[PATCH v8 10/12] NFSD: Combine direct I/O feasibility check with iterator setup

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

When direct I/O is not feasible (due to missing alignment info,
too-small writes, or no alignment possible), pack the entire
write payload into a single non-DIO segment and follow the usual
direct write I/O path.

This simplifies nfsd_direct_write() by eliminating the fallback path
and the separate nfsd_buffered_write() call - all writes now go
through nfsd_issue_write_dio() which handles both DIO and buffered
segments.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/nfsd/vfs.c | 69 +++++++++++++++++++++------------------------------
 1 file changed, 28 insertions(+), 41 deletions(-)

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index a872be300c9f..be0688f2ab3d 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1304,30 +1304,6 @@ nfsd_find_dio_aligned_offset(struct nfsd_file *nf, loff_t file_offset,
 	return SIZE_MAX;  /* No alignment found */
 }
 
-/*
- * Check if the underlying file system implements direct I/O.
- */
-static bool
-nfsd_is_write_dio_possible(loff_t offset, unsigned long len,
-			   struct nfsd_write_dio_args *args)
-{
-	u32 offset_align = args->nf->nf_dio_offset_align;
-	u32 mem_align = args->nf->nf_dio_mem_align;
-
-	if (unlikely(!mem_align || !offset_align))
-		return false;
-
-	/*
-	 * Need enough data to potentially find an aligned segment.
-	 * In the worst case, we might need up to
-	 * lcm(offset_align, mem_align) bytes for the prefix.
-	 */
-	if (unlikely(len < max(offset_align, mem_align)))
-		return false;
-
-	return true;
-}
-
 static void
 nfsd_write_dio_seg_init(struct nfsd_write_dio_seg *segment,
 			struct bio_vec *bvec, unsigned int nvecs,
@@ -1340,22 +1316,31 @@ nfsd_write_dio_seg_init(struct nfsd_write_dio_seg *segment,
 	segment->use_dio = false;
 }
 
-static bool
-nfsd_setup_write_dio_iters(struct bio_vec *bvec, unsigned int nvecs,
-			   loff_t offset, unsigned long total,
-			   struct nfsd_write_dio_args *args)
+static void
+nfsd_write_dio_iters_init(struct bio_vec *bvec, unsigned int nvecs,
+			  loff_t offset, unsigned long total,
+			  struct nfsd_write_dio_args *args)
 {
 	u32 offset_align = args->nf->nf_dio_offset_align;
+	u32 mem_align = args->nf->nf_dio_mem_align;
 	unsigned long mem_offset = bvec->bv_offset;
 	loff_t prefix_end, orig_end, middle_end;
 	size_t prefix, middle, suffix;
 
 	args->nsegs = 0;
 
+	/*
+	 * Check if direct I/O is feasible for this write request.
+	 * If alignments are not available, the write is too small,
+	 * or no alignment can be found, fall back to buffered I/O.
+	 */
+	if (unlikely(!mem_align || !offset_align) ||
+	    unlikely(total < max(offset_align, mem_align)))
+		goto no_dio;
 	prefix = nfsd_find_dio_aligned_offset(args->nf, offset, mem_offset,
 					     total);
 	if (prefix == SIZE_MAX)
-		return false;	/* No alignment possible */
+		goto no_dio;
 
 	prefix_end = offset + prefix;
 	orig_end = offset + total;
@@ -1371,7 +1356,7 @@ nfsd_setup_write_dio_iters(struct bio_vec *bvec, unsigned int nvecs,
 	}
 
 	if (!middle)
-		return false;	/* No aligned region for DIO */
+		goto no_dio;
 
 	nfsd_write_dio_seg_init(&args->segment[args->nsegs], bvec, nvecs,
 				total, prefix, middle);
@@ -1384,7 +1369,13 @@ nfsd_setup_write_dio_iters(struct bio_vec *bvec, unsigned int nvecs,
 		++args->nsegs;
 	}
 
-	return true;
+	return;
+
+no_dio:
+	/* No alignment possible - pack into single non-DIO segment */
+	nfsd_write_dio_seg_init(&args->segment[0], bvec, nvecs, total,
+				0, total);
+	args->nsegs = 1;
 }
 
 static int
@@ -1405,7 +1396,7 @@ nfsd_buffered_write(struct svc_rqst *rqstp, struct file *file,
 }
 
 static int
-nfsd_issue_write_dio(struct svc_rqst *rqstp, struct svc_fh *fhp, u32 *stable_how,
+nfsd_issue_dio_write(struct svc_rqst *rqstp, struct svc_fh *fhp, u32 *stable_how,
 		     struct kiocb *kiocb, unsigned int nvecs, unsigned long *cnt,
 		     struct nfsd_write_dio_args *args)
 {
@@ -1413,10 +1404,6 @@ nfsd_issue_write_dio(struct svc_rqst *rqstp, struct svc_fh *fhp, u32 *stable_how
 	ssize_t host_err;
 	unsigned int i;
 
-	if (!nfsd_setup_write_dio_iters(rqstp->rq_bvec, nvecs, kiocb->ki_pos,
-					*cnt, args))
-		return nfsd_buffered_write(rqstp, file, nvecs, cnt, kiocb);
-
 	/*
 	 * Any buffered IO issued here will be misaligned, use
 	 * sync IO to ensure it has completed before returning.
@@ -1425,6 +1412,9 @@ nfsd_issue_write_dio(struct svc_rqst *rqstp, struct svc_fh *fhp, u32 *stable_how
 	kiocb->ki_flags |= (IOCB_DSYNC|IOCB_SYNC);
 	*stable_how = NFS_FILE_SYNC;
 
+	nfsd_write_dio_iters_init(rqstp->rq_bvec, nvecs, kiocb->ki_pos,
+				  *cnt, args);
+
 	*cnt = 0;
 	for (i = 0; i < args->nsegs; i++) {
 		if (args->segment[i].use_dio) {
@@ -1463,11 +1453,8 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	if (args.nf->nf_file->f_op->fop_flags & FOP_DONTCACHE)
 		kiocb->ki_flags |= IOCB_DONTCACHE;
 
-	if (nfsd_is_write_dio_possible(kiocb->ki_pos, *cnt, &args))
-		return nfsd_issue_write_dio(rqstp, fhp, stable_how, kiocb,
-					    nvecs, cnt, &args);
-
-	return nfsd_buffered_write(rqstp, args.nf->nf_file, nvecs, cnt, kiocb);
+	return nfsd_issue_dio_write(rqstp, fhp, stable_how, kiocb, nvecs,
+				    cnt, &args);
 }
 
 /**
-- 
2.51.0