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

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Message-ID: <20180724090800.g43mmfnuuqwczzb2@kshutemo-mobl1>
Date:   Tue, 24 Jul 2018 12:08:00 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>, 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 Mon, Jul 23, 2018 at 02:33:08PM -0700, David Rientjes wrote:
> 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?

Why not a separate files?

-- 
 Kirill A. Shutemov