	 * to do so.  On the direct path itself the flag is inert.
+	 */
+	if (nf->nf_file->f_op->fop_flags & FOP_DONTCACHE)
+		segments[nsegs].flags |= IOCB_DONTCACHE;
 	nsegs++;
 
 	if (suffix)
-- 
2.52.0