Memory locking limit and zero-copy migrations

[Milan Zamazal]

Fangge Jin <fjin at redhat.com> writes:

> On Fri, Aug 19, 2022 at 4:08 AM Milan Zamazal <mzamazal at redhat.com> wrote:
>
>> > Not sure whether you already know this, but I had a hard time
>> > differentiating the two concepts:
>> > 1. memlock hard limit(shown by prlimit): the hard limit for locked host
>> > memory
>> > 2. memtune hard limit(memtune->hard_limit): the hard limit for in-use
>> host
>> > memory, this memory can be swapped out.
>>
>> No, I didn't know it, thank you for pointing this out.  Indeed, 2. is
>> what both the libvirt and kernel documentation seem to say, although not
>> so clearly.
>>
>> But when I add <memtune> with <hard_limit> to the domain XML and then
>> start the VM, I can see the limit shown by `prlimit -l' is increased
>> accordingly.  This is good for my use case, but does it match what you
>> say about the two concepts?
>
> memtune->hard_limit(hard limit of in-use memory) actually takes effect via
> cgroup,
> you can check the value by:
> # virsh memtune uefi1
> hard_limit     : 134217728
> soft_limit     : unlimited
> swap_hard_limit: unlimited
> # cat
> /sys/fs/cgroup/memory/machine.slice/machine-qemu\\x2d6\\x2duefi1.scope/libvirt/memory.limit_in_bytes
>
> 137438953472
>
> When vm starts with memtune->hard_limit set in domain XML, memlock
> hard limit( hard_limit of locked memory, shown by 'prlimit -l')will be
> set to the value of memtune->hard_limit. This's probably because
> memlock hard limit must be less than memtune->hard_limit.

Well, increasing the memlock limit to keep it within memtune->hard_limit
wouldn't make much sense, but thank you for confirming that setting
memtune->hard_limit adjusts both the limits to the requested value.