linux-kernel - Re: [PATCH 07/14] mm: consider zone which is not fully populated to have holes

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Date:   Thu, 18 May 2017 18:42:10 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Mel Gorman <mgorman@...e.de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        qiuxishi@...wei.com, Kani Toshimitsu <toshi.kani@....com>,
        slaoub@...il.com, Joonsoo Kim <js1304@...il.com>,
        Andi Kleen <ak@...ux.intel.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>
Subject: Re: [PATCH 07/14] mm: consider zone which is not fully populated to
 have holes

On Thu 18-05-17 18:14:39, Vlastimil Babka wrote:
> On 05/15/2017 10:58 AM, Michal Hocko wrote:
[...]
> >  #ifdef CONFIG_MEMORY_HOTPLUG
> > +/*
> > + * Return page for the valid pfn only if the page is online. All pfn
> > + * walkers which rely on the fully initialized page->flags and others
> > + * should use this rather than pfn_valid && pfn_to_page
> > + */
> > +#define pfn_to_online_page(pfn)				\
> > +({							\
> > +	struct page *___page = NULL;			\
> > +							\
> > +	if (online_section_nr(pfn_to_section_nr(pfn)))	\
> > +		___page = pfn_to_page(pfn);		\
> > +	___page;					\
> > +})
> 
> This seems to be already assuming pfn_valid() to be true. There's no
> "pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS" check and the comment
> suggests as such, but...

Yes, we should check the validity of the section number. We do not have
to check whether the section is valid because online sections are a
subset of those that are valid.

> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index 05796ee974f7..c3a146028ba6 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -929,6 +929,9 @@ static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
> >  	unsigned long i;
> >  	unsigned long onlined_pages = *(unsigned long *)arg;
> >  	struct page *page;
> > +
> > +	online_mem_sections(start_pfn, start_pfn + nr_pages);
> 
> Shouldn't this be moved *below* the loop that initializes struct pages?
> In the offline case you do mark sections offline before "tearing" struct
> pages, so that should be symmetric.

You are right! Andrew, could you fold the following intot the patch?
---
>From 0550b61203d6970b47fd79f5e6372dccd143cbec Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
Date: Thu, 18 May 2017 18:38:24 +0200
Subject: [PATCH] fold me "mm: consider zone which is not fully populated to
 have holes"

- check valid section number in pfn_to_online_page - Vlastimil
- mark sections online after all struct pages are initialized in
  online_pages_range - Vlastimil

Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 include/linux/memory_hotplug.h | 3 ++-
 mm/memory_hotplug.c            | 5 +++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index aec88657ec49..9e0249d0f5e4 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -22,8 +22,9 @@ struct resource;
 #define pfn_to_online_page(pfn)				\
 ({							\
 	struct page *___page = NULL;			\
+	unsigned long ___nr = pfn_to_section_nr(pfn);	\
 							\
-	if (online_section_nr(pfn_to_section_nr(pfn)))	\
+	if (___nr < NR_MEM_SECTIONS && online_section_nr(___nr))\
 		___page = pfn_to_page(pfn);		\
 	___page;					\
 })
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index de30822642c6..a31b547c11f0 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -746,14 +746,15 @@ static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
 	unsigned long onlined_pages = *(unsigned long *)arg;
 	struct page *page;
 
-	online_mem_sections(start_pfn, start_pfn + nr_pages);
-
 	if (PageReserved(pfn_to_page(start_pfn)))
 		for (i = 0; i < nr_pages; i++) {
 			page = pfn_to_page(start_pfn + i);
 			(*online_page_callback)(page);
 			onlined_pages++;
 		}
+
+	online_mem_sections(start_pfn, start_pfn + nr_pages);
+
 	*(unsigned long *)arg = onlined_pages;
 	return 0;
 }
-- 
2.11.0


-- 
Michal Hocko
SUSE Labs