Re: [QUICKLIST 0/4] Arch independent quicklists V2

[Andrew Morton <akpm@linux-foundation.org>]

> On Tue, 13 Mar 2007 23:01:11 +1100 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> Andrew Morton wrote:
> >>On Tue, 13 Mar 2007 22:30:19 +1100 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> >>We don't actually have to zap_pte_range the entire page table in
> >>order to free it (IIRC we used to have to, before the 4lpt patches).
> > 
> > 
> > I'm trying to remember why we ever would have needed to zero out the pagetable
> > pages if we're taking down the whole mm?  Maybe it's because "oh, the
> > arch wants to put this page into a quicklist to recycle it", which is
> > all rather circular.
> > 
> > It would be interesting to look at a) leave the page full of random garbage
> > if we're releasing the whole mm and b) return it straight to the page allocator.
> 
> Well we have the 'fullmm' case, which avoids all the locked pte operations
> (for those architectures where hardware pt walking requires atomicity).

I suspect there are some tlb operations which could be skipped in that case
too.

> However we still have to visit those to-be-unmapped parts of the page table
> to find the pages and free them. So we still at least need to bring it into
> cache for the read... at which point, the store probably isn't a big burden.

It means all that data has to be written back.  Yes, I expect it'll prove
to be less costly than the initial load.