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    SubjectRe: [PATCH -mm] vmscan: make mapped executable pages the first class citizen
    On Fri, May 08, 2009 at 12:04:27PM -0400, Rik van Riel wrote:
    > Elladan wrote:
    >
    >>> Nobody (except you) is proposing that we completely disable
    >>> the eviction of executable pages. I believe that your idea
    >>> could easily lead to a denial of service attack, with a user
    >>> creating a very large executable file and mmaping it.
    >>>
    >>> Giving executable pages some priority over other file cache
    >>> pages is nowhere near as dangerous wrt. unexpected side effects
    >>> and should work just as well.
    >>
    >> I don't think this sort of DOS is relevant for a single user or trusted user
    >> system.
    >
    > Which not all systems are, meaning that the mechanism
    > Christoph proposes can never be enabled by default and
    > would have to be tweaked by the user.
    >
    > I prefer code that should work just as well 99% of the
    > time, but can be enabled by default for everybody.
    > That way people automatically get the benefit.

    I read Christopher's proposal as essentially, "have a desktop switch which
    won't evict executable pages unless they're using more than some huge
    percentage of RAM" (presumably, he wants anonymous pages to get special
    treatment too) -- this would essentially be similar to mlocking all your
    executables, only with a safety net if you go above x% and without affecting
    non-executable file maps.

    Given that, the DOS possibility you proposed seemed to just be one where a user
    could push a lot of unprotected pages out quickly and make the system run slow.

    I don't see how that's any different than just asking malloc() for a lot of ram
    and then touching it a lot to make it appear very hot to the VM. Any user can
    trivially do that already, and some apps (eg. a jvm) happily do that for you.
    The pathology is the same, and if anything an executable mmap is harder.

    -E


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