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

[Stefan Hajnoczi]

On Wed, May 01, 2019 at 07:59:17PM +0100, Dr. David Alan Gilbert wrote:
> * Stefan Hajnoczi (stefanha at redhat.com) wrote:
> > On Thu, Apr 25, 2019 at 05:21:58PM -0400, Vivek Goyal wrote:
> > > On Thu, Apr 25, 2019 at 03:33:23PM +0100, Stefan Hajnoczi wrote:
> > > > On Tue, Apr 23, 2019 at 11:49:15AM -0700, Liu Bo wrote:
> > > > > 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.
> > > 
> > > Stefan,
> > > 
> > > Can this be emulated by sending a request to qemu? If virtiofsd can detect
> > > that source/destination of READ/WRITE is not guest RAM, can it forward
> > > message to qemu to do this operation (which has access to all the DAX
> > > windows)?
> > > 
> > > This probably will mean introducing new messages like
> > > setupmapping/removemapping messages between virtiofsd/qemu.  
> > 
> > Yes, interesting idea!
> > 
> > When virtiofsd is unable to map the virtqueue iovecs due to addresses
> > outside guest RAM, it could forward READ/WRITE requests to QEMU along
> > with the file descriptor.  It would be slow but fixes the problem.
> > 
> > Implementing this is a little tricky because the libvhost-user code
> > probably fails before fuse_lowlevel.c is able to parse the FUSE request
> > header.  It will require reworking libvhost-user and fuse_virtio.c code,
> > I think.
> 
> Yes, this doesn't look too bad; I need to tweak
>    vu_queue_pop->vu_queue_map_desc->virtqueue_map_desc
> to give back a list of unmappable parts of the iovec
> (assuming that the first few elements of the iovec are
> mappable then the rest aren't and not allowing weird mixes).
> 
> One thing that worries me a bit is that if we do a read() or write()
> in the qemu code, it might block on the mmap'd backing file.

Even a memcpy (no read()/write()) could block if the page is not
resident :(.  We have to expect that this operation could block.

Stefan
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: not available
URL: <http://listman.redhat.com/archives/virtio-fs/attachments/20190502/2b7d7c5f/attachment.sig>