Subject: [PATCH V4 2/2] arm: mm: Switch back to L_PTE_WRITE

From: [email-masked] (Will Deacon)
Date: Fri, 20 Jun 2014 10:21:35 +0100

On Mon, Jun 16, 2014 at 03:32:39PM +0100, Steve Capper wrote:
> For LPAE, we have the following means for encoding writable or dirty
> ptes:
>                               L_PTE_DIRTY       L_PTE_RDONLY
>     !pte_dirty && !pte_write        0               1
>     !pte_dirty && pte_write         0               1
>     pte_dirty && !pte_write         1               1
>     pte_dirty && pte_write          1               0
> 
> So we can't distinguish between writable clean ptes and read only
> ptes. This can cause problems with ptes being incorrectly flagged as
> read only when they are writable but not dirty.
> 
> This patch re-introduces the L_PTE_WRITE bit for both short descriptors
> and long descriptors, by reverting
>   36bb94b ARM: pgtable: provide RDONLY page table bit rather than WRITE bit
> 
> For short descriptors the L_PTE_RDONLY bit is renamed to L_PTE_WRITE
> and the pertinent logic changed. For long descriptors, L_PTE_WRITE is
> implemented as a new software bit.
> 
> HugeTLB pages will use the L_PTE_WRITE semantics automatically.
> 
> We need to add some logic to Transparent HugePages to ensure that they
> correctly interpret the revised pgprot permissions.

I think this look alright, but it certainly needs some stress testing. Have
you given it a good hammering? If so, this could use some exposure in -next.

  Reviewed-by: Will Deacon <[email-masked]>

Will