From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f198.google.com (mail-pg1-f198.google.com [209.85.215.198]) by kanga.kvack.org (Postfix) with ESMTP id C24836B0006 for ; Mon, 23 Jul 2018 17:50:16 -0400 (EDT) Received: by mail-pg1-f198.google.com with SMTP id q12-v6so1104710pgp.6 for ; Mon, 23 Jul 2018 14:50:16 -0700 (PDT) Received: from out4436.biz.mail.alibaba.com (out4436.biz.mail.alibaba.com. [47.88.44.36]) by mx.google.com with ESMTPS id e22-v6si8830318pfi.184.2018.07.23.14.50.14 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 23 Jul 2018 14:50:15 -0700 (PDT) Subject: Re: [PATCH] mm: thp: remove use_zero_page sysfs knob References: <1532110430-115278-1-git-send-email-yang.shi@linux.alibaba.com> <20180720123243.6dfc95ba061cd06e05c0262e@linux-foundation.org> <3238b5d2-fd89-a6be-0382-027a24a4d3ad@linux.alibaba.com> <20180722035156.GA12125@bombadil.infradead.org> From: Yang Shi Message-ID: <7d96258c-2973-2df5-08d4-828875058be1@linux.alibaba.com> Date: Mon, 23 Jul 2018 14:49:34 -0700 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: David Rientjes , Matthew Wilcox Cc: Andrew Morton , kirill@shutemov.name, hughd@google.com, aaron.lu@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 7/23/18 2:33 PM, 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? Sounds worth trying to me :-)A It might be worth making it persistent by default. Keeping 2MB memory unreclaimable sounds not harmful for the use case which prefer to use THP.