[PATCH] nfs: add 'noextend' option for lock-less 'lost writes' prevention

[Dan Aloni <dan.aloni@vastdata.com>]

There are some applications that write to predefined non-overlapping
file offsets from multiple clients and therefore don't need to rely on
file locking. However, NFS file system behavior of extending writes to
to deal with write fragmentation, causes those clients to corrupt each
other's data.

To help these applications, this change adds the `noextend` parameter to
the mount options, and handles this case in `nfs_can_extend_write`.

Clients can additionally add the 'noac' option to ensure page cache
flush on read for modified files.

Signed-off-by: Dan Aloni <dan.aloni@vastdata.com>
---
 fs/nfs/fs_context.c       | 8 ++++++++
 fs/nfs/super.c            | 3 +++
 fs/nfs/write.c            | 3 +++
 include/linux/nfs_fs_sb.h | 1 +
 4 files changed, 15 insertions(+)

diff --git a/fs/nfs/fs_context.c b/fs/nfs/fs_context.c
index 6c9f3f6645dd..509718bc5b24 100644
--- a/fs/nfs/fs_context.c
+++ b/fs/nfs/fs_context.c
@@ -49,6 +49,7 @@ enum nfs_param {
 	Opt_bsize,
 	Opt_clientaddr,
 	Opt_cto,
+	Opt_extend,
 	Opt_fg,
 	Opt_fscache,
 	Opt_fscache_flag,
@@ -149,6 +150,7 @@ static const struct fs_parameter_spec nfs_fs_parameters[] = {
 	fsparam_u32   ("bsize",		Opt_bsize),
 	fsparam_string("clientaddr",	Opt_clientaddr),
 	fsparam_flag_no("cto",		Opt_cto),
+	fsparam_flag_no("extend",	Opt_extend),
 	fsparam_flag  ("fg",		Opt_fg),
 	fsparam_flag_no("fsc",		Opt_fscache_flag),
 	fsparam_string("fsc",		Opt_fscache),
@@ -592,6 +594,12 @@ static int nfs_fs_context_parse_param(struct fs_context *fc,
 		else
 			ctx->flags |= NFS_MOUNT_TRUNK_DISCOVERY;
 		break;
+	case Opt_extend:
+		if (result.negated)
+			ctx->flags |= NFS_MOUNT_NO_EXTEND;
+		else
+			ctx->flags &= ~NFS_MOUNT_NO_EXTEND;
+		break;
 	case Opt_ac:
 		if (result.negated)
 			ctx->flags |= NFS_MOUNT_NOAC;
diff --git a/fs/nfs/super.c b/fs/nfs/super.c
index cbbd4866b0b7..f27fd3858913 100644
--- a/fs/nfs/super.c
+++ b/fs/nfs/super.c
@@ -549,6 +549,9 @@ static void nfs_show_mount_options(struct seq_file *m, struct nfs_server *nfss,
 	else
 		seq_puts(m, ",local_lock=posix");
 
+	if (nfss->flags & NFS_MOUNT_NO_EXTEND)
+		seq_puts(m, ",noextend");
+
 	if (nfss->flags & NFS_MOUNT_WRITE_EAGER) {
 		if (nfss->flags & NFS_MOUNT_WRITE_WAIT)
 			seq_puts(m, ",write=wait");
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 2329cbb0e446..ed76c317b349 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -1315,7 +1315,10 @@ static int nfs_can_extend_write(struct file *file, struct folio *folio,
 	struct file_lock_context *flctx = locks_inode_context(inode);
 	struct file_lock *fl;
 	int ret;
+	unsigned int mntflags = NFS_SERVER(inode)->flags;
 
+	if (mntflags & NFS_MOUNT_NO_EXTEND)
+		return 0;
 	if (file->f_flags & O_DSYNC)
 		return 0;
 	if (!nfs_folio_write_uptodate(folio, pagelen))
diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
index 92de074e63b9..f6d8a4f63e50 100644
--- a/include/linux/nfs_fs_sb.h
+++ b/include/linux/nfs_fs_sb.h
@@ -157,6 +157,7 @@ struct nfs_server {
 #define NFS_MOUNT_WRITE_WAIT		0x02000000
 #define NFS_MOUNT_TRUNK_DISCOVERY	0x04000000
 #define NFS_MOUNT_SHUTDOWN			0x08000000
+#define NFS_MOUNT_NO_EXTEND		0x10000000
 
 	unsigned int		fattr_valid;	/* Valid attributes */
 	unsigned int		caps;		/* server capabilities */
-- 
2.39.3