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SubjectRe: [PATCH] mm: thp: remove use_zero_page sysfs knob
On Mon, 23 Jul 2018, David Rientjes wrote:

> > > The huge zero page can be reclaimed under memory pressure and, if it is,
> > > it is attempted to be allocted again with gfp flags that attempt memory
> > > compaction that can become expensive. If we are constantly under memory
> > > pressure, it gets freed and reallocated millions of times always trying to
> > > compact memory both directly and by kicking kcompactd in the background.
> > >
> > > It likely should also be per node.
> >
> > Have you benchmarked making the non-huge zero page per-node?
> >
>
> Not since we disable it :) I will, though. The more concerning issue for
> us, modulo CVE-2017-1000405, is the cpu cost of constantly directly
> compacting memory for allocating the hzp in real time after it has been
> reclaimed. We've observed this happening tens or hundreds of thousands
> of times on some systems. It will be 2MB per node on x86 if the data
> suggests we should make it NUMA aware, I don't think the cost is too high
> to leave it persistently available even under memory pressure if
> use_zero_page is enabled.
>

Measuring access latency to 4GB of memory on Naples I observe ~6.7%
slower access latency intrasocket and ~14% slower intersocket.

use_zero_page is currently a simple thp flag, meaning it rejects writes
where val != !!val, so perhaps it would be best to overload it with
additional options? I can imagine 0x2 defining persistent allocation so
that the hzp is not freed when the refcount goes to 0 and 0x4 defining if
the hzp should be per node. Implementing persistent allocation fixes our
concern with it, so I'd like to start there. Comments?

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