Re: [PATCH v2 08/20] KVM: s390: KVM page table management functions: allocation

Claudio Imbrenda
2025-09-17T14:00:18Z

On Wed, 17 Sep 2025 15:26:33 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> Am 17.09.25 um 15:11 schrieb Claudio Imbrenda:
> > On Wed, 17 Sep 2025 14:30:06 +0200
> > Heiko Carstens <hca@linux.ibm.com> wrote:
> >
> >> On Wed, Sep 17, 2025 at 01:25:56PM +0200, Claudio Imbrenda wrote:
> >>> On Wed, 17 Sep 2025 09:27:33 +0200
> >>> Heiko Carstens <hca@linux.ibm.com> wrote:
> >>>
> >>>> On Tue, Sep 16, 2025 at 07:36:44PM +0200, Heiko Carstens wrote:
> >>>>> On Tue, Sep 16, 2025 at 07:06:06PM +0200, Christian Borntraeger wrote:
> >>>>>>
> >>>>>> Am 16.09.25 um 19:05 schrieb Claudio Imbrenda:
> >>>>>>
> >>>>>>>>> I think GFP_ATOMIC actually gives more guarantees?
> >>>>>>>>
> >>>>>>>> In real life GFP_ATOMIC can fail, GFP_KERNEL does not.All gfp allocation failures
> >>>>>>>> are usually the atomic ones.
> >>>>>>>
> >>>>>>> interesting... then I guess I need GFP_KERNEL | GFP_ATOMIC ?
> >>>>>>
> >>>>>> No. ATOMIC always means: can fail.
> >>>
> >>> my issue is that GFP_KERNEL can sleep, and this allocation is sometimes
> >>> called from atomic contexts (e.g. while holding spinlocks)
> >>>
> >>> the right way to do this would be with mempools, to allocate memory
> >>> (and potentially sleep) when we are not in atomic context, and use it
> >>> whenever needed. this is on my to-do list for the future, but right now
> >>> I'd like to avoid having to refactor a ton of code.
> >>
> >> I doubt this is accetable even for an intermediate solution. As soon
> >> as the host is under memory pressure and starts doing I/O to free up
> >> memory, you will end up in -ENOMEM situations for simple guest page
> >> allocations.
> >>
> >> What happens with a guest in such a situation? Is this gracefully
> >> handled without that the guest is terminated?
> >
> > well, we return -ENOMEM to userspace (and qemu will probably kill the
> > guest)
> >
> > but if we can't even allocate 16kB, probably we're already in a pretty
> > bad situation
> >
> > if you think this is not acceptable, I guess I'll have to implement
> > mempools
>
> This is not acceptable. 16k atomic allocations are pretty much guaranteed
> to fail after a while of high workload.
> What are the callers of this allocation?

literally anything that touches the gmap page tables, since we need to
hold kvm->mmu_lock, which is an rw spinlock

I'll use mempools