Re: large page patch

[Andrew Morton]

"David S. Miller" wrote:
> 
>    From: Andrew Morton <[e-mail]>
>    Date: Thu, 01 Aug 2002 17:37:46 -0700
> 
>    Some observations which have been made thus far:
> 
>    - Minimal impact on the VM and MM layers
> 
> Well the downside of this is that it means it isn't transparent
> to userspace.  For example, specfp2000 results aren't going to
> improve after installing these changes.  Some of the other large
> page implementations would.
> 
>    - The change to MAX_ORDER is unneeded
> 
> This is probably done to increase the likelyhood that 4MB page orders
> are available.  If we collapse 4MB pages deeper, they are less likely
> to be broken up because smaller orders would be selected first.

This is leakage from ia64, which supports up to 256k pages.

> Maybe it doesn't make a difference....
> 
>    - swapping of large pages and making them pagecache-coherent is
>      unpopular.
> 
> Swapping them is easy, any time you hit a large PTE you unlarge it.
> This is what some of other large page implementations do.  Basically
> the implementation is that set_pte() breaks apart large ptes when
> necessary.

As far as mm/*.c is concerned, there is no pte.  It's just a vma
which is marked "don't touch"  These pages aren't on the LRU, nothing
knows about them.

Apparently a page-table based representation could not be used by PPC.