[PATCH v9 08/12] NFSD: Simplify nfsd_iov_iter_aligned_bvec()

[Chuck Lever <cel@kernel.org>]

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

Pages in the RPC receive buffer are contiguous. Practically
speaking, this means that, after the first bvec, the bv_offset
field is always zero.

The loop in nfsd_iov_iter_aligned_bvec() also sets "skip" to zero
after the first bvec is checked. Thus, for bvecs following the first
one, bv_offset = 0 and skip = 0, and the check becomes:

  (0 + 0) & addr_mask = 0

This always passes regardless of the alignment mask.

Since all bvecs after the first one start at bv_offset zero (page-
aligned), they are inherently aligned for DIO. Therefore, only the
first bvec needs to be checked for DIO alignment.

Note that for RDMA transports, the incoming payload always starts on
page alignment, since svcrdma sets up the RDMA Read sink buffers
that way. For RDMA, this loop visits every bvec in each payload, and
never finds an unaligned bvec.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/nfsd/vfs.c | 26 ++++++++------------------
 1 file changed, 8 insertions(+), 18 deletions(-)

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 5d6efcceb8c9..37353fb48d58 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1288,30 +1288,20 @@ nfsd_is_write_dio_possible(loff_t offset, unsigned long len,
 	return true;
 }
 
+/*
+ * Check if the bvec iterator is aligned for direct I/O.
+ *
+ * bvecs generated from RPC receive buffers are contiguous: After the first
+ * bvec, all subsequent bvecs start at bv_offset zero (page-aligned).
+ * Therefore, only the first bvec is checked.
+ */
 static bool
 nfsd_iov_iter_aligned_bvec(const struct nfsd_file *nf, const struct iov_iter *i)
 {
-	unsigned int len_mask = nf->nf_dio_offset_align - 1;
 	unsigned int addr_mask = nf->nf_dio_mem_align - 1;
 	const struct bio_vec *bvec = i->bvec;
-	size_t skip = i->iov_offset;
-	size_t size = i->count;
 
-	if (size & len_mask)
-		return false;
-	do {
-		size_t len = bvec->bv_len;
-
-		if (len > size)
-			len = size;
-		if ((unsigned long)(bvec->bv_offset + skip) & addr_mask)
-			return false;
-		bvec++;
-		size -= len;
-		skip = 0;
-	} while (size);
-
-	return true;
+	return !((unsigned long)(bvec->bv_offset + i->iov_offset) & addr_mask);
 }
 
 static void
-- 
2.51.0