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

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Date:   Tue, 27 Sep 2022 11:46:44 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Tycho Andersen <tycho@...ho.pizza>
Cc:     Eric Biederman <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, fuse-devel@...ts.sourceforge.net,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH] fuse: In fuse_flush only wait if someone wants the return code

On Thu, 1 Sept 2022 at 16:07, Tycho Andersen <tycho@...ho.pizza> wrote:
>
> From: "Eric W. Biederman" <ebiederm@...ssion.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/

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