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    SubjectRe: [PATCH 2/6] vmtrunc: restore unmap_vmas zap_bytes

    * Hugh Dickins <hugh@veritas.com> wrote:

    > The low-latency unmap_vmas patch silently moved the zap_bytes test
    > after the TLB finish and lockbreak and regather: why? That not only
    > makes zap_bytes redundant (might as well use ZAP_BLOCK_SIZE), it makes
    > the unmap_vmas level redundant too - it's all about saving TLB flushes
    > when unmapping a series of small vmas.

    the problem was latency generated by pages queueing up in tlb_gather's
    queue and being freed in one loop, causing latencies. So there was no
    latency-break, only a shifting of the queue.

    > Move zap_bytes test back before the lockbreak, and delete the curious
    > comment that a small zap block size doesn't matter: it's true
    > need_flush prevents TLB flush when no page has been unmapped, but
    > unmapping pages in small blocks involves many more TLB flushes than in
    > large blocks.

    could we perhaps free those pages outside of the lock? That would be
    just as good for me.

    Ingo
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