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

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Message-ID: <20210609164919.GA1938@pc638.lan>
Date:   Wed, 9 Jun 2021 18:49:19 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     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,
        borntraeger@...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 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?

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.

--
Vlad Rezki