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Re: [Xen-devel] An issue in sharing the pages again in xen-memshare



Hi Sepanta,
memsharing in Xen is fairly rough around the edges. Not being able to
share all pages between VMs is normal though.  Last time I checked you
could still share a fairly large chunk of memory between a VM and its
clone created via save/restore. Just keep in mind that the origin VM
can't be unpaused after its memory is shared to a clone and that the
clone also needs a CoW disk. If you really believe the page you are
trying to share should be sharable you will need to debug it further
on your own and submit a patch with a fix that I would be happy to
review.

Tamas

On Fri, Oct 5, 2018 at 11:26 PM sepanta s <sapanta992@xxxxxxxxx> wrote:
>
>
> Dear Tamas,
>
> I have got consecutive problem in memory sharing in Xen. I would be so much 
> grateful if you could give me a hand.
>
> Firstly, I have written a user-level application that deduplicates unshared 
> pages after T miliseconds. The problem with this application is that it can't 
> share all the pages even though the pages are numerous. As I went through 
> debugging the Xen code, I figured out that reference count of those pages 
> were negative. So, by searching more I got that an overflow occurs for those 
> pages in that part of my Xen version 4.7.
> I decided to migrate to a newer stable release, which I chose 4.11. In that 
> version, I created a raw image, installed my windows and run the virtual 
> machine. Then, I created a clone of the vm. I paused both the source and its 
> clone and tried to run memsharetool to share pages between the source and the 
> clone. Here, the Xen host hangs and I cannot do anything. I reported the 
> problem as a bug or something similar to Xen mailing list.  I asked Razvan, 
> he told me to debug and send the report but my PC doesn't have a serial port. 
> As the other PCs I have access to do not have serial ports, I tried to 
> install Xen in a VM in Vbox to use the emulated serial ports to debug from 
> another Vbox VM. But, that comes other problems as I cannot create VM in Xen 
> 4.11 in ubuntu 17.04 as dom0.
>
> You know I am just moving from one problem  to another. I just need your help.
> I am looking forward to hearing from you soon.
>
>
> With Regards,
> Sepanta
>
> ---------- Forwarded message ---------
> From: sepanta s <sapanta992@xxxxxxxxx>
> Date: Mon, Sep 17, 2018 at 8:13 AM
> Subject: Re: [Xen-devel] An issue in sharing the pages again in xen-memshare
> To: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>
>
> Thanks a lot!
>
> On Sun, Sep 16, 2018 at 6:10 PM Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> 
> wrote:
>>
>> Hello,
>>
>> I don't know what the problem might be (as mentioned before, I've done
>> no work at all with memsharing). However, to see why your host hangs you
>> should use a serial port connection to another computer and see if
>> there's a backtrace.
>>
>> https://wiki.xenproject.org/wiki/Xen_Serial_Console
>> https://wiki.xenproject.org/wiki/Debugging_Xen
>>
>> I think your best bet is Tamas - if he's got time to help.
>>
>>
>> Good luck,
>> Razvan
>>
>> On 9/16/18 12:05 PM, sepanta s wrote:
>> > Hi Razvan,
>> >
>> > I checked through the problem I mentioned before. The problem was due to
>> > a vulnerability in reference counters in Xen 4.9. I compiled the last
>> > release of Xen 4.9 as I checked the patches had been applied.
>> > But a new problem arises from something else. You know, before I want to
>> > share the memory of two VMs, I create one and then create a clone of it
>> > with save/restore. Then I use "range" option in memsharetool in Xen to
>> > share the pages, but the host hangs. I guess there is a bug in Xen in
>> > sharing two VMs which one has been created via save/restoring. I checked
>> > this for Xen 4.11 with Ubuntu 17.04 as host and Windows 7 as guest.
>> > Can you help me or should I post this on the Xen developers mailing list?'
>> >
>> > I look forward to hearing from you.
>> >
>> > P.S: I am Phd student and this is my Master's project which was not
>> > implemented. I am working on it on and off but want to finish it. That's
>> > why there is long period of time between each question . Just thought
>> > being on and off is not really good! So, I shared the reason with you.
>> >
>> > With Regards,
>> > Sepanta
>> >
>> >
>> >
>> >
>> > On Fri, Aug 24, 2018 at 2:29 PM sepanta s <sapanta992@xxxxxxxxx
>> > <mailto:sapanta992@xxxxxxxxx>> wrote:
>> >
>> >     Thanks.
>> >
>> >     On Fri, Aug 24, 2018 at 2:40 PM Razvan Cojocaru
>> >     <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>> wrote:
>> >
>> >         No problem, I was just trying to clear things up.
>> >
>> >         Good luck with the debugging!
>> >
>> >         On 8/24/18 8:31 AM, sepanta s wrote:
>> >         > Thanks for your reply, I understand.
>> >         > Unfortunately, He didn't reply the previous one and I am stuck 
>> > in.
>> >         > I will not reply the email to you again.
>> >         >
>> >         > Sorry.
>> >         >
>> >         > On Fri, Aug 24, 2018 at 2:19 PM Razvan Cojocaru
>> >         > <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>
>> >         <mailto:rcojocaru@xxxxxxxxxxxxxxx
>> >         <mailto:rcojocaru@xxxxxxxxxxxxxxx>>> wrote:
>> >         >
>> >         >     On 8/24/18 8:07 AM, sepanta s wrote:
>> >         >     > I debugged the kernel codes and found out the root cause.
>> >         >     > The sharing is not done for some pages causing error
>> >         -E2BIG which
>> >         >     is due
>> >         >     > to reference count being more than one. This prevents
>> >         hypervor to
>> >         >     > nominate the page. As much as I know about this issue,
>> >         the page that I
>> >         >     > want to nominate and I receive this error from
>> >         make_page_sharable
>> >         >     > function in mem_sharing.c is pointed by more than one
>> >         pages. So, is
>> >         >     > there any idea to resolve this issue? I though that I
>> >         can put the page
>> >         >     > number in a buffer and share it again after t
>> >         milliseconds but
>> >         >     still the
>> >         >     > nomination return -E2BIG error.  Am I missing something?
>> >         >
>> >         >     As stated before, unfortunately I've never used this
>> >         feature and don't
>> >         >     know what to suggest. Again, maybe Tamas will chime in.
>> >         >
>> >
>>

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