Re: [PATCH] x86: create array based interface to change page attribute

["Thomas Hellström" <thomas@tungstengraphics.com>]

Arjan van de Ven wrote:
> Thomas Hellström wrote:
>> Given this problem, the previously mentioned use-case, and the fact 
>> that we mostly really use user-space mappings,
>> Is there a possibility we could add the following functions to Dave's 
>> patch (provided they would work as intended, of course, namely 
>> invalidate / bring back the kernel mapping).
>
> sadly there are multiple mappings, both in theory and practice.
> Especially the _np / _p functions specifically work on only the 
> mapping you specify.
>
> For this to work we would need to somehow make a "mark all mappings 
> NP, but please only do the kernel ones" kind of thing.
> The semantics of that are... lets say messy at best.
Hmm, I'm not sure I follow you here. Are you saying that it's illegal to 
have an NP mapping of a page (which, If I understand it correctly, means 
no mapping at all) at the same time as you have a, say user-space WC 
mapping pointing to the same physical page?

Or are you saying that it's very hard to keep track of all mappings to a 
page, and change only the ones you really want to change?

If you mean the latter, for the DRM case, the DRM memory manager has 
already made sure all user-space mappings of a page are killed before 
calling CPA on it, (using unmap_mapping_range()) and they are faulted 
back once CPA is done.

I was under the impression that calling CPA on the kernel mapping of 
that page would do the rest?

/Thomas