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    SubjectRe: x86: 4kstacks default
    On Thu, 24 Apr 2008 09:36:52 +1000
    David Chinner <dgc@sgi.com> wrote:

    > On Wed, Apr 23, 2008 at 03:27:01PM +1000, Benjamin Herrenschmidt
    > wrote:
    > > On Sat, 2008-04-19 at 16:23 +0200, Ingo Molnar wrote:
    > > > * Andrew Morton <akpm@linux-foundation.org> wrote:
    > > >
    > > > > > config 4KSTACKS
    > > > > > bool "Use 4Kb for kernel stacks instead of 8Kb"
    > > > > > - depends on DEBUG_KERNEL
    > > > > > depends on X86_32
    > > > > > + default y
    > > > >
    > > > > This patch will cause kernels to crash.
    > > >
    > > > what mainline kernels crash and how will they crash? Fedora and
    > > > other distros have had 4K stacks enabled for years:
    > > >
    > > > $ grep 4K /boot/config-2.6.24-9.fc9
    > > > CONFIG_4KSTACKS=y
    > > >
    > > > and we've conducted tens of thousands of bootup tests with all
    > > > sorts of drivers and kernel options enabled and have yet to see a
    > > > single crash due to 4K stacks. So basically the kernel default
    > > > just follows the common distro default now. (distros and users
    > > > can still disable it)
    > >
    > > Do we routinely test nasty scenarii such as a GFP_KERNEL allocation
    > > deep in a call stack trying to swap something out to NFS ?
    >
    > I doubt it, because this is the place that a local XFS filesystem
    > typically blows a 4k stack (direct memory reclaim triggering
    > ->writepage). Boot testing does nothing to exercise the potential
    > paths for stack overflows....
    >

    THe good news is that direct reclaim is.. rare.
    And I also doubt XFS is unique here; imagine the whole stacking thing on x86-64 just the same ...

    I wonder if the direct reclaim path should avoid direct reclaim if the stack has only X bytes left.
    (where the value of X is... well we can figure that one out later)

    The rarity of direct reclaim during normal use ought to make this not a performance problem per se,
    and the benefits go further than just "XFS" or "4K stacks".

    --
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