Re: [PATCH 1/2] nfs: ensure correct writeback errors are returned on close()

[Trond Myklebust <trondmy@hammerspace.com>]

On Fri, 2020-07-31 at 13:46 -0400, Scott Mayhew wrote:
> nfs_wb_all() calls filemap_write_and_wait(), which uses
> filemap_check_errors() to determine the error to return.
> filemap_check_errors() only looks at the mapping->flags and will
> therefore only return either -ENOSPC or -EIO.  To ensure that the
> correct error is returned on close(), nfs{,4}_file_flush() should
> call
> file_check_and_advance_wb_err() which looks at the errseq value in
> mapping->wb_err.
> 
> Fixes: 6fbda89b257f ("NFS: Replace custom error reporting mechanism
> with
> generic one")
> Signed-off-by: Scott Mayhew <smayhew@redhat.com>
> ---
>  fs/nfs/file.c     | 3 ++-
>  fs/nfs/nfs4file.c | 3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nfs/file.c b/fs/nfs/file.c
> index f96367a2463e..eeef6580052f 100644
> --- a/fs/nfs/file.c
> +++ b/fs/nfs/file.c
> @@ -148,7 +148,8 @@ nfs_file_flush(struct file *file, fl_owner_t id)
>  		return 0;
>  
>  	/* Flush writes to the server and return any errors */
> -	return nfs_wb_all(inode);
> +	nfs_wb_all(inode);
> +	return file_check_and_advance_wb_err(file);
>  }
>  
>  ssize_t
> diff --git a/fs/nfs/nfs4file.c b/fs/nfs/nfs4file.c
> index 8e5d6223ddd3..77bf9c12734c 100644
> --- a/fs/nfs/nfs4file.c
> +++ b/fs/nfs/nfs4file.c
> @@ -125,7 +125,8 @@ nfs4_file_flush(struct file *file, fl_owner_t id)
>  		return filemap_fdatawrite(file->f_mapping);
>  
>  	/* Flush writes to the server and return any errors */
> -	return nfs_wb_all(inode);
> +	nfs_wb_all(inode);
> +	return file_check_and_advance_wb_err(file);
>  }
>  
>  #ifdef CONFIG_NFS_V4_2

I don't think this one is correct. The contract with POSIX is that we
always deliver the error on fsync(). If we call
file_check_and_advance_wb_err() here in nfs_file_flush(), then that
means we eat the error before it can get delivered to fsync().

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com