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

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Date:   Mon, 16 Mar 2020 09:30:24 +0100
From:   gregkh <gregkh@...uxfoundation.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>,
        Marc Zyngier <maz@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.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 Mon, Mar 16, 2020 at 09:22:43AM +0100, 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. How about using get_free_pages()
> to implement this?

Even if it was guaranteed, a pointless wrapper like this is not needed
or ok, and shouldn't be created, just use kmalloc.

thanks,

greg k-h