linux-kernel - Re: [PATCH v6 04/10] arm64: hyperv: Add memory alloc/free functions for Hyper-V size pages

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Message-ID: <632eb459dbe53a9b69df2a4f030a755b@kernel.org>
Date:   Mon, 16 Mar 2020 08:30:42 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Michael Kelley <mikelley@...rosoft.com>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        gregkh <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
        linux-efi <linux-efi@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>, olaf@...fle.de,
        Andy Whitcroft <apw@...onical.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jason Wang <jasowang@...hat.com>, marcelo.cerri@...onical.com,
        "K. Y. Srinivasan" <kys@...rosoft.com>, sunilmut@...rosoft.com,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH v6 04/10] arm64: hyperv: Add memory alloc/free functions
 for Hyper-V size pages

On 2020-03-16 08:22, Arnd Bergmann wrote:
> On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@...rosoft.com> 
> wrote:
>>  /*
>> + * Functions for allocating and freeing memory with size and
>> + * alignment HV_HYP_PAGE_SIZE. These functions are needed because
>> + * the guest page size may not be the same as the Hyper-V page
>> + * size. We depend upon kmalloc() aligning power-of-two size
>> + * allocations to the allocation size boundary, so that the
>> + * allocated memory appears to Hyper-V as a page of the size
>> + * it expects.
>> + *
>> + * These functions are used by arm64 specific code as well as
>> + * arch independent Hyper-V drivers.
>> + */
>> +
>> +void *hv_alloc_hyperv_page(void)
>> +{
>> +       BUILD_BUG_ON(PAGE_SIZE <  HV_HYP_PAGE_SIZE);
>> +       return kmalloc(HV_HYP_PAGE_SIZE, GFP_KERNEL);
>> +}
>> +EXPORT_SYMBOL_GPL(hv_alloc_hyperv_page);
> 
> I don't think there is any guarantee that kmalloc() returns 
> page-aligned
> allocations in general.

I believe that guarantee came with 59bb47985c1db ("mm, sl[aou]b: 
guarantee
natural alignment for kmalloc(power-of-two)").

> How about using get_free_pages() to implement this?

This would certainly work, at the expense of a lot of wasted memory when
PAGE_SIZE isn't 4k.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...