Re: XIP (Rev. 2)

[Matt Thomas <matt%3am-software.com@localhost>]

On Nov 9, 2010, at 9:06 AM, Masao Uebayashi wrote:

> On Tue, Nov 09, 2010 at 08:39:16AM -0800, Matt Thomas wrote:
>> 
>> On Nov 8, 2010, at 11:25 PM, Masao Uebayashi wrote:
>> 
>>>     http://uebayasi.dyndns.org/~uebayasi/tmp/uebayasi-xip-20101109.txt
>> 
>> Besides the churn (and there is a lot of it), I think my fundamental
>> problem with this incarnation of XIP is that it took a wrong approach.
>> It has tried to fit itself under uvm_vnodeops and I think that's a
>> fatal flaw by requiring invasive changes to contort to that decision.
>> 
>> Instead, XIP should have its own pager ops uvm_xipops and vnodes should
>> be set to use that in vnalloc/insmntque which is easily done since you 
>> can just check for MNT_XIP in the passed mount point.
> 
> I understand having a separate pager would work too.  If you go
> that route, you have to give up COW.  The two layered amap/uobj is
> the fundamental design of UVM.

You don't have to give up COW, you have to deal with it.  And those
changes will be far less pervasive than the changes you've had to make.

> I'd also point out that pgo_fault() is prepaired only for *special*
> purposes.  My plan is to rewrite those backends to use pgo_get().
> Then use single pmap_enter().

And XIP isn't special?  But I disagree that pgo_fault is only for
those purposes.  It could be used for more but that hasn't been
needed.

> Both of yours and mine are possible, and there're pros and cons.

That's true.