[POWERPC] Clear sub-page HPTE present bits when demoting page size

[Benjamin Herrenschmidt]

On Wed, 2008-06-18 at 16:40 +1000, Paul Mackerras wrote:
> When we demote a slice from 64k to 4k, and we are about to insert an
> HPTE for a 4k subpage and we notice that there is an existing 64k
> HPTE, we first invalidate that HPTE before inserting the new 4k
> subpage HPTE.  Since the bits that encode which hash bucket the old
> HPTE was in overlap with the bits that encode which of the 16 subpages
> have HPTEs, we need to clear out the subpage HPTE-present bits before
> starting to insert HPTEs for the 4k subpages.  If we don't do that, we
> can erroneously think that a subpage already has an HPTE when it
> doesn't.
> 
> That in itself wouldn't be such a problem except that when we go to
> update the HPTE that we think is present on machines with a
> hypervisor, the hypervisor can tell us that the HPTE we think is there
> is actually there even though it isn't, which can lead to a process
> getting stuck in a loop, continually faulting.  The reason for the
> confusion is that the AVPN (abbreviated virtual page number) we are
> looking for in the HPTE for a 4k subpage can actually match the AVPN
> in a stale HPTE for another 64k page.  For example, the HPTE for
> the 4k subpage at 0x84000f000 will be in the same hash bucket and have
> the same AVPN as the HPTE for the 64k page at 0x8400f0000.
> 
> This fixes the code to clear out the subpage HPTE-present bits.
>
> Signed-off-by: Paul Mackerras <paulus at samba.org>

Acked-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>

> ---
> diff --git a/arch/powerpc/mm/hash_low_64.S b/arch/powerpc/mm/hash_low_64.S
> index 21d2484..70f4c83 100644
> --- a/arch/powerpc/mm/hash_low_64.S
> +++ b/arch/powerpc/mm/hash_low_64.S
> @@ -568,6 +568,10 @@ htab_inval_old_hpte:
>  	ld	r7,STK_PARM(r9)(r1)	/* ssize */
>  	ld	r8,STK_PARM(r8)(r1)	/* local */
>  	bl	.flush_hash_page
> +	/* Clear out _PAGE_HPTE_SUB bits in the new linux PTE */
> +	lis	r0,_PAGE_HPTE_SUB at h
> +	ori	r0,r0,_PAGE_HPTE_SUB at l
> +	andc	r30,r30,r0
>  	b	htab_insert_pte
>  	
>  htab_bail_ok:
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