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

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Message-ID: <20200311125923.GA83257@redhat.com>
Date:   Wed, 11 Mar 2020 08:59:23 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Liu Bo <bo.liu@...ux.alibaba.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 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.

> 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)