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

[Trond Myklebust <trondmy@hammerspace.com>]

On Fri, 2020-07-31 at 15:43 -0400, Scott Mayhew wrote:
> On Fri, 31 Jul 2020, Trond Myklebust wrote:
> 
> > 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().
> 
> I was looking at callers of the flush f_op and the only one I saw was
> filp_close(), so I assumed that there wouldn't be any other calls to
> fsync() for that struct file... I guess that's not the case if the
> file
> descriptor was duplicated though.
> 
> Would a solution using filemap_sample_wb_err() &
> filemap_check_wb_err()
> be acceptable (like in the 2nd patch)?
> 

I think that would be more appropriate, yes.

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