Querying the size of devdax devices from userspace

Querying the size of devdax devices from userspace

[Stefan Hajnoczi]

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How can userspace applications query the size of devdax character
devices?

stat(1) doesn't know how large the device is:

  # stat /dev/dax0.0
    File: /dev/dax0.0
    Size: 0         	Blocks: 0          IO Block: 4096   character special file
  Device: 6h/6d	Inode: 56764       Links: 1     Device type: fa,d

ndctl(1) finds out by digging in /sys:

  # ndctl list
  [
    {
      "dev":"namespace0.0",
      "mode":"devdax",
      "map":"dev",
      "size":2111832064,
      "uuid":"235acf4d-503f-46be-bf64-f26faf9777ef",
      "chardev":"dax0.0"
    }
  ]

I'm not sure how to do it without enumerating all nvdimms in /sys.  Is
there a mapping from devdax major/minor number to a /sys path?

The use case I have in mind is that QEMU currently takes the size as a
command-line parameter.  The failure mode is ugly when users get this
value wrong: the guest gets a softlockup and there is no error message
on the host side.

I'd like QEMU to detect the size or at least reject size values that are
too large.  In order to do that userspace needs a convenient way of
querying the size.  Any ideas?

Stefan

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[Qemu-devel] Querying the size of devdax devices from userspace

[Stefan Hajnoczi]

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How can userspace applications query the size of devdax character
devices?

stat(1) doesn't know how large the device is:

  # stat /dev/dax0.0
    File: /dev/dax0.0
    Size: 0         	Blocks: 0          IO Block: 4096   character special file
  Device: 6h/6d	Inode: 56764       Links: 1     Device type: fa,d

ndctl(1) finds out by digging in /sys:

  # ndctl list
  [
    {
      "dev":"namespace0.0",
      "mode":"devdax",
      "map":"dev",
      "size":2111832064,
      "uuid":"235acf4d-503f-46be-bf64-f26faf9777ef",
      "chardev":"dax0.0"
    }
  ]

I'm not sure how to do it without enumerating all nvdimms in /sys.  Is
there a mapping from devdax major/minor number to a /sys path?

The use case I have in mind is that QEMU currently takes the size as a
command-line parameter.  The failure mode is ugly when users get this
value wrong: the guest gets a softlockup and there is no error message
on the host side.

I'd like QEMU to detect the size or at least reject size values that are
too large.  In order to do that userspace needs a convenient way of
querying the size.  Any ideas?

Stefan

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Re: Querying the size of devdax devices from userspace

[Dan Williams]

On Sun, Feb 3, 2019 at 11:56 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> How can userspace applications query the size of devdax character
> devices?
>
> stat(1) doesn't know how large the device is:
>
>   # stat /dev/dax0.0
>     File: /dev/dax0.0
>     Size: 0             Blocks: 0          IO Block: 4096   character special file
>   Device: 6h/6d Inode: 56764       Links: 1     Device type: fa,d
>
> ndctl(1) finds out by digging in /sys:
>
>   # ndctl list
>   [
>     {
>       "dev":"namespace0.0",
>       "mode":"devdax",
>       "map":"dev",
>       "size":2111832064,
>       "uuid":"235acf4d-503f-46be-bf64-f26faf9777ef",
>       "chardev":"dax0.0"
>     }
>   ]
>
> I'm not sure how to do it without enumerating all nvdimms in /sys.  Is
> there a mapping from devdax major/minor number to a /sys path?
>
> The use case I have in mind is that QEMU currently takes the size as a
> command-line parameter.  The failure mode is ugly when users get this
> value wrong: the guest gets a softlockup and there is no error message
> on the host side.
>
> I'd like QEMU to detect the size or at least reject size values that are
> too large.  In order to do that userspace needs a convenient way of
> querying the size.  Any ideas?

You're looking for /sys/dev/char. That will wake you from major:minor
to sysfs. Perhaps borrow fio's implementation for the same:

   http://git.kernel.dk/cgit/fio/tree/engines/dev-dax.c#n258
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Re: [Qemu-devel] Querying the size of devdax devices from userspace

[Dan Williams]

On Sun, Feb 3, 2019 at 11:56 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> How can userspace applications query the size of devdax character
> devices?
>
> stat(1) doesn't know how large the device is:
>
>   # stat /dev/dax0.0
>     File: /dev/dax0.0
>     Size: 0             Blocks: 0          IO Block: 4096   character special file
>   Device: 6h/6d Inode: 56764       Links: 1     Device type: fa,d
>
> ndctl(1) finds out by digging in /sys:
>
>   # ndctl list
>   [
>     {
>       "dev":"namespace0.0",
>       "mode":"devdax",
>       "map":"dev",
>       "size":2111832064,
>       "uuid":"235acf4d-503f-46be-bf64-f26faf9777ef",
>       "chardev":"dax0.0"
>     }
>   ]
>
> I'm not sure how to do it without enumerating all nvdimms in /sys.  Is
> there a mapping from devdax major/minor number to a /sys path?
>
> The use case I have in mind is that QEMU currently takes the size as a
> command-line parameter.  The failure mode is ugly when users get this
> value wrong: the guest gets a softlockup and there is no error message
> on the host side.
>
> I'd like QEMU to detect the size or at least reject size values that are
> too large.  In order to do that userspace needs a convenient way of
> querying the size.  Any ideas?

You're looking for /sys/dev/char. That will wake you from major:minor
to sysfs. Perhaps borrow fio's implementation for the same:

   http://git.kernel.dk/cgit/fio/tree/engines/dev-dax.c#n258

Re: Querying the size of devdax devices from userspace

[Stefan Hajnoczi]

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On Mon, Feb 04, 2019 at 12:06:32AM -0800, Dan Williams wrote:
> On Sun, Feb 3, 2019 at 11:56 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > How can userspace applications query the size of devdax character
> > devices?
> >
> > stat(1) doesn't know how large the device is:
> >
> >   # stat /dev/dax0.0
> >     File: /dev/dax0.0
> >     Size: 0             Blocks: 0          IO Block: 4096   character special file
> >   Device: 6h/6d Inode: 56764       Links: 1     Device type: fa,d
> >
> > ndctl(1) finds out by digging in /sys:
> >
> >   # ndctl list
> >   [
> >     {
> >       "dev":"namespace0.0",
> >       "mode":"devdax",
> >       "map":"dev",
> >       "size":2111832064,
> >       "uuid":"235acf4d-503f-46be-bf64-f26faf9777ef",
> >       "chardev":"dax0.0"
> >     }
> >   ]
> >
> > I'm not sure how to do it without enumerating all nvdimms in /sys.  Is
> > there a mapping from devdax major/minor number to a /sys path?
> >
> > The use case I have in mind is that QEMU currently takes the size as a
> > command-line parameter.  The failure mode is ugly when users get this
> > value wrong: the guest gets a softlockup and there is no error message
> > on the host side.
> >
> > I'd like QEMU to detect the size or at least reject size values that are
> > too large.  In order to do that userspace needs a convenient way of
> > querying the size.  Any ideas?
> 
> You're looking for /sys/dev/char. That will wake you from major:minor
> to sysfs. Perhaps borrow fio's implementation for the same:
> 
>    http://git.kernel.dk/cgit/fio/tree/engines/dev-dax.c#n258

Excellent, thanks!

Stefan

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Re: [Qemu-devel] Querying the size of devdax devices from userspace

[Stefan Hajnoczi]

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On Mon, Feb 04, 2019 at 12:06:32AM -0800, Dan Williams wrote:
> On Sun, Feb 3, 2019 at 11:56 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > How can userspace applications query the size of devdax character
> > devices?
> >
> > stat(1) doesn't know how large the device is:
> >
> >   # stat /dev/dax0.0
> >     File: /dev/dax0.0
> >     Size: 0             Blocks: 0          IO Block: 4096   character special file
> >   Device: 6h/6d Inode: 56764       Links: 1     Device type: fa,d
> >
> > ndctl(1) finds out by digging in /sys:
> >
> >   # ndctl list
> >   [
> >     {
> >       "dev":"namespace0.0",
> >       "mode":"devdax",
> >       "map":"dev",
> >       "size":2111832064,
> >       "uuid":"235acf4d-503f-46be-bf64-f26faf9777ef",
> >       "chardev":"dax0.0"
> >     }
> >   ]
> >
> > I'm not sure how to do it without enumerating all nvdimms in /sys.  Is
> > there a mapping from devdax major/minor number to a /sys path?
> >
> > The use case I have in mind is that QEMU currently takes the size as a
> > command-line parameter.  The failure mode is ugly when users get this
> > value wrong: the guest gets a softlockup and there is no error message
> > on the host side.
> >
> > I'd like QEMU to detect the size or at least reject size values that are
> > too large.  In order to do that userspace needs a convenient way of
> > querying the size.  Any ideas?
> 
> You're looking for /sys/dev/char. That will wake you from major:minor
> to sysfs. Perhaps borrow fio's implementation for the same:
> 
>    http://git.kernel.dk/cgit/fio/tree/engines/dev-dax.c#n258

Excellent, thanks!

Stefan

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