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

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 16.09.25 um 18:47 schrieb Claudio Imbrenda:
> On Tue, 16 Sep 2025 18:26:53 +0200
> Heiko Carstens <hca@linux.ibm.com> wrote:
> 
>> On Wed, Sep 10, 2025 at 08:07:34PM +0200, Claudio Imbrenda wrote:
>>> Add page table management functions to be used for KVM guest (gmap)
>>> page tables.
>>>
>>> This patch adds the boilerplate and functions for the allocation and
>>> deallocation of DAT tables.
>>>
>>> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
>>> ---
>>>   arch/s390/kvm/Makefile     |  1 +
>>>   arch/s390/kvm/dat.c        | 91 ++++++++++++++++++++++++++++++++++++++
>>>   arch/s390/kvm/dat.h        |  4 ++
>>>   arch/s390/mm/page-states.c |  1 +
>>>   4 files changed, 97 insertions(+)
>>>   create mode 100644 arch/s390/kvm/dat.c
>>
>> ...
>>
>>> +static inline struct page_table *dat_alloc_pt_noinit(void)
>>> +{
>>> +	struct page *page;
>>> +	void *virt;
>>> +
>>> +	page = alloc_pages(GFP_ATOMIC, 0);
>>> +	if (!page)
>>> +		return NULL;
>>> +
>>> +	virt = page_to_virt(page);
>>> +	__arch_set_page_dat(virt, 1);
>>> +	return virt;
>>> +}
>>
>> Is GFP_ATOMIC a typo, and this should have been GFP_KERNEL?
>>
>> Otherwise I would guess this will cause problems in the future when
>> under memory pressure allocating guest page tables fails easily,
>> while before this change such allocations never failed.
> 
> how so? the documentation in gfp_types.h says:
> 
>   * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
>   * watermark is applied to allow access to "atomic reserves".
>   * The current implementation doesn't support NMI and few other strict
>   * non-preemptive contexts (e.g. raw_spin_lock). The same applies to %GFP_NOWAIT.
>   *
>   * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
>   * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
> 
> 
> 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.