Re: [PATCH v3 02/14] KVM: s390: pv: avoid stall notifications for some UVCs

David Hildenbrand
2021-08-06T07:30:28Z

On 04.08.21 17:40, Claudio Imbrenda wrote:
> Improve make_secure_pte to avoid stalls when the system is heavily
> overcommitted. This was especially problematic in kvm_s390_pv_unpack,
> because of the loop over all pages that needed unpacking.
>
> Also fix kvm_s390_pv_init_vm to avoid stalls when the system is heavily
> overcommitted.

I suggest splitting this change into a separate patch and adding a bit
more meat to the description why using the other variant is possible in
the called context. I was kind of surprise to find that change buried in
this patch.

Then, you can give both patches a more descriptive patch subject.

>
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests")
> ---
> arch/s390/kernel/uv.c | 29 +++++++++++++++++++++++------
> arch/s390/kvm/pv.c | 2 +-
> 2 files changed, 24 insertions(+), 7 deletions(-)
>
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index aeb0a15bcbb7..68a8fbafcb9c 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -180,7 +180,7 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
> {
> pte_t entry = READ_ONCE(*ptep);
> struct page *page;
> - int expected, rc = 0;
> + int expected, cc = 0;
>
> if (!pte_present(entry))
> return -ENXIO;
> @@ -196,12 +196,25 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
> if (!page_ref_freeze(page, expected))
> return -EBUSY;
> set_bit(PG_arch_1, &page->flags);
> - rc = uv_call(0, (u64)uvcb);
> + /*
> + * If the UVC does not succeed or fail immediately, we don't want to
> + * loop for long, or we might get stall notifications.
> + * On the other hand, this is a complex scenario and we are holding a lot of
> + * locks, so we can't easily sleep and reschedule. We try only once,
> + * and if the UVC returned busy or partial completion, we return
> + * -EAGAIN and we let the callers deal with it.
> + */
> + cc = __uv_call(0, (u64)uvcb);
> page_ref_unfreeze(page, expected);
> - /* Return -ENXIO if the page was not mapped, -EINVAL otherwise */
> - if (rc)
> - rc = uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
> - return rc;
> + /*
> + * Return -ENXIO if the page was not mapped, -EINVAL for other errors.
> + * If busy or partially completed, return -EAGAIN.
> + */
> + if (cc == UVC_CC_OK)
> + return 0;
> + else if (cc == UVC_CC_BUSY || cc == UVC_CC_PARTIAL)
> + return -EAGAIN;
> + return uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
> }

That looks conceptually like the right thing to me.

--
Thanks,

David / dhildenb