Re: [PATCH] fuse: In fuse_flush only wait if someone wants the return code

["Serge E. Hallyn" <serge@hallyn.com>]

On Tue, Sep 27, 2022 at 11:46:44AM +0200, Miklos Szeredi wrote:
> On Thu, 1 Sept 2022 at 16:07, Tycho Andersen <tycho@tycho.pizza> wrote:
> >
> > From: "Eric W. Biederman" <ebiederm@xmission.com>
> >
> > In my very light testing this resolves a hang where a thread of the
> > fuse server was accessing the fuse filesystem (the fuse server is
> > serving up), when the fuse server is killed.
> >
> > The practical problem is that the fuse server file descriptor was
> > being closed after the file descriptor into the fuse filesystem so
> > that the fuse filesystem operations were being blocked for instead of
> > being aborted.  Simply skipping the unnecessary wait resolves this
> > issue.
> >
> > This is just a proof of concept and someone should look to see if the
> > fuse max_background limit could cause a problem with this approach.
> 
> Maybe you missed my comments here:
> 
> https://lore.kernel.org/all/CAJfpegsTmiO-sKaBLgoVT4WxDXBkRES=HF1YmQN1ES7gfJEJ+w@mail.gmail.com/

That's odd - fwiw I too had completely missed that reply, sorry.

> I'm generally okay with this, but please write a proper changelog for
> the patch, also mentioning the issues related to posix locks.
> 
> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -464,6 +464,67 @@ static void fuse_sync_writes(struct inode *inode)
> >         fuse_release_nowrite(inode);
> >  }
> >
> > +struct fuse_flush_args {
> > +       struct fuse_args args;
> > +       struct fuse_flush_in inarg;
> > +       struct inode *inode;
> > +       struct fuse_file *ff;
> > +};
> > +
> > +static void fuse_flush_end(struct fuse_mount *fm, struct fuse_args *args, int err)
> > +{
> > +       struct fuse_flush_args *fa = container_of(args, typeof(*fa), args);
> > +
> > +       if (err == -ENOSYS) {
> > +               fm->fc->no_flush = 1;
> > +               err = 0;
> > +       }
> > +
> > +       /*
> > +        * In memory i_blocks is not maintained by fuse, if writeback cache is
> > +        * enabled, i_blocks from cached attr may not be accurate.
> > +        */
> > +       if (!err && fm->fc->writeback_cache)
> > +               fuse_invalidate_attr_mask(fa->inode, STATX_BLOCKS);
> > +
> > +
> > +       iput(fa->inode);
> > +       fuse_file_put(fa->ff, false, false);
> > +       kfree(fa);
> > +}
> > +
> > +static int fuse_flush_async(struct file *file, fl_owner_t id)
> > +{
> > +       struct inode *inode = file_inode(file);
> > +       struct fuse_mount *fm = get_fuse_mount(inode);
> > +       struct fuse_file *ff = file->private_data;
> > +       struct fuse_flush_args *fa;
> > +       int err;
> > +
> > +       fa = kzalloc(sizeof(*fa), GFP_KERNEL);
> > +       if (!fa)
> > +               return -ENOMEM;
> > +
> > +       fa->inarg.fh = ff->fh;
> > +       fa->inarg.lock_owner = fuse_lock_owner_id(fm->fc, id);
> > +       fa->args.opcode = FUSE_FLUSH;
> > +       fa->args.nodeid = get_node_id(inode);
> > +       fa->args.in_numargs = 1;
> > +       fa->args.in_args[0].size = sizeof(fa->inarg);
> > +       fa->args.in_args[0].value = &fa->inarg;
> > +       fa->args.force = true;
> > +       fa->args.nocreds = true;
> > +       fa->args.end = fuse_flush_end;
> > +       fa->inode = igrab(inode);
> 
> Grabbing the inode should already taken care of by fuse_file_release().
> 
> Also please try to reduce duplication in both the above functions.
> 
> Thanks,
> Miklos