Re: [PATCH 3/4] add replace_page(): change the page pte is pointing to.

[Izik Eidus <ieidus@redhat.com>]

Andrea Arcangeli wrote:
> On Tue, Apr 14, 2009 at 03:09:25PM -0700, Andrew Morton wrote:
>   
>> On Thu,  9 Apr 2009 06:58:40 +0300
>> Izik Eidus <ieidus@redhat.com> wrote:
>>
>>     
>>> replace_page() allow changing the mapping of pte from one physical page
>>> into diffrent physical page.
>>>       
>> At a high level, this is very similar to what page migration does.  Yet
>> this implementation shares nothing with the page migration code.
>>
>> Can this situation be improved?
>>     
>
> This was discussed last time too. Basically the thing is that using
> migration entry with its special page fault paths, for this looks a
> bit of an overkill complexity and unnecessary dependency on the
> migration code. 

I agree about that.

> All we need is to mark the pte readonly. replace_page
> is a no brainer then. The brainer part is page_wrprotect
> (page_wrprotect is like fork).
>
> The data visibility in the final memcmp you mentioned in the other
> mail is supposedly taken care of by page_wrprotect too. It already
> does flush_cache_page for the virtual indexed and not physically
> tagged caches. page_wrprotect has to also IPI all CPUs to nuke any not
> wrprotected tlb entry. I don't think we need further smp memory
> barriers when we're guaranteed all tlb entries are wrprotected in the
> other cpus and an IPI and invlpg run in them, to be sure we read the
> data stable during memcmp even if we read through the kernel
> pagetables and the last userland write happened through userland ptes
> before they become effective wrprotected by the IPI.
>   

Yup agree.