[Question] A question about arm64 pte

[Steve Capper]

On Mon, Jan 16, 2017 at 08:39:56PM +0800, Yisheng Xie wrote:
> hi Catalin,
> Thank you so much for you reply.
> 
> On 2017/1/16 19:56, Catalin Marinas wrote:
> > On Mon, Jan 16, 2017 at 06:08:47PM +0800, Yisheng Xie wrote:
> >> I have question about arm64 pte.
> > 
> > I assume the context is ARMv8.0 (without hardware DBM support).
> 
> Yes.
> > 
> >> For arm64, PTE_WRITE（== PTE_DBM） is to mark whether the page is writable,
> >> and PTE_DIRTY is to mark whether the page is dirty.
> >> However, PTE_RDONLY is only cleared when both PTE_WRITE and PTE_DIRTY are set.
> > 
> > That's what set_pte_at() does.
> > 
> 
> So if we mmap a memory region use /dev/mem like:
>    fildes = open("/dev/mem", O_RDWR | O_CREAT, 0777);
>    addr = mmap(NULL, LEN, PROT_READ | PROT_WRITE, MAP_SHARED, fildes, offset);
> 
> The PTE_RDONLY will be set， Right ?
> However，when use memset to write the region it still works well, and the bit PTE_RDONLY is also cleared.
> Is there anywhere clear the PTE_RDONLY before write that page ?
> 

Hi Yisheng,
Out of interest, why is /dev/mem being accessed directly from userspace?

The case above will have subtley different logic (mmap_mem will affect
how things are actually mapped); which I'm trying to understand...

Cheers,
-- 
Steve