Re: [PATCH] mm: thp: remove use_zero_page sysfs knob

[Yang Shi]

On 7/20/18 1:02 PM, David Rientjes wrote:
> On Fri, 20 Jul 2018, Andrew Morton wrote:
>
> > > By digging into the original review, it looks use_zero_page sysfs knob
> > > was added to help ease-of-testing and give user a way to mitigate
> > > refcounting overhead.
> > >
> > > It has been a few years since the knob was added at the first place, I
> > > think we are confident that it is stable enough. And, since commit
> > > 6fcb52a56ff60 ("thp: reduce usage of huge zero page's atomic counter"),
> > > it looks refcounting overhead has been reduced significantly.
> > >
> > > Other than the above, the value of the knob is always 1 (enabled by
> > > default), I'm supposed very few people turn it off by default.
> > >
> > > So, it sounds not worth to still keep this knob around.
> > Probably OK.  Might not be OK, nobody knows.
> >
> > It's been there for seven years so another six months won't kill us.
> > How about as an intermediate step we add a printk("use_zero_page is
> > scheduled for removal.  Please contact linux-mm@xxxxxxxxx if you need
> > it").
> We disable the huge zero page through this interface, there were issues
> related to the huge zero page shrinker (probably best to never free a
> per-node huge zero page after allocated) and CVE-2017-1000405 for huge
> dirty COW.

Thanks for the information. It looks the CVE has been resolved by commit 
a8f97366452ed491d13cf1e44241bc0b5740b1f0 ("mm, thp: Do not make page 
table dirty unconditionally in touch_p[mu]d()"), which is in 4.15 already.

What was the shrinker related issue? I'm supposed it has been resolved, 
right?