Re: x86_64: potential critical issue with quicklists and page table pages - Kernel

This is a discussion on Re: x86_64: potential critical issue with quicklists and page table pages - Kernel ; On Fri, 2007-09-21 at 12:21 -0700, Linus Torvalds wrote: > Yeah, and the whole thing seems totally bogus. It totally depends on > mmu_gather doing everything right (which very much includes the dependency > on mmu gathering disabling preempt). > ...

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Thread: Re: x86_64: potential critical issue with quicklists and page table pages

  1. Re: x86_64: potential critical issue with quicklists and page table pages


    On Fri, 2007-09-21 at 12:21 -0700, Linus Torvalds wrote:

    > Yeah, and the whole thing seems totally bogus. It totally depends on
    > mmu_gather doing everything right (which very much includes the dependency
    > on mmu gathering disabling preempt).
    >
    > For exmaple, if we were to go back to the original small tlb_gather with a
    > simple quicklist on the stack, rather than the per-cpu datastructure, the
    > quicklists would immediately break horribly - simply because they are
    > incorrectly now depending on the internal semantics of that tlb-gather.
    >
    > As it is, the quicklists try to be something separate, but by virtue of
    > being separate, they will always be buggy.
    >
    > The only way to fix it would be to integrate the quicklist stuff *with*
    > the mmu_gather stuff, so that these kinds of implementation issues are
    > explicitly shown in the relationship, instead of havign two "independent"
    > pieces of code where one piece very subtly depends on the exact
    > implementation of the other.


    I've been mostly offline since KS (since a bit before in fact), so I
    missed some of those discussions but so that you know, I'm toying a bit
    with mmu gather and page table accessors at the moment, and one of the
    things I've been contemplating is just that ... integrating a quicklist
    in the gather to handle just that (and possibly other issues that have
    been overlooked on some archs).

    I'd suggest just reverting the patch for now (well, I see from the
    commit list that you did just that) and I'll try to come up with
    something better.

    Christoph, I'd be happy if you didn't start butchering mmu_gather just
    right now since I'm doing just that and it will collide all over the
    place :-) Or if you want something specific done, please throw
    ideas/patches at me and I'll integrate that in my serie.

    Cheers,
    Ben.


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  2. Re: x86_64: potential critical issue with quicklists and page table pages

    On Tue, 25 Sep 2007, Benjamin Herrenschmidt wrote:

    > I'd suggest just reverting the patch for now (well, I see from the
    > commit list that you did just that) and I'll try to come up with
    > something better.


    That would be great. Note that the reversal of the x86_64 quicklist
    support patch does not address the issue that is (as pointed out by
    Siddha) in core code and not in x86_64 arch dode. The fix that I posted
    fixes the core issue.

    > Christoph, I'd be happy if you didn't start butchering mmu_gather just
    > right now since I'm doing just that and it will collide all over the
    > place :-) Or if you want something specific done, please throw
    > ideas/patches at me and I'll integrate that in my serie.


    Any "butchering" was done for 2.6.22. 2.6.23 only added arch support for
    x86_64 since Andi forgot to merge it for .22. I'd be glad if you could
    look into this and make quicklists interoperate nicely withh mmu gather.
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  3. Re: x86_64: potential critical issue with quicklists and page table pages


    On Mon, 2007-09-24 at 14:42 -0700, Christoph Lameter wrote:
    > On Tue, 25 Sep 2007, Benjamin Herrenschmidt wrote:
    >
    > > I'd suggest just reverting the patch for now (well, I see from the
    > > commit list that you did just that) and I'll try to come up with
    > > something better.

    >
    > That would be great. Note that the reversal of the x86_64 quicklist
    > support patch does not address the issue that is (as pointed out by
    > Siddha) in core code and not in x86_64 arch dode. The fix that I posted
    > fixes the core issue.
    >
    > > Christoph, I'd be happy if you didn't start butchering mmu_gather just
    > > right now since I'm doing just that and it will collide all over the
    > > place :-) Or if you want something specific done, please throw
    > > ideas/patches at me and I'll integrate that in my serie.

    >
    > Any "butchering" was done for 2.6.22. 2.6.23 only added arch support for
    > x86_64 since Andi forgot to merge it for .22. I'd be glad if you could
    > look into this and make quicklists interoperate nicely withh mmu gather.


    I'll try, I'll let you know. Give me a few days to finish catching up
    with backlog of things before i get back to it though.

    Cheers,
    Ben.


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