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

[Liu Bo]

On Tue, Apr 23, 2019 at 01:09:19PM +0100, Stefan Hajnoczi wrote:
> 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.
>

Can you please elaborate how the buffer is outside guest RAM?
Is it also via direct IO?

> > 
> > 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.
> 

In case of direct IO, kvm is able to make sure that guest's dax
mapping is sync'd with the underlying host mmap region, isn't it?


thanks,
-liubo