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

Christian Borntraeger
2025-09-17T14:05:52Z

Am 17.09.25 um 16:00 schrieb Claudio Imbrenda:
> 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

So how is x86 allocating page table memory. I cant see GPF_ATOMIC or mempool over there.