Re: [PATCH -next 2/2] nfs: nfs_file_write() check writeback errors correctly

[Trond Myklebust <trondmy@hammerspace.com>]

On Sat, 2022-03-05 at 20:46 +0800, ChenXiaoSong wrote:
> If nobody has seen the writeback error yet, then
> filemap_sample_wb_err()
> always return 0. Even if there is no new writeback error between
> filemap_sample_wb_err() and filemap_check_wb_err(),
> filemap_check_wb_err() will return the old error.
> 
> Fix this by using file->f_mapping->wb_err as the old error.
> 
> Fixes: ce368536dd61 ("nfs: nfs_file_write() should check for
> writeback errors")
> Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
> ---
>  fs/nfs/file.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/nfs/file.c b/fs/nfs/file.c
> index 83d63bce9596..8763f89c176a 100644
> --- a/fs/nfs/file.c
> +++ b/fs/nfs/file.c
> @@ -635,7 +635,7 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct
> iov_iter *from)
>  
>         nfs_clear_invalid_mapping(file->f_mapping);
>  
> -       since = filemap_sample_wb_err(file->f_mapping);
> +       since = file->f_mapping->wb_err;
>         nfs_start_io_write(inode);
>         result = generic_write_checks(iocb, from);
>         if (result > 0) {
> @@ -669,7 +669,7 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct
> iov_iter *from)
>                 goto out;
>  
>         /* Return error values */
> -       error = filemap_check_wb_err(file->f_mapping, since);
> +       error = errseq_check(&file->f_mapping->wb_err, since);
>         if (nfs_need_check_write(file, inode, error)) {
>                 int err = nfs_wb_all(inode);
>                 if (err < 0)

Hmm... Why isn't this considered a bug with filemap_sample_wb_err()? If
what you say is true, then do_dentry_open() could be picking up
existing errors from the filesystem and from the inode and propagating
them to random processes.

It basically means everyone who cares about correctness of the error
return values needs to do a fsync() immediately after open() in order
to sync up the value in file->f_wb_err.

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