[PATCH v8 7/9] nfs/direct: add misaligned WRITE handling

[Mike Snitzer <snitzer@kernel.org>]

Because the NFS client will already happily handle misaligned O_DIRECT
IO (by sending it out to NFSD via RPC) this commit's new capabilities
are for the benefit of LOCALIO and require the nfs modparam:
  localio_O_DIRECT_align_misaligned_IO=Y

When enabled, a misaligned DIO WRITE is split into a head, middle and
tail as needed. The large middle extent is DIO-aligned and the head
and/or tail are misaligned (due to each being a partial page).

The misaligned head and/or tail extents are issued using buffered IO
and the DIO-aligned middle is issued using O_DIRECT.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 fs/nfs/direct.c | 40 +++++++++++++++++++++++++++++++++++-----
 1 file changed, 35 insertions(+), 5 deletions(-)

diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
index fc011571c5d29..3803289a94793 100644
--- a/fs/nfs/direct.c
+++ b/fs/nfs/direct.c
@@ -963,8 +963,15 @@ static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
 		if (result < 0)
 			break;
 
-		bytes = result;
-		npages = (result + pgbase + PAGE_SIZE - 1) / PAGE_SIZE;
+		/* Limit the amount of bytes serviced each iteration to aligned batches */
+		if (pos < dreq->middle_offset && dreq->start_len)
+			bytes = min_t(size_t, dreq->start_len, result);
+		else if (pos < dreq->end_offset && dreq->middle_len)
+			bytes = min_t(size_t, dreq->middle_len, result);
+		else
+			bytes = result;
+		npages = (bytes + pgbase + PAGE_SIZE - 1) / PAGE_SIZE;
+
 		for (i = 0; i < npages; i++) {
 			struct nfs_page *req;
 			unsigned int req_len = min_t(size_t, bytes, PAGE_SIZE - pgbase);
@@ -983,6 +990,7 @@ static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
 			}
 
 			pgbase = 0;
+			result -= req_len;
 			bytes -= req_len;
 			requested_bytes += req_len;
 			pos += req_len;
@@ -992,9 +1000,28 @@ static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
 				continue;
 			}
 
+			/* Issue IO if this req was the end of the start or middle */
+			if (bytes == 0) {
+				if ((dreq->start_len &&
+				     pos == dreq->middle_offset && result >= dreq->middle_len) ||
+				    (dreq->end_len &&
+				     pos == dreq->end_offset && result == dreq->end_len))
+					desc.pg_doio_now = 1;
+			}
+
 			nfs_lock_request(req);
-			if (nfs_pageio_add_request(&desc, req))
+			if (nfs_pageio_add_request(&desc, req)) {
+				if (desc.pg_doio_now) {
+					/* Reset and handle iter to next aligned boundary */
+					iov_iter_revert(iter, result);
+					desc.pg_doio_now = 0;
+					break;
+				}
 				continue;
+			}
+
+			if (unlikely(desc.pg_doio_now))
+				desc.pg_doio_now = 0;
 
 			/* Exit on hard errors */
 			if (desc.pg_error < 0 && desc.pg_error != -EAGAIN) {
@@ -1092,8 +1119,11 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, struct iov_iter *iter,
 		goto out;
 
 	dreq->inode = inode;
-	dreq->max_count = count;
-	dreq->io_start = pos;
+	if (swap || !nfs_analyze_dio(pos, count, dreq)) {
+		dreq->max_count = count;
+		dreq->io_start = pos;
+	}
+
 	dreq->ctx = get_nfs_open_context(nfs_file_open_context(iocb->ki_filp));
 	l_ctx = nfs_get_lock_context(dreq->ctx);
 	if (IS_ERR(l_ctx)) {
-- 
2.44.0