linux-kernel - Re: [PATCH v2 1/2] mm/vmalloc: export __vmalloc_node

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Date:   Wed, 9 Jun 2021 19:47:43 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, frankja@...ux.ibm.com,
        cohuck@...hat.com, david@...hat.com, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Uladzislau Rezki <urezki@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v2 1/2] mm/vmalloc: export __vmalloc_node_range

On 09.06.21 18:28, Claudio Imbrenda wrote:
> On Wed, 9 Jun 2021 16:59:17 +0100
> Christoph Hellwig <hch@...radead.org> wrote:
> 
>> On Tue, Jun 08, 2021 at 08:06:17PM +0200, Claudio Imbrenda wrote:
>>> The recent patches to add support for hugepage vmalloc mappings
>>> added a flag for __vmalloc_node_range to allow to request small
>>> pages. This flag is not accessible when calling vmalloc, the only
>>> option is to call directly __vmalloc_node_range, which is not
>>> exported.
>>>
>>> This means that a module can't vmalloc memory with small pages.
>>>
>>> Case in point: KVM on s390x needs to vmalloc a large area, and it
>>> needs to be mapped with small pages, because of a hardware
>>> limitation.
>>>
>>> This patch exports __vmalloc_node_range so it can be used in modules
>>> too.
>>
>> No.  I spent a lot of effort to mak sure such a low-level API is
>> not exported.
> 
> ok, but then how can we vmalloc memory with small pages from KVM?

An alternative would be to provide a vmalloc_no_huge function in generic
code  (similar to  vmalloc_32) (or if preferred in s390 base architecture code)
Something like

void *vmalloc_no_huge(unsigned long size)
{
         return __vmalloc_node_flags(size, NUMA_NO_NODE,VM_NO_HUGE_VMAP |
                                 GFP_KERNEL | __GFP_ZERO);
}
EXPORT_SYMBOL(vmalloc_no_huge);

or a similar vzalloc variant.