Re: Removing kernel thread stack swapping

From: Brooks Davis (brooks_at_one-eyed-alien.net)
Date: 03/03/05

  • Next message: David Xu: "Re: Removing kernel thread stack swapping" 
    Date: Thu, 3 Mar 2005 08:58:25 -0800
To: John Baldwin <jhb@freebsd.org>

    On Thu, Mar 03, 2005 at 09:54:07AM -0500, John Baldwin wrote:
    > On Thursday 03 March 2005 02:42 am, David Schultz wrote:
    > > Any objections to the idea of removing the feature of swapping out
    > > kernel stacks? Unlike disabling UAREA swapping, this has the
    > > small downside that it wastes 16K (give or take a power of 2) of
    > > wired memory per kernel thread that we would otherwise have
    > > swapped out. However, this disadvantage is probably negligible by
    > > today's standards, and there are several advantages:
    > >
    > > 1. David Xu found that some kernel code stores externally-accessible
    > > data structures on the stack, then goes to sleep and allows the
    > > stack to become swappable. This can result in a kernel panic.
    >
    > He found one instance.
    >
    > > 2. We don't know how many instances of the above problem there are.
    > > Selectively disabling swapping for the right threads at the
    > > right times would decrease maintainability.
    >
    > Probably 1. Note that since at least FreeBSD 1.0 programmers have had to
    > realize that the stack can be swapped out. The signal code in pre-5.x stores
    > part of the signal state in struct proc directly in order to support swapped
    > out stacks. In 5.x we just malloc the whole signal state directly since we
    > killed the u-area. sigwait() has a bug that should be fixed, let's not
    > engage in overkill and throw the baby out with the bath water. In general we
    > need to discourage use of stack variables anyway because when people use
    > stack space rather than malloc() space the failure case for running out is
    > much worse, i.e. kernel panic when you overflow your stack (even though KVM
    > may be available) vs. waiting until some memory is available or returning
    > NULL.
    >
    > Hence, don't kill this whole feature just because someone is too lazy
    > to fix a bug.

    It would be very useful and informative if someone were to write a
    high level description of the ways in which the kernel is not a POSIX C
    programming environment. In addition to providing somewhere to point
    people who wonder why -lbigcomplicatedlibrary doesn't work with their
    kernel source, such a list would force us to enumerate those differences
    and make sure they are based on design decisions that make sense.

    -- Brooks 

    -- 
Any statement of the form "X is the one, true Y" is FALSE.
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  • Next message: David Xu: "Re: Removing kernel thread stack swapping"

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