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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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