[RFC PATCH 1/6] arm64: vmemmap: use virtual projection of linear region

[Will Deacon]

On Thu, Feb 25, 2016 at 08:02:00AM +0100, Ard Biesheuvel wrote:
> On 24 February 2016 at 17:21, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> > Commit dd006da21646 ("arm64: mm: increase VA range of identity map") made
> > some changes to the memory mapping code to allow physical memory to reside
> > at an offset that exceeds the size of the virtual address space.
> >
> > However, since the size of the vmemmap area is proportional to the size of
> > the VA area, but it is populated relative to the physical space, we may
> > end up with the struct page array being mapped outside of the vmemmap
> > region. For instance, on my Seattle A0 box, I can see the following output
> > in the dmesg log.
> >
> >    vmemmap : 0xffffffbdc0000000 - 0xffffffbfc0000000   (     8 GB maximum)
> >              0xffffffbfc0000000 - 0xffffffbfd0000000   (   256 MB actual)
> >
> > We can fix this by deciding that the vmemmap region is not a projection of
> > the physical space, but of the virtual space above PAGE_OFFSET, i.e., the
> > linear region. This way, we are guaranteed that the vmemmap region is of
> > sufficient size, and we can also reduce its size by half.
> >
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 7 ++++---
> >  arch/arm64/mm/init.c             | 4 ++--
> >  2 files changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index a440f5a85d08..8e6baea0ff61 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -34,18 +34,19 @@
> >  /*
> >   * VMALLOC and SPARSEMEM_VMEMMAP ranges.
> >   *
> > - * VMEMAP_SIZE: allows the whole VA space to be covered by a struct page array
> > + * VMEMAP_SIZE: allows the whole linear region to be covered by a struct page array
> >   *     (rounded up to PUD_SIZE).
> >   * VMALLOC_START: beginning of the kernel vmalloc space
> >   * VMALLOC_END: extends to the available space below vmmemmap, PCI I/O space,
> >   *     fixed mappings and modules
> >   */
> > -#define VMEMMAP_SIZE           ALIGN((1UL << (VA_BITS - PAGE_SHIFT)) * sizeof(struct page), PUD_SIZE)
> > +#define VMEMMAP_SIZE           ALIGN((1UL << (VA_BITS - PAGE_SHIFT - 1)) * sizeof(struct page), PUD_SIZE)
> >
> >  #define VMALLOC_START          (MODULES_END)
> >  #define VMALLOC_END            (PAGE_OFFSET - PUD_SIZE - VMEMMAP_SIZE - SZ_64K)
> >
> > -#define vmemmap                        ((struct page *)(VMALLOC_END + SZ_64K))
> > +#define VMEMMAP_START          (VMALLOC_END + SZ_64K)
> > +#define vmemmap                        ((struct page *)(VMEMMAP_START - memstart_addr / sizeof(struct page)))
> >
> 
> Note that with the linear region randomization which is now in -next,
> this division needs to be signed (since memstart_addr can wrap).
> 
> So I should either update the definition of memstart_addr to s64 in
> this patch, or cast to (s64) in the expression above

Can you avoid the division altogether by doing something like:

(struct page *)(VMEMMAP_START - (PHYS_PFN(memstart_addr) * sizeof(struct page)))

or have I misunderstood how this works?

Will