linux-kernel - Re: [PATCH] mm: thp: remove use_zero

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Message-ID: <3238b5d2-fd89-a6be-0382-027a24a4d3ad@linux.alibaba.com>
Date:   Fri, 20 Jul 2018 13:37:47 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     kirill@...temov.name, hughd@...gle.com, aaron.lu@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: thp: remove use_zero_page sysfs knob



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@...ck.org 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?