[PATCH v10 7/7] NFS: add basic STATX_DIOALIGN and STATX_DIO_READ_ALIGN support

[Mike Snitzer <snitzer@kernel.org>]

NFS doesn't have DIO alignment constraints, so have NFS respond with
accommodating DIO alignment attributes (rather than plumb in GETATTR
support for STATX_DIOALIGN and STATX_DIO_READ_ALIGN).

The most coarse-grained dio_offset_align is the most accommodating
(e.g. PAGE_SIZE, in future larger may be supported).

Now that NFS has support, NFS reexport will now handle unaligned DIO
(NFSD's NFSD_IO_DIRECT support requires the underlying filesystem
support STATX_DIOALIGN and/or STATX_DIO_READ_ALIGN).

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 fs/nfs/inode.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index 49df9debb1a69..84bf3d21c25cc 100644
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -1073,6 +1073,21 @@ int nfs_getattr(struct mnt_idmap *idmap, const struct path *path,
 	if (S_ISDIR(inode->i_mode))
 		stat->blksize = NFS_SERVER(inode)->dtsize;
 	stat->btime = NFS_I(inode)->btime;
+
+	/* Special handling for STATX_DIOALIGN and STATX_DIO_READ_ALIGN
+	 * - NFS doesn't have DIO alignment constraints, avoid getting
+	 *   these DIO attrs from remote and just respond with most
+	 *   accommodating limits (so client will issue supported DIO).
+	 * - this is unintuitive, but the most coarse-grained
+	 *   dio_offset_align is the most accommodating.
+	 */
+	if ((request_mask & (STATX_DIOALIGN | STATX_DIO_READ_ALIGN)) &&
+	    S_ISREG(inode->i_mode)) {
+		stat->result_mask |= STATX_DIOALIGN | STATX_DIO_READ_ALIGN;
+		stat->dio_mem_align = 4; /* 4-byte alignment */
+		stat->dio_offset_align = PAGE_SIZE;
+		stat->dio_read_offset_align = stat->dio_offset_align;
+	}
 out:
 	trace_nfs_getattr_exit(inode, err);
 	return err;
-- 
2.44.0