linux-kernel - Re: [PATCH 0/6] mm: make movable onlining suck less

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Date:   Tue, 4 Apr 2017 10:23:02 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Reza Arbab <arbab@...ux.vnet.ibm.com>, Mel Gorman <mgorman@...e.de>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        Tang Chen <tangchen@...fujitsu.com>, qiuxishi@...wei.com,
        Kani Toshimitsu <toshi.kani@....com>, slaoub@...il.com,
        Joonsoo Kim <js1304@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Zhang Zhen <zhenzhang.zhang@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dan Williams <dan.j.williams@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Lai Jiangshan <laijs@...fujitsu.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 0/6] mm: make movable onlining suck less

On Tue 04-04-17 09:34:12, Michal Hocko wrote:
> On Tue 04-04-17 09:23:29, Michal Hocko wrote:
> > [Let's add Gary who as introduced this code c04fc586c1a48]
> 
> OK, so Gary's email doesn't exist anymore. Does anybody can comment on
> this? I suspect this code is just-in-case... Mel?
>  
> > On Mon 03-04-17 15:42:13, Reza Arbab wrote:
> [...]
> > > Almost there. I'm seeing the memory in the correct node now, but the
> > > /sys/devices/system/node/nodeX/memoryY links are not being created.
> > > 
> > > I think it's tripping up here, in register_mem_sect_under_node():
> > > 
> > > 		page_nid = get_nid_for_pfn(pfn);
> > > 		if (page_nid < 0)
> > > 			continue;
> > 
> > Huh, this code is confusing. How can we have a memblock spanning more
> > nodes? If not then the loop over all sections in the memblock seem
> > pointless as well.  Also why do we require page_initialized() in
> > get_nid_for_pfn? The changelog doesn't explain that and there are no
> > comments that would help either.

OK, so I've been thinkin about that and I believe that page_initialized
check in get_nid_for_pfn is just bogus. There is nothing to rely on the
page::lru to be already initialized. So I will go with the following as
a separate preparatory patch.

I believe the whole code should be revisited and I have put that on my
ever growing todo list because I suspect that it is more complex than
necessary. I suspect that memblock do not span more nodes and all this
is just-in-case code (e.g. the onlining code assumes a single zone aka
node. But let's do that later.

---
>From fd2e3b6eca1cf7766527203d23db6aca5957a3f1 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
Date: Tue, 4 Apr 2017 10:05:06 +0200
Subject: [PATCH] mm: drop page_initialized check from get_nid_for_pfn

c04fc586c1a4 ("mm: show node to memory section relationship with
symlinks in sysfs") has added means to export memblock<->node
association into the sysfs. It has also introduced get_nid_for_pfn
which is a rather confusing counterpart of pfn_to_nid which checks also
whether the pfn page is already initialized (page_initialized).  This
is done by checking page::lru != NULL which doesn't make any sense at
all. Nothing in this path really relies on the lru list being used or
initialized. Just remove it

Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 drivers/base/node.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/drivers/base/node.c b/drivers/base/node.c
index 5548f9686016..ee080a35e869 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -368,8 +368,6 @@ int unregister_cpu_under_node(unsigned int cpu, unsigned int nid)
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG_SPARSE
-#define page_initialized(page)  (page->lru.next)
-
 static int __ref get_nid_for_pfn(unsigned long pfn)
 {
 	struct page *page;
@@ -380,9 +378,6 @@ static int __ref get_nid_for_pfn(unsigned long pfn)
 	if (system_state == SYSTEM_BOOTING)
 		return early_pfn_to_nid(pfn);
 #endif
-	page = pfn_to_page(pfn);
-	if (!page_initialized(page))
-		return -1;
 	return pfn_to_nid(pfn);
 }
 
-- 
2.11.0

-- 
Michal Hocko
SUSE Labs