linux-kernel - Re: [PATCH 20/20] fuse,virtiofs: Add logic to free up a memory range

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Date:   Thu, 12 Mar 2020 01:24:30 +0800
From:   Liu Bo <bo.liu@...ux.alibaba.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, virtio-fs@...hat.com, miklos@...redi.hu,
        stefanha@...hat.com, dgilbert@...hat.com, mst@...hat.com
Subject: Re: [PATCH 20/20] fuse,virtiofs: Add logic to free up a memory range

On Wed, Mar 11, 2020 at 08:59:23AM -0400, Vivek Goyal wrote:
> On Wed, Mar 11, 2020 at 01:16:42PM +0800, Liu Bo wrote:
> 
> [..]
> > > @@ -719,6 +723,7 @@ void fuse_conn_put(struct fuse_conn *fc)
> > >  	if (refcount_dec_and_test(&fc->count)) {
> > >  		struct fuse_iqueue *fiq = &fc->iq;
> > >  
> > > +		flush_delayed_work(&fc->dax_free_work);
> > 
> > Today while debugging another case, I realized that flushing work here
> > at the very last fuse_conn_put() is a bit too late, here's my analysis,
> > 
> >          umount                                                   kthread
> > 
> > deactivate_locked_super
> >   ->virtio_kill_sb                                            try_to_free_dmap_chunks
> >     ->generic_shutdown_super                                    ->igrab()
> >                                                                 ...
> >      ->evict_inodes()  -> check all inodes' count
> >      ->fuse_conn_put                                            ->iput
> >  ->virtio_fs_free_devs
> >    ->fuse_dev_free
> >      ->fuse_conn_put // vq1
> >    ->fuse_dev_free
> >      ->fuse_conn_put // vq2
> >        ->flush_delayed_work
> > 
> > The above can end up with a warning message reported by evict_inodes()
> > about stable inodes.
> 
> Hi Liu Bo,
> 
> Which warning is that? Can you point me to it in code.
>

Hmm, it was actually in generic_shutdow_super,
---
              printk("VFS: Busy inodes after unmount of %s. "
                           "Self-destruct in 5 seconds.  Have a nice day...\n",
---

> > So I think it's necessary to put either
> > cancel_delayed_work_sync() or flush_delayed_work() before going to
> > generic_shutdown_super().
> 
> In general I agree that shutting down memory range freeing worker
> earling in unmount/shutdown sequence makes sense. It does not seem
> to help to let it run while filesystem is going away. How about following
> patch.
> 
> ---
>  fs/fuse/inode.c     |    1 -
>  fs/fuse/virtio_fs.c |    5 +++++
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> Index: redhat-linux/fs/fuse/virtio_fs.c
> ===================================================================
> --- redhat-linux.orig/fs/fuse/virtio_fs.c	2020-03-10 14:11:10.970284651 -0400
> +++ redhat-linux/fs/fuse/virtio_fs.c	2020-03-11 08:27:08.103330039 -0400
> @@ -1295,6 +1295,11 @@ static void virtio_kill_sb(struct super_
>  	vfs = fc->iq.priv;
>  	fsvq = &vfs->vqs[VQ_HIPRIO];
>  
> +	/* Stop dax worker. Soon evict_inodes() will be called which will
> +	 * free all memory ranges belonging to all inodes.
> +	 */
> +	flush_delayed_work(&fc->dax_free_work);
> +
>  	/* Stop forget queue. Soon destroy will be sent */
>  	spin_lock(&fsvq->lock);
>  	fsvq->connected = false;
> Index: redhat-linux/fs/fuse/inode.c
> ===================================================================
> --- redhat-linux.orig/fs/fuse/inode.c	2020-03-10 09:13:35.132565666 -0400
> +++ redhat-linux/fs/fuse/inode.c	2020-03-11 08:22:02.685330039 -0400
> @@ -723,7 +723,6 @@ void fuse_conn_put(struct fuse_conn *fc)
>  	if (refcount_dec_and_test(&fc->count)) {
>  		struct fuse_iqueue *fiq = &fc->iq;
>  
> -		flush_delayed_work(&fc->dax_free_work);
>  		if (fc->dax_dev)
>  			fuse_free_dax_mem_ranges(&fc->free_ranges);
>  		if (fiq->ops->release)

Looks good, it should be safe now, but I feel like
cancel_delayed_work_sync() would be a good alternative for "stop dax
worker".

Reviewed-by: Liu Bo <bo.liu@...ux.alibaba.com>

Fine with either folding directly or a new patch, thanks for fixing it.

thanks,
-liubo