Re: [RFC PATCH 1/2] mm: hugetlb: Copy huge_pmd_share from x86 to mm.

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Apr 29, 2013 at 04:47:49PM +0100, Steve Capper wrote:
> On Mon, Apr 29, 2013 at 04:26:41PM +0100, Catalin Marinas wrote:
> > On Mon, Apr 29, 2013 at 03:55:55PM +0100, Steve Capper wrote:
> > > Under x86, multiple puds can be made to reference the same bank of
> > > huge pmds provided that they represent a full PUD_SIZE of shared
> > > huge memory that is aligned to a PUD_SIZE boundary.
> > > 
> > > The code to share pmds does not require any architecture specific
> > > knowledge other than the fact that pmds can be indexed, thus can
> > > be beneficial to some other architectures.
> > > 
> > > This patch copies the huge pmd sharing (and unsharing) logic from
> > > x86/ to mm/ and introduces a new config option to activate it:
> > > CONFIG_ARCH_WANTS_HUGE_PMD_SHARE.
> > 
> > Just wondering whether more of it could be shared. The following look
> > pretty close to what you'd write for arm64:
> > 
> > - huge_pte_alloc()
> > - huge_pte_offset() (there is a pud_large macro on x86 which checks for
> >   present & huge, we can replace it with just pud_huge in this function
> >   as it already checks for present)
> > - follow_huge_pud()
> > - follow_huge_pmd()
> 
> I did do something like this initially, then reined it back a bit
> as it placed implicit restrictions on x86 and arm64.
> 
> If we enable 64K pages on arm64 for instance, we obviate the need
> to share pmds (pmd_index doesn't exist for 64K pages). So I have a
> slightly different huge_pte_alloc function to account for this.

I guess with 64K pages on arm64 (two levels of page tables), you can't
share the pmds anyway. pud_none() is defined as 0 in pgtable-nopmd.h.
huge_pte_alloc() can probably be the same with 64K pages since pud_alloc
always succeeds (pmd/pud/pgd are all the same).

So with some #ifdef __PAGETABLE_PMD_FOLDED (not nice but it allows for
some more code sharing) you can add empty (NULL-returning)
huge_pmd_share/huge_pmd_unshare functions and avoid the compiler error
for pmd_index.

-- 
Catalin

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