Re: [rfc] mm: mmu gather in-place

From: Nick Piggin
Date: Fri Mar 17 2006 - 09:13:20 EST

Nick Piggin wrote:

I'm embarrassed to release this patch in such a state, but I am
because a) I won't have much time to work on it in the short term;
b) it would take a lot of work to polish so I'd like to see what
people think before going too far; c) so I have something other than
boring lockless pagecache to talk about at Ottawa.

The basic idea is this: replace the heavyweight per-CPU mmu_gather
structure with a lightweight stack based one which is missing the
big page vector. Instead of the vector, use Linux pagetables to
store the pages-to-be-freed. Pages and pagetables are first unmapped,
then tlbs are flushed, then pages and pagetables are freed.

There is a downside: walking the page table can be anywhere from
slightly to a lot less efficient than walking the vector, depending
on density, and this adds a 2nd pagetable walk to unmapping (but
removes the vector walk, of course).

Upsides: mmu_gather is preemptible, horrible mmu_gather breaking
code can be removed, artificial disparity between PREEMPT tlb
flush batching and non-PREEMPT disappears (preempt can now have
good performance and non-preempt can have good latency). tlb flush
batching is possibly much closer to perfect though on non-PREEMPT
that may not be noticable (for PREEMPT, it appears to be spending
5x less time in tlb flushing on kbuild)

- nonlinear mappings don't work yet
- hugepages don't work yet
- i386 only

Note that in theory it should be usable by any architecture of course.
Actually those ones for which hardware doesn't natively grok the Linux
page tables can even be more creative than i386...

SUSE Labs, Novell Inc.
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