Re: DAX can not work on virtual nvdimm device

[Xiao Guangrong]

Hi Ross,

Sorry for the delay, i just returned back from KVM Forum.

On 08/20/2016 02:30 AM, Ross Zwisler wrote:
> On Fri, Aug 19, 2016 at 07:59:29AM -0700, Dan Williams wrote:
> > On Fri, Aug 19, 2016 at 4:19 AM, Xiao Guangrong
> > <guangrong.xiao@xxxxxxxxxxxxxxx> wrote:
> > > Hi Dan,
> > >
> > > Recently, Redhat reported that nvml test suite failed on QEMU/KVM,
> > > more detailed info please refer to:
> > >    https://bugzilla.redhat.com/show_bug.cgi?id=1365721
> > >
> > > The reason for this bug is that the memory region created by mmap()
> > > on the dax-based file was gone so that the region can not be found
> > > in /proc/self/smaps during the runtime.
> > >
> > > This is a simple way to trigger this issue:
> > >    mount -o dax /dev/pmem0 /mnt/pmem/
> > >    vim /mnt/pmem/xxx
> > > then 'vim' is crashed due to segment fault.
> > >
> > > This bug can be reproduced on your tree, the top commit is
> > > 10d7902fa0e82b (dax: unmap/truncate on device shutdown), the kernel
> > > configure file is attached.
> > >
> > > Your thought or comment is highly appreciated.
> >
> > I'm going to be offline until Tuesday, but I will investigate when I'm
> > back.  In the meantime if Ross or Vishal had an opportunity to take a
> > look I wouldn't say "no" :).
>
> I haven't been able to reproduce this vim segfault.  I'm using QEMU v2.6.0,
> and the kernel commit you mentioned, and your kernel config.
>
> Here's my QEMU command line:
>
> sudo ~/qemu/bin/qemu-system-x86_64 /var/lib/libvirt/images/alara.qcow2 \
> -machine pc,nvdimm -m 8G,maxmem=100G,slots=100  -object \
> memory-backend-file,id=mem1,share,mem-path=/dev/pmem0,size=8G -device \
> nvdimm,memdev=mem1,id=nv1 -smp 6 -machine pc,accel=kvm
>
> With this I'm able to mkfs the guest's /dev/pmem0, mount it with -o dax, and
> write a file with vim.

Thanks for your test. That's strange...

> Can you reproduce your results with a pmem device created via a memmap kernel
> command line parameter in the guest?  You'll need to update your kernel
> config to enable CONFIG_X86_PMEM_LEGACY and CONFIG_X86_PMEM_LEGACY_DEVICE.

Okay, i tested it with mmap=6G!10G, it failed too. So it looks like it's a
filesystem or DAX issue.

More precisely, i figured out the root case that read() returns a wrong value
when it reaches the end of the file, following test case can trigger it:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

int main(int argc, char *argv[])
{
	char *filename;

	if (argc < 2) {
		printf("arg: filename.\n");
		return -1;
	}

	filename = argv[1];
	printf("test on %s.\n", filename);

	int fd = open(filename, O_RDWR);

	if (fd < 0) {
		perror("open");
		return -1;
	}

	int count = 0;

	while (1) {
		ssize_t ret;
		char buf;

		ret = read(fd, &buf, sizeof(buf));
		if (ret < 0) {
			perror("READ");
			return -1;
		}

		if (ret == 0)
			break;
		if (ret != sizeof(buf)) {
			printf("Count %x Ret %lx sizeof(buf) %lx.\n",
				count, ret, sizeof(buf));
			return -1;
		}

		count++;
		printf("%c", buf);
	}

	printf("\n Good Read.\n");
	return 0;
}



It will fail at "ret != sizeof(buf)", for example, the error output on my
test env is:
      Count 1000 Ret 22f84200 sizeof(buf) 1.