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Re: Merge of rmind-uvmplock branch
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.
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