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Subject: Re: [PATCH v2 1/2] mm/vmalloc: export __vmalloc_node_range
To:     Uladzislau Rezki <urezki@gmail.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org, frankja@linux.ibm.com,
        cohuck@redhat.com, david@redhat.com, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        David Rientjes <rientjes@google.com>
References: <20210608180618.477766-1-imbrenda@linux.ibm.com>
 <20210608180618.477766-2-imbrenda@linux.ibm.com>
 <YMDlVdB8m62AhbB7@infradead.org> <20210609182809.7ae07aad@ibm-vm>
 <20210609164919.GA1938@pc638.lan>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <1fec60f1-6479-39a6-f47f-f47166f754f6@de.ibm.com>
Date:   Wed, 9 Jun 2021 19:50:27 +0200
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On 09.06.21 18:49, Uladzislau Rezki wrote:
> On Wed, Jun 09, 2021 at 06:28:09PM +0200, Claudio Imbrenda wrote:
>> On Wed, 9 Jun 2021 16:59:17 +0100
>> Christoph Hellwig <hch@infradead.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?
> Does the s390x support CONFIG_HAVE_ARCH_HUGE_VMALLOC what is arch
> specific?

Not yet, but we surely want that for almost everything on s390.
Only this particular firmware interface does not handle large pages
for donated memory.

> 
> If not then small pages are used. Or am i missing something?
> 
> I agree with Christoph that exporting a low level internals
> is not a good idea.