[Xen-devel] Issues with Xenpaging in Xen-4.2.1

[sahilsuneja <sahilsuneja@xxxxxxxxx>]

Hi,

I know Xenpaging is still at an experimental stage, but I still wanted to
test it out. Sadly, I could not get it to work reliably.

I have an AMD RVI enabled linux 3.2 host, and I am using linux 2.6.35-22 HVM
guests. After booting a few guests normally upto the host RAM limit, I do
the following to enable paging for a few guests:

/>/usr/lib/xen/bin/xenpaging -f /var/lib/xen/xenpaging/pagefile-hvm-guest -d
1 &
>xenstore-write /local/domain/1/memory/target-tot_pages $((1024*512))/

(here '1' is the dom_id and pagefile-hvm-guest is the backing paging file
for the hvm guest)

I am then able to boot more VMs and I can see the corresponding pagefiles
increase in size for the xenpaging enabled guests.

The problem is when I go back to the guests on which I enabled xenpaging, I
see the following errors when I run any basic commands like vim, top, less
/proc/meminfo:

/xc: error: Error populating page 100: Internal error
xc: error: Error loading 100 during page-in (22 = Invalid argument):
Internal error/

This happens even when there is paged-out memory from other guests to be
had, and the system is not out of RAM for paging in guest's memory.

It seems like a page-in error on gfn 100, but that is strange as, from I
what I gathered from the xenpaging code, policy_default.c starts paging only
after half of the guest's gfns : current_gfn = max_pages / 2 (inside
policy_default.c: policy_init())

I was expecting there to be other stability issues rather that this kind of
an error.

Could someone please help me understand this and let me know if I am doing
something incorrectly. 

Also, there do not seem to be any interesting messages in xm dmesg or in
/var/log/xen*.

Thanks,

Sahil Suneja
Ph.D. Student
University of Toronto



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