Libguestfs - Sep 2017 - Re: [PATCH] lib: direct: Disable qemu locking when opening drives readonly.

Incomplete fix for https://bugzilla.redhat.com/show_bug.cgi?id=1417306

The full fix is waiting for a libvirt change, but this can still go
upstream.

Rich.

Since qemu 2.10, qemu locks all drives even when they are opened
readonly.  This disables this locking in this safe case.

Incomplete fix for
https://bugzilla.redhat.com/show_bug.cgi?id=1417306
---
 lib/launch-direct.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lib/launch-direct.c b/lib/launch-direct.c
index 3b848165c..e5465539d 100644
--- a/lib/launch-direct.c
+++ b/lib/launch-direct.c
@@ -221,8 +221,11 @@ add_drive_standard_params (guestfs_h *g, struct
backend_direct_data *data,
     file = guestfs_int_drive_source_qemu_param (g, &drv->src);
     append_list_format ("file=%s", file);
 
-    if (drv->readonly)
+    if (drv->readonly) {
       append_list ("snapshot=on");
+      if (guestfs_int_version_ge (&data->qemu_version, 2, 10, 0))
+        append_list ("locking=off");
+    }
     append_list_format ("cache=%s",
                         drv->cachemode ? drv->cachemode :
"writeback");
     if (drv->src.format)
-- 
2.13.2

On Tue, Sep 05, 2017 at 03:55:07PM +0100, Richard W.M. Jones
wrote:
> Incomplete fix for https://bugzilla.redhat.com/show_bug.cgi?id=1417306
> 
> The full fix is waiting for a libvirt change, but this can still go
> upstream.
I talked to Dan and apparently this is a full fix, since libvirt
will intuit the correct locking parameters.

Rich.

-- 
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On Tuesday, 5 September 2017 16:55:08 CEST Richard W.M. Jones
wrote:
> Since qemu 2.10, qemu locks all drives even when they are opened
> readonly.  This disables this locking in this safe case.
> 
> Incomplete fix for
> https://bugzilla.redhat.com/show_bug.cgi?id=1417306
> ---
I guess that, according to the other message in this thread, the commit
message will be amended to use RHBZ# in the subject, and drop the last
paragraph?
>  lib/launch-direct.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/launch-direct.c b/lib/launch-direct.c
> index 3b848165c..e5465539d 100644
> --- a/lib/launch-direct.c
> +++ b/lib/launch-direct.c
> @@ -221,8 +221,11 @@ add_drive_standard_params (guestfs_h *g, struct
backend_direct_data *data,
>      file = guestfs_int_drive_source_qemu_param (g, &drv->src);
>      append_list_format ("file=%s", file);
>  
> -    if (drv->readonly)
> +    if (drv->readonly) {
>        append_list ("snapshot=on");
> +      if (guestfs_int_version_ge (&data->qemu_version, 2, 10, 0))
> +        append_list ("locking=off");
> +    }
>      append_list_format ("cache=%s",
>                          drv->cachemode ? drv->cachemode :
"writeback");
LGTM.  IMHO locking=off ought to be used also for the appliance disk,
otherwise starting two or more libguestfs instances for the same user
will fail, no?  Even if not, disabling the locking should still be good,
as in not making qemu do more work than needed.

-- 
Pino Toscano

On Wed, Sep 06, 2017 at 10:41:24AM +0300, Roman Kagan
wrote:
> On Tue, Sep 05, 2017 at 03:55:08PM +0100, Richard W.M. Jones wrote:
> > Since qemu 2.10, qemu locks all drives even when they are opened
> > readonly.  This disables this locking in this safe case.
> > 
> > Incomplete fix for
> > https://bugzilla.redhat.com/show_bug.cgi?id=1417306
> > ---
> >  lib/launch-direct.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/lib/launch-direct.c b/lib/launch-direct.c
> > index 3b848165c..e5465539d 100644
> > --- a/lib/launch-direct.c
> > +++ b/lib/launch-direct.c
> > @@ -221,8 +221,11 @@ add_drive_standard_params (guestfs_h *g, struct
backend_direct_data *data,
> >      file = guestfs_int_drive_source_qemu_param (g, &drv->src);
> >      append_list_format ("file=%s", file);
> >  
> > -    if (drv->readonly)
> > +    if (drv->readonly) {
> >        append_list ("snapshot=on");
> > +      if (guestfs_int_version_ge (&data->qemu_version, 2, 10,
0))
> > +        append_list ("locking=off");
> 
> Don't we have a way to enumerate supported options?
Yes, although it's difficult to do:

https://www.redhat.com/archives/libguestfs/2017-September/msg00039.html

Rich.
> Depending on QEMU version is not going to play nice with vendor-shipped
> QEMU where features are often backported.
> 
> > +    }
> >      append_list_format ("cache=%s",
> >                          drv->cachemode ? drv->cachemode :
"writeback");
> >      if (drv->src.format)
> 
> Roman.
-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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