Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Wed, 9 Jun 2021 18:49:19 +0200
To:     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,
        borntraeger@de.ibm.com, cohuck@redhat.com, david@redhat.com,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Uladzislau Rezki <urezki@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        David Rientjes <rientjes@google.com>
Subject: Re: [PATCH v2 1/2] mm/vmalloc: export __vmalloc_node_range
Message-ID: <20210609164919.GA1938@pc638.lan>
References: <20210608180618.477766-1-imbrenda@linux.ibm.com>
 <20210608180618.477766-2-imbrenda@linux.ibm.com>
 <YMDlVdB8m62AhbB7@infradead.org>
 <20210609182809.7ae07aad@ibm-vm>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20210609182809.7ae07aad@ibm-vm>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

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?

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