ACK: [SRU][J][I][F][PATCH 0/1] KVM nesting support leaks too much memory, might result in stalls during cleanup (LP: 1974017)

[Marcelo Henrique Cerri]

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Acked-by: Marcelo Henrique Cerri <marcelo.cerri at canonical.com>

On Thu, May 19 2022, frank.heimes at canonical.com wrote:
> BugLink: https://bugs.launchpad.net/bugs/1974017
>
> SRU Justification:
>
> [Impact]
>
>  * If running KVM with nesting support (e.g. 'kvm.nested=1' on the kernel
>    command line), the shadow page table code will produce too many entries
>    in the shadow code.
>
>  * The below mentioned upstream fix will prevent the entries from being
>    piled up, by checking for existing entries at insert time.
>
>  * This measurably reduces the list length and is faster than traversing
>    the list at shutdown time only.
>
> [Fix]
>
>  * a06afe8383080c630a7a528b8382fc6bb4925b61 a06afe838308
>    "KVM: s390: vsie/gmap: reduce gmap_rmap overhead"
>
> [Test Plan]
>
>  * A IBM zSystems or LinuxONE LPAR on a z13 or newer is needed.
>
>  * Ubuntu focal, impish or jammy needs to be installed
>    and the Ubuntu LPAR setup as (1st level) KVM host,
>    allowing nested virtualization.
>
>  * Now setup one (or more) KVM virtual machines,
>    with similar Ubuntu releases,
>    and define one or more of them again as (2nd level) KVM host.
>
>  * Define several KVM virtual machines on this (2nd level) KVM host
>    in a memory constraint fashion,
>    so that a lot of memory mapping is caused.
>
>  * Let such a system run for a while under load.
>
>  * Now shutdown one (or more) 2nd level VMs and notice the
>    time it takes.
>
>  * With the patch in place this time should be considerably
>    quicker than without.
>
>  * The result is reduced mapping (gmap_rmap) overhead,
>    less danger of leaking memory
>    and a better responding system.
>
> [Where problems could occur]
>
>  * In case wrong entries are freed up this will harm the virtual
>    memory management and may even lead to crashes.
>
>  * In case the pointer handling is not done properly,
>    again crashes may occur.
>
>  * But with net just five new lines the patch is pretty short, readable
>    and the modifications traceable in arch/s390/mm/gmap.c only.
>
>  * The changes are limited to s390/mm only,
>    hence don't affect other architectures.
>
> [Other Info]
>
>  * The commit was upstream accepted in v5.18-rc6.
>
>  * Since the planned target kernel for kinetic is 5.19,
>    the kinetic kernel does not need to be patched.
>
>  * Hence the SRUs are for jammy, impish and focal.
>
> Christian Borntraeger (1):
>   KVM: s390: vsie/gmap: reduce gmap_rmap overhead
>
>  arch/s390/mm/gmap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> --
> 2.25.1


- --
Regards,
Marcelo
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