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    SubjectRe: [PATCh] x86: overmapped fix when 4K pages on tail - 64bit
    Andi Kleen wrote:
    > First I was only commenting on one specific patch, nothing more.
    > My point is full rounding to 4K on all corners is wasteful because the
    > CPUs have to handle that case anyways and every split costs precious
    > TLB entries in direct mapping accesses.

    Well, the CPU *does* handle them... by splitting the larger pages into
    smaller pages. They still end up in the small-page TLB, so there is no
    real difference if done in the CPU or in software.

    > And I might be old fashioned, but I still think minimizing TLB misses
    > in the kernel is still quite important since the TLBs of modern x86
    > CPUs are still comparatively small.
    > btw that is why I was also quite disappointed that the new cpa eliminated
    > reassembly. It means that on a long uptime system even with moderate
    > traffic of CPA page allocation/free eventually the completely direct mapping
    > will be all 4K. And there will be TLB miss galore on each system call
    > when user space is TLB intensive.
    > Ok in that light Yinghai's patch is perhaps not so bad after longer
    > uptime in that scenario. Still performance directly after boot up is
    > also something that shouldn't be ignored and I'm still hopefully that
    > reassembly will be readded at some point anyways.

    Memory state transitions are (fortunately) relatively rare and
    long-lived, but of course having reassembly is a nice thing to have in
    the long run. Such reassembly also would rather naturally handle any
    small-page effects of boundary cases.


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