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

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Date:   Wed, 9 Jun 2021 19:50:27 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Uladzislau Rezki <urezki@...il.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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>,
        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 18:49, 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@...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?
> 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.