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

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2024-06-27 at 13:01 +0300, Dan Aloni wrote:
> 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, if these applications want non-aligned offsets
> and sizes they need to either use locks or risk data corruption, as
> the
> NFS client defaults to extending writes to whole pages.
> 
> This commit adds a new mount option `noalignwrite`, which allows to
> turn
> that off and avoid the need of locking, as long as these applications
> don't overlap on offsets.
> 
> 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..7e000d782e28 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_alignwrite,
>  	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("alignwrite",	Opt_alignwrite),
>  	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_alignwrite:
> +		if (result.negated)
> +			ctx->flags |= NFS_MOUNT_NO_ALIGNWRITE;
> +		else
> +			ctx->flags &= ~NFS_MOUNT_NO_ALIGNWRITE;
> +		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..1382ae19bba4 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_ALIGNWRITE)
> +		seq_puts(m, ",noalignwrite");
> +
>  	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..cfe8061bf005 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_ALIGNWRITE)
> +		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..4d28b4a328a7 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_ALIGNWRITE		0x10000000
>  
>  	unsigned int		fattr_valid;	/* Valid attributes
> */
>  	unsigned int		caps;		/* server
> capabilities */

While I hate that we need a new mount option for this, I do understand
the need. I do wonder if we'd be better served by a new fadvise64()
flag or something. Either way, this would probably be needed in the
interim, so...

Reviewed-by: Jeff Layton <jlayton@kernel.org>