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

[Marc Zyngier]

On 2020-03-16 08:22, Arnd Bergmann wrote:
> On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@xxxxxxxxxxxxx> 
> 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...