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Subject: Re: [PATCH v2 1/2] mm/vmalloc: export __vmalloc_node_range
To:     Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Christoph Hellwig <hch@infradead.org>
Cc:     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>,
        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>
References: <20210608180618.477766-1-imbrenda@linux.ibm.com>
 <20210608180618.477766-2-imbrenda@linux.ibm.com>
 <YMDlVdB8m62AhbB7@infradead.org> <20210609182809.7ae07aad@ibm-vm>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <6bf6fb06-0930-8cae-3e2b-8cb3237a6197@de.ibm.com>
Date:   Wed, 9 Jun 2021 19:47:43 +0200
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On 09.06.21 18:28, 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?

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.