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Re: [libvirt] [PATCH v4 06/14] qemu_ns: Allow /dev/mapper/control for PR




On 04/10/2018 10:58 AM, Michal Privoznik wrote:
> If qemu-pr-helper is compiled with multipath support the first
> thing it does is opening /dev/mapper/control. Since we're going

s/opening/open

> to be running it inside qemu namespace we need to create it
> there. Unfortunately, we don't know if it was compiled with or
> without multipath so we have to create it anyway.
> 

Not sure this explains whether it's multipath or namespaces that's the
focus/cause of this patch. I thought multipath allows multiple ways to
address the same LUN and namespace provides a mechanism to "control"
device events and protections so that something like udev doesn't change
something behind our back. For PR it's a mechanism to allow certain
ioctl calls to be able to control ownership and/or write processing to
the LUN so that it's not possible to have two writers.

So does this patch add /dev/mapper/control to the namespace list because
that's what will "control" the ioctl's to the LUN used by PR ???

> BTW: This might be the ugliest piece of code I've ever written
> but @devMapperControl really needs to be type of char * otherwise
> some crazy check in VIR_APPEND_ELEMENT fails.

This hunk probably doesn't belong in a commit message...  and well
doesn't necessarily provide the confidence level for acceptance ;-)!

> 
> Signed-off-by: Michal Privoznik <mprivozn redhat com>
> ---
>  src/qemu/qemu_domain.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
> index 0856f04406..6fe4eb57e1 100644
> --- a/src/qemu/qemu_domain.c
> +++ b/src/qemu/qemu_domain.c
> @@ -108,6 +108,7 @@ VIR_ENUM_IMPL(qemuDomainNamespace, QEMU_DOMAIN_NS_LAST,
>  #define PROC_MOUNTS "/proc/mounts"
>  #define DEVPREFIX "/dev/"
>  #define DEV_VFIO "/dev/vfio/vfio"
> +#define DEVICE_MAPPER_CONTROL_PATH "/dev/mapper/control"
>  
>  
>  struct _qemuDomainLogContext {
> @@ -10269,6 +10270,11 @@ qemuDomainSetupDisk(virQEMUDriverConfigPtr cfg ATTRIBUTE_UNUSED,
>              goto cleanup;
>      }
>  
> +    /* qemu-pr-helper might require access to /dev/mapper/control. */
> +    if (virStoragePRDefIsEnabled(disk->src->pr) &&
> +        qemuDomainCreateDevice(DEVICE_MAPPER_CONTROL_PATH, data, true) < 0)
> +        goto cleanup;
> +
>      ret = 0;
>   cleanup:
>      VIR_FREE(dst);
> @@ -11281,6 +11287,9 @@ qemuDomainNamespaceSetupDisk(virDomainObjPtr vm,
>      const char **paths = NULL;
>      size_t npaths = 0;
>      int ret = -1;
> +    /* This is very nasty but we need it to work around some
> +     * stupid checks in VIR_APPEND_ELEMENT macro. */
> +    char *devMapperControl = (char *) DEVICE_MAPPER_CONTROL_PATH;

alternatively

    char *dmPath = NULL;

>  
>      if (!qemuDomainNamespaceEnabled(vm, QEMU_DOMAIN_NS_MOUNT))
>          return 0;
> @@ -11296,6 +11305,11 @@ qemuDomainNamespaceSetupDisk(virDomainObjPtr vm,
>              goto cleanup;
>      }
>  
> +    /* qemu-pr-helper might require access to /dev/mapper/control. */
> +    if (virStoragePRDefIsEnabled(src->pr) &&
> +        VIR_APPEND_ELEMENT_COPY(paths, npaths, devMapperControl) < 0)

Alternatively:

    VIR_STRDUP(dmPath, DEVICE_MAPPER_CONTROL_PATH) < 0) &&
    VIR_APPEND_ELEMENT_COPY(paths, npaths, dmPath)

> +        goto cleanup;
> +
>      if (qemuDomainNamespaceMknodPaths(vm, paths, npaths) < 0)
>          return -1;

BTW: Existing, but this leaks @paths
>  
> 

    VIR_FREE(dmPath);


John


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