Re: [PATCH -next,v2] fuse: return the more nuanced writeback error on close()

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2022-05-30 at 14:13 +0200, Miklos Szeredi wrote:
> On Mon, 23 May 2022 at 03:35, ChenXiaoSong <chenxiaosong2@huawei.com>
> wrote:
> > 
> > As filemap_check_errors() only report -EIO or -ENOSPC, we return
> > more nuanced
> > writeback error -(file->f_mapping->wb_err & MAX_ERRNO).
> > 
> >   filemap_write_and_wait
> >     filemap_write_and_wait_range
> >       filemap_check_errors
> >         -ENOSPC or -EIO
> >   filemap_check_wb_err
> >     errseq_check
> >       return -(file->f_mapping->wb_err & MAX_ERRNO)
> > 
> > Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
> > ---
> >  fs/fuse/file.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> > index f18d14d5fea1..9917bc2795e6 100644
> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -488,10 +488,10 @@ static int fuse_flush(struct file *file,
> > fl_owner_t id)
> >         inode_unlock(inode);
> > 
> >         err = filemap_check_errors(file->f_mapping);
> > +       /* return more nuanced writeback errors */
> >         if (err)
> > -               return err;
> > +               return filemap_check_wb_err(file->f_mapping, 0);
> 
> I'm wondering if this should be file_check_and_advance_wb_err()
> instead.
> 
> Is there a difference between ->flush() and ->fsync()?
> 
> Jeff, can you please help?
> 

Hi Miklos,

We just went through this discussion for the case of NFS.

The point is that ->flush() is only called on close(). While you can
report errors in close(), the 'man 2 fsync' manpage documents that
post-Linux 4.13, the writeback errors are required to be returned on -
>fsync(). You should therefore not be calling
file_check_and_advance_wb_err() in anything that is being called as
part of close() since that will clear the error from the errseq_t and
prevent it from being reported in a future fsync() call from a dup()ed
file descriptor, etc.

NFS also wants to make a special case out of write() when we know that
the error is one of EDQUOT, EFBIG or ENOSPC, in which case we will also
use  file_check_and_advance_wb_err() to return the error immediately,
and clear it from the errseq_t.

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