[Virtio-fs] [PATCH 3/4] virtiofsd: use file-backend memory region for virtiofsd's cache area

[Stefan Hajnoczi]

On Wed, Apr 17, 2019 at 03:51:21PM +0100, Dr. David Alan Gilbert wrote:
> * Liu Bo (bo.liu at linux.alibaba.com) wrote:
> > From: Xiaoguang Wang <xiaoguang.wang at linux.alibaba.com>
> > 
> > When running xfstests test case generic/413, we found such issue:
> >     1, create a file in one virtiofsd mount point with dax enabled
> >     2, mmap this file, get virtual addr: A
> >     3, write(fd, A, len), here fd comes from another file in another
> >        virtiofsd mount point without dax enabled, also note here write(2)
> >        is direct io.
> >     4, this direct io will hang forever, because the virtiofsd has crashed.
> > Here is the stack:
> > [  247.166276] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [  247.167171] t_mmap_dio      D    0  2335   2102 0x00000000
> > [  247.168006] Call Trace:
> > [  247.169067]  ? __schedule+0x3d0/0x830
> > [  247.170219]  schedule+0x32/0x80
> > [  247.171328]  schedule_timeout+0x1e2/0x350
> > [  247.172416]  ? fuse_direct_io+0x2e5/0x6b0 [fuse]
> > [  247.173516]  wait_for_completion+0x123/0x190
> > [  247.174593]  ? wake_up_q+0x70/0x70
> > [  247.175640]  fuse_direct_IO+0x265/0x310 [fuse]
> > [  247.176724]  generic_file_read_iter+0xaa/0xd20
> > [  247.177824]  fuse_file_read_iter+0x81/0x130 [fuse]
> > [  247.178938]  ? fuse_simple_request+0x104/0x1b0 [fuse]
> > [  247.180041]  ? fuse_fsync_common+0xad/0x240 [fuse]
> > [  247.181136]  __vfs_read+0x108/0x190
> > [  247.181930]  vfs_read+0x91/0x130
> > [  247.182671]  ksys_read+0x52/0xc0
> > [  247.183454]  do_syscall_64+0x55/0x170
> > [  247.184200]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > 
> > And virtiofsd crashed because vu_gpa_to_va() can not handle guest physical
> > address correctly. For a memory mapped area in dax mode, indeed the page
> > for this area points virtiofsd's cache area, or rather virtio pci device's
> > cache bar. In qemu, currently this cache bar is implemented with an anonymous
> > memory and will not pass this cache bar's address info to vhost-user backend,
> > so vu_gpa_to_va() will fail.
> > 
> > To fix this issue, we create this vhost cache area with a file backend
> > memory area.
> 
> Thanks,
>   I know there was another case of the daemon trying to access the
> buffer that Stefan and Vivek hit, but fixed by persuading the kernel
> not to do it;  Stefan/Vivek: What do you think?

That case happened with cache=none and the dax mount option.

The general problem is when FUSE_READ/FUSE_WRITE is sent and the buffer
is outside guest RAM.

> 
> It worries me a little exposing the area back to the daemon; the guest
> can write the BAR and change the mapping, I doubt anything would notice
> that (but also I doubt it happens much).

If two virtiofsd processes are involved then it's even harder since they
do not have up-to-date access the other's DAX window.

Stefan
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