Re: Merge of rmind-uvmplock branch

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

On Jun 14, 2011, at 8:52 AM, Andrew Doran wrote:

> Mindaugas, thank you very much for your hard work!!
> 
> On Tue, Jun 07, 2011 at 03:16:06AM +0000, YAMAMOTO Takashi wrote:
> 
>> the idea is to protect pv chains with object lock, right?
> 
> That was the initial driver for me anyway.
> 
> Now that this work is in, there are some changes that can be built upon it
> to realise the full value -- and hopefully make exit() etc. very cheap.
> Looking at this from an x86 perspective but some of the ideas will apply
> to other ports:
> 
> - My initial idea was to kill the global PV hash and associated locks.  To
>  replace this we would embed a list head in each uvm_map_entry (or wherever
>  else a mapping is managed).  This would be supplied by the caller on each
>  relevant pmap call - pmap_enter() and so on.  PV entries would be added to
>  and removed from this structure by the pmap module.  An initial
>  implementation could get away with a dumb linked list I think.

So if there is no global P->V list for a page, how do pmap_page_protect and 
friends work?  That is the point of the PV entry after all.

For doing V->P, the phys_addr is gotten from accessing the page table and then 
a P->V lookup is done.

> - So then PV entries tracked with the mapping instead of globally. Once
>  pmap_remove_all() has been called pmap_remove() could switch to a
>  "shortcut" mode and become very quick.  From memory I believe all it would
>  need to do is tear down the software PV entries, and not touch any page or
>  pmap state.  Tearing down pmap state would be deferred to pmap_destroy(). 
>  At that point we can then clear out the pmap's uvm_objects and free all
>  pmap pages in bulk.  This would avoid potentially thousands of expensive
>  scans and atomic updates to the hardware paging structures, which account
>  for the bulk of expense during exit() etc.  If the system is short on
>  memory we might want a mechanism to switch CPUs away from this pmap if
>  they are hanging onto it as pmap_kernel - i.e. preventing pmap_destroy()
>  from being called.

Still don't have a good algorithm for pmap_remove_all.  Walking the page tables 
isn't very fast since the p->v overhead is annoying.