Re: [PATCH 4/10] alternate 4-level page tables patches

[Linus Torvalds <torvalds@osdl.org>]

On Sat, 18 Dec 2004, William Lee Irwin III wrote:
>
> William Lee Irwin III wrote:
> >> vmas are unmapped one-by-one during process destruction.
> 
> On Sun, Dec 19, 2004 at 11:05:22AM +1100, Nick Piggin wrote:
> > Yeah but clear_page_tables isn't called for each vma that is unmapped
> > at exit time. Rather, one big one is called at the end - I suspect
> > this is usually more efficient.
> 
> For clear_page_tables() you want to scan as little as possible. The
> exit()-time performance issue is tlb_finish_mmu().

Note that the fact that we share lots of code between "unmap" and "exit" 
is likely a (performance) bug.

The exit case is really a lot simpler, not just because we get rid of the 
whole VM, but because nobody else can be reading the page tables at the 
same time, and in particular we do not have a lot of the races that a 
simple unmap can have. The whole "gather/flush" thing is overkill, I 
think.

Actually, looking at the code, I wonder why we haven't marked the exit 
case to be "fast". We have this special optimization for single-CPU which 
doesn't bunch pages up and free them in chunks, and we should probably 
mark the exit case to use the fast-case where we can flush the TLB's 
early. Hmm?

Ingo, is there any reason we don't do this:

--- 1.24/include/asm-generic/tlb.h	2004-07-10 17:14:00 -07:00
+++ edited/include/asm-generic/tlb.h	2004-12-18 17:30:43 -08:00
@@ -58,7 +58,7 @@
 	tlb->mm = mm;
 
 	/* Use fast mode if only one CPU is online */
-	tlb->nr = num_online_cpus() > 1 ? 0U : ~0U;
+	tlb->nr = num_online_cpus() > 1 && !full_mm_flush ? 0U : ~0U;
 
 	tlb->fullmm = full_mm_flush;
 	tlb->freed = 0;

which should make the exit case TLB handling go much faster. Was there 
some race in that too? Nobody should be using the VM any more at that 
point, so it _should_ be safe, no?

		Linus
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