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

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Date:   Tue, 31 May 2022 08:33:26 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Jeff Layton" <jlayton@...nel.org>
Cc:     "Miklos Szeredi" <miklos@...redi.hu>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        "ChenXiaoSong" <chenxiaosong2@...wei.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        liuyongqiang13@...wei.com, "zhangyi (F)" <yi.zhang@...wei.com>,
        zhangxiaoxu5@...wei.com, "Steve French" <smfrench@...il.com>
Subject: Re: [PATCH -next,v2] fuse: return the more nuanced writeback error on close()

On Tue, 31 May 2022, Jeff Layton wrote:
> On Mon, 2022-05-30 at 14:13 +0200, Miklos Szeredi wrote:
> > On Mon, 23 May 2022 at 03:35, ChenXiaoSong <chenxiaosong2@...wei.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@...wei.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.
> > 
> 
> I think that it probably shouldn't be, actually. Reason below...
> 
> > Is there a difference between ->flush() and ->fsync()?
> > 
> > Jeff, can you please help?
> > 
> > 
> 
> The main difference is that ->flush is called from filp_close, so it's
> called when a file descriptor (or equivalent) is being torn down out,
> whereas ->fsync is (obviously) called from the fsync codepath.

->flush is for cache coherence. It is best-effort
->fsync is for data safety. Obviously errors are important.

> 
> We _must_ report writeback errors on fsync, but reporting them on the
> close() syscall is less clear. The thing about close() is that it's
> going be successful no matter what is returned. The file descriptor will
> no longer work afterward regardless.
> 
> fsync also must also initiate writeback of all the buffered data, but
> it's not required for filesystems to do that on close() (and in fact,
> there are good reasons not to if you can). A successful close() tells
> you nothing about whether your data made it to the backing store. It
> might just not have been synced out yet.
> 
> Personally, I think it's probably best to _not_ return writeback errors
> on close at all. The only "legitimate" error on close is -EBADF.
> Arguably, we should make ->flush be void return. Note that most
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Excellent idea!

NeilBrown

> filp_close callers ignore the error anyway, so it's not much of a
> stretch.
> 
> In any case, if you do decide to return errors in fuse_flush, then
> advancing the cursor would also have the effect of masking writeback
> errors on dup()'ed file descriptors, and I don't think you want to do
> that.
> -- 
> Jeff Layton <jlayton@...nel.org>
>