Re: [PATCH 0/2] change_protection(): Count the number of pages affected

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, Nov 14, 2012 at 12:50 AM, Ingo Molnar <mingo@kernel.org> wrote:
> What do you guys think about this mprotect() optimization?

Hmm..

If this is mainly about just avoiding the TLB flushing, I do wonder if
it might not be more interesting to try to be much more aggressive.

As noted elsewhere, we should just notice when vm_page_prot doesn't
change at all - even if 'flags' change, it is possible that the actual
low-level page protection bits do not (due to the X=R issue).

But even *more* aggressively, how about looking at

 - not flushing the TLB at all if the bits become  more permissive
(taking the TLB micro-fault and letting the CPU just update it on its
own)

 - even *more* aggressive: if the bits become strictly more
restrictive, how about not flushing the TLB at all, *and* not even
changing the page tables, and just teaching the page fault code to do
it lazily at fault time?

Now, the "change protections lazily" might actually be a huge
performance problem with the page fault overhead dwarfing any TLB
flush costs, but we don't really know, do we? It might be worth trying
out.

               Linus

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