Re: [Xen-devel] [PATCHv1] xen/balloon: disable memory hotplug in PV guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/11/2015 10:42 AM, David Vrabel wrote:
> On 10/03/15 13:35, Boris Ostrovsky wrote:
> > On 03/10/2015 07:40 AM, David Vrabel wrote:
> > > On 09/03/15 14:10, David Vrabel wrote:
> > > > Memory hotplug doesn't work with PV guests because:
> > > >
> > > >     a) The p2m cannot be expanded to cover the new sections.
> > > Broken by 054954eb051f35e74b75a566a96fe756015352c8 (xen: switch to
> > > linear virtual mapped sparse p2m list).
> > >
> > > This one would be non-trivial to fix.  We'd need a sparse set of
> > > vm_area's for the p2m or similar.
> > >
> > > >     b) add_memory() builds page tables for the new sections which means
> > > >        the new pages must have valid p2m entries (or a BUG occurs).
> > > After some more testing this appears to be broken by:
> > >
> > > 25b884a83d487fd62c3de7ac1ab5549979188482 (x86/xen: set regions above the
> > > end of RAM as 1:1) included 3.16.
> > >
> > > This one can be trivially fixed by setting the new sections in the p2m
> > > to INVALID_P2M_ENTRY before calling add_memory().
> > Have you tried 3.17? As I said yesterday, it worked for me (with 4.4 Xen).
> No.  But there are three bugs that prevent it from working in 3.16+ so
> I'm really not sure how you had a working in a 3.17 PV guest.
This is what I have:

[build@build-mk2 linux-boris]$ ssh root@tst008 cat 
/mnt/lab/bootstrap-x86_64/test_small.xm
extra="console=hvc0 debug earlyprintk=xen "
kernel="/mnt/lab/bootstrap-x86_64/vmlinuz"
ramdisk="/mnt/lab/bootstrap-x86_64/initramfs.cpio.gz"
memory=1024
maxmem = 4096
vcpus=1
maxvcpus=3
name="bootstrap-x86_64"
on_crash="preserve"
vif = [ 'mac=00:0F:4B:00:00:68, bridge=switch' ]
vnc=1
vnclisten="0.0.0.0"
disk=['phy:/dev/guests/bootstrap-x86_64,xvda,w']
[build@build-mk2 linux-boris]$ ssh root@tst008 xl create 
/mnt/lab/bootstrap-x86_64/test_small.xm
Parsing config from /mnt/lab/bootstrap-x86_64/test_small.xm
[build@build-mk2 linux-boris]$ ssh root@tst008 xl list |grep 
bootstrap-x86_64
bootstrap-x86_64                             2  1024     1 -b----       5.4
[build@build-mk2 linux-boris]$ ssh root@g-pvops uname -r
3.17.0upstream
[build@build-mk2 linux-boris]$ ssh root@g-pvops dmesg|grep paravirtualized
[    0.000000] Booting paravirtualized kernel on Xen
[build@build-mk2 linux-boris]$ ssh root@g-pvops grep MemTotal /proc/meminfo
MemTotal:         968036 kB
[build@build-mk2 linux-boris]$ ssh root@tst008 xl mem-set 
bootstrap-x86_64 2048
[build@build-mk2 linux-boris]$ ssh root@tst008 xl list |grep 
bootstrap-x86_64
bootstrap-x86_64                             2  2048     1 -b----       5.7
[build@build-mk2 linux-boris]$ ssh root@g-pvops grep MemTotal /proc/meminfo
MemTotal:        2016612 kB
[build@build-mk2 linux-boris]$


> Regardless, it definitely doesn't work now because of the linear p2m
> change.  What do you want to do about this?
Since backing out p2m changes is not an option I guess your patch is the 
only short-term alternative.
But this still looks like a regression so perhaps Juergen can take a 
look to see how it can be fixed.
-boris


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