linux-kernel - Re: [PATCH v2 18/20] fuse: Release file in process context

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Date:   Mon, 10 Aug 2020 11:48:30 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtio-fs-list <virtio-fs@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v2 18/20] fuse: Release file in process context

On Mon, Aug 10, 2020 at 10:29:13AM +0200, Miklos Szeredi wrote:
> On Fri, Aug 7, 2020 at 9:55 PM Vivek Goyal <vgoyal@...hat.com> wrote:
> >
> > fuse_file_put(sync) can be called with sync=true/false. If sync=true,
> > it waits for release request response and then calls iput() in the
> > caller's context. If sync=false, it does not wait for release request
> > response, frees the fuse_file struct immediately and req->end function
> > does the iput().
> >
> > iput() can be a problem with DAX if called in req->end context. If this
> > is last reference to inode (VFS has let go its reference already), then
> > iput() will clean DAX mappings as well and send REMOVEMAPPING requests
> > and wait for completion. (All the the worker thread context which is
> > processing fuse replies from daemon on the host).
> >
> > That means it blocks worker thread and it stops processing further
> > replies and system deadlocks.
> 
> Is this reasoning specific to DAX?  Seems to me this is a virtio-fs
> specific issue.

I would think it is virtio-fs + DAX issues. virtio-fs without DAX does
not have this problem.

If making this conditional on DAX an issue, I am wondering, can
we now set args->may_block instead. Now virtiofs will do completion
in a separate worker thread if ->may_block is set. That means, 
worker will block till REMOVEMAPPING completes and then be woken
up.

Do let me know if you like setting args->may_block approach better.
I can give that a try.

Vivek