Messages in this thread | | | From | Nick Piggin <> | Subject | Re: [PATCH 1/2] mm: implement remap_pfn_range with apply_to_page_range | Date | Fri, 14 Nov 2008 18:35:16 +1100 |
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On Friday 14 November 2008 16:22, Jeremy Fitzhardinge wrote: > Nick Piggin wrote: > > On Friday 14 November 2008 13:56, Jeremy Fitzhardinge wrote: > >> Nick Piggin wrote: > >>> This isn't performance critical to anyone? > >> > >> The only difference should be between having the specialized code and an > >> indirect function call, no? > > > > Indirect function call per pte. It's going to be slower surely. > > Yes, though changing the calling convention to handle (up to) a whole > page worth of ptes in one call would be fairly simple I think.
Yep. And leaving it alone is even simpler and still faster :)
> > It is accepted practice to (carefully) duplicate the page table walking > > functions in memory management code. I don't think that's a problem, > > there is already so many instances of them (just be sure to stick to > > exactly the same form and variable names, and any update or bugfix to > > any of them is trivially applicable to all). > > I think that's pretty awful practice, frankly, and I'd much prefer there > to be a single iterator function which everyone uses.
I think its pretty nice. It means you can make the loops fairly optimal even if they might have slightly different requirements (different arguments, latency breaks, copy_page_range etc).
> The open-coded > iterators everywhere just makes it completely impractical to even think > about other kinds of pagetable structures. (Of course we have at least > two "general purpose" pagetable walkers now...)
I think that's being way over dramatic. When switching to a different page table structure, I assure you that copying and pasting your new walking algorithm a few times will be the least of your worries :)
It's not meant to be pluggable. Actually this came up last I think when the UNSW wanted to add page table accessors to abstract this. They came up with a good set of things, but in the end you can't justify slowing things down in these paths unless you actually have a replacement page table structure that gets you a *net win*. So far, I haven't heard from them again.
No, adding a cycle here or an indirect function call there IMO is not acceptable in core mm/ code without a good reason.
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