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Message-ID: <20100720152809.GW13117@csn.ul.ie>
Date: Tue, 20 Jul 2010 16:28:09 +0100
From: Mel Gorman <mel@....ul.ie>
To: Minchan Kim <minchan.kim@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Russell King <linux@....linux.org.uk>,
Johannes Weiner <hannes@...xchg.org>,
linux-mm <linux-mm@...ck.org>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
LKML <linux-kernel@...r.kernel.org>,
Kukjin Kim <kgene.kim@...sung.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v2
On Sun, Jul 18, 2010 at 07:18:31PM +0900, Minchan Kim wrote:
> Kukjin reported oops happen while he change min_free_kbytes
> http://www.spinics.net/lists/arm-kernel/msg92894.html
> It happen by memory map on sparsemem.
>
First off, thanks for working on this.
> The system has a memory map following as.
> section 0 section 1 section 2
> 0x20000000-0x25000000, 0x40000000-0x50000000, 0x50000000-0x58000000
> SECTION_SIZE_BITS 28(256M)
>
> It means section 0 is an incompletely filled section.
> Nontheless, current pfn_valid of sparsemem checks pfn loosely.
> It checks only mem_section's validation but ARM can free mem_map on hole
> to save memory space. So in above case, pfn on 0x25000000 can pass pfn_valid's
> validation check. It's not what we want.
>
> We can match section size to smallest valid size.(ex, above case, 16M)
> But Russell doesn't like it due to mem_section's memory overhead with different
> configuration(ex, 512K section).
>
> I tried to add valid pfn range in mem_section but everyone doesn't like it
> due to size overhead.
Also IIRC, it was vunerable to a hole being punched in the middle of the
section.
> This patch is suggested by KAMEZAWA-san.
> I just fixed compile error and change some naming.
>
> This patch registers address of mem_section to memmap itself's page struct's
> pg->private field. This means the page is used for memmap of the section.
> Otherwise, the page is used for other purpose and memmap has a hole.
>
> This patch is based on mmotm-2010-07-01-12-19.
>
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Reported-by: Kukjin Kim <kgene.kim@...sung.com>
> ---
> arch/arm/mm/init.c | 9 ++++++++-
> include/linux/mmzone.h | 21 ++++++++++++++++++++-
> mm/Kconfig | 5 +++++
> mm/sparse.c | 41 +++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 74 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index cfe4c5e..4586f40 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -234,6 +234,11 @@ static void __init arm_bootmem_free(struct meminfo *mi)
> arch_adjust_zones(zone_size, zhole_size);
>
> free_area_init_node(0, zone_size, min, zhole_size);
> +
> + for_each_bank(i, mi) {
> + mark_memmap_hole(bank_pfn_start(&mi->bank[i]),
> + bank_pfn_end(&mi->bank[i]), true);
> + }
> }
Why do we need to mark banks both valid and invalid? Is it not enough to
just mark the holes in free_memmap() and assume it is valid otherwise?
>
> #ifndef CONFIG_SPARSEMEM
> @@ -386,8 +391,10 @@ free_memmap(unsigned long start_pfn, unsigned long end_pfn)
> * If there are free pages between these,
> * free the section of the memmap array.
> */
> - if (pg < pgend)
> + if (pg < pgend) {
> + mark_memmap_hole(pg >> PAGE_SHIFT, pgend >> PAGE_SHIFT, false);
> free_bootmem(pg, pgend - pg);
> + }
> }
>
> /*
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 9ed9c45..2ed9728 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -15,6 +15,7 @@
> #include <linux/seqlock.h>
> #include <linux/nodemask.h>
> #include <linux/pageblock-flags.h>
> +#include <linux/mm_types.h>
> #include <generated/bounds.h>
> #include <asm/atomic.h>
> #include <asm/page.h>
> @@ -1047,11 +1048,29 @@ static inline struct mem_section *__pfn_to_section(unsigned long pfn)
> return __nr_to_section(pfn_to_section_nr(pfn));
> }
>
> +void mark_memmap_hole(unsigned long start, unsigned long end, bool valid);
> +
The naming is confusing with the "valid" parameter.
What's a "valid hole"? I can see that one being a cause of head
scratching in the future :)
> +#ifdef CONFIG_SPARSEMEM_HAS_HOLE
Why not use CONFIG_ARCH_HAS_HOLES_MEMORYMODEL ?
> +static inline int page_valid(struct mem_section *ms, unsigned long pfn)
> +{
> + struct page *page = pfn_to_page(pfn);
> + struct page *__pg = virt_to_page(page);
> + return __pg->private == (unsigned long)ms;
> +}
> +#else
> +static inline int page_valid(struct mem_section *ms, unsigned long pfn)
> +{
> + return 1;
> +}
> +#endif
> +
> static inline int pfn_valid(unsigned long pfn)
> {
> + struct mem_section *ms;
> if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
> return 0;
> - return valid_section(__nr_to_section(pfn_to_section_nr(pfn)));
> + ms = __nr_to_section(pfn_to_section_nr(pfn));
> + return valid_section(ms) && page_valid(ms, pfn);
> }
So it appears here that we unconditionally check page_valid() but we know
which sections had holes in them at the time we called mark_memmap_hole(). Can
the sections with holes be tagged so that only some sections need to call
page_valid()? As it is, ARM will be taking a an performance hit just in case
the section has holes but it should only need to take a performance hit
on the corner case where a section is not fully populated.
>
> static inline int pfn_present(unsigned long pfn)
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 527136b..959ac1d 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -128,6 +128,11 @@ config SPARSEMEM_VMEMMAP
> pfn_to_page and page_to_pfn operations. This is the most
> efficient option when sufficient kernel resources are available.
>
> +config SPARSEMEM_HAS_HOLE
> + bool "allow holes in sparsemem's memmap"
> + depends on ARM && SPARSEMEM && !SPARSEMEM_VMEMMAP
> + default n
> +
> # eventually, we can have this option just 'select SPARSEMEM'
> config MEMORY_HOTPLUG
> bool "Allow for memory hot-add"
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 95ac219..76d5012 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -615,6 +615,47 @@ void __init sparse_init(void)
> free_bootmem(__pa(usemap_map), size);
> }
>
> +#ifdef CONFIG_SPARSEMEM_HAS_HOLE
> +/*
> + * Fill memmap's pg->private with a pointer to mem_section.
> + * pfn_valid() will check this later. (see include/linux/mmzone.h)
> + * Evenry arch should call
> + * mark_memmap_hole(start, end, true) # for all allocated mem_map
> + * and, after that,
> + * mark_memmap_hole(start, end, false) # for all holes in mem_map.
> + * please see usage in ARM.
> + */
> +void mark_memmap_hole(unsigned long start, unsigned long end, bool valid)
> +{
> + struct mem_section *ms;
> + unsigned long pos, next;
> + struct page *pg;
> + void *memmap, *mapend;
> +
> + for (pos = start; pos < end; pos = next) {
> + next = (pos + PAGES_PER_SECTION) & PAGE_SECTION_MASK;
> + ms = __pfn_to_section(pos);
> + if (!valid_section(ms))
> + continue;
> +
> + for (memmap = (void*)pfn_to_page(pos),
> + /* The last page in section */
> + mapend = pfn_to_page(next-1);
> + memmap < mapend; memmap += PAGE_SIZE) {
> + pg = virt_to_page(memmap);
> + if (valid)
> + pg->private = (unsigned long)ms;
> + else
> + pg->private = 0;
> + }
> + }
> +}
> +#else
> +void mark_memmap_hole(unsigned long start, unsigned long end, bool valid)
> +{
> +}
> +#endif
> +
The patch should also delete memmap_valid_within() and replace it with a
call to pfn_valid_within(). The reason memmap_valid_within() existed was
because sparsemem had holes punched in it but I'd rather not see use of
that function grow.
> #ifdef CONFIG_MEMORY_HOTPLUG
> #ifdef CONFIG_SPARSEMEM_VMEMMAP
> static inline struct page *kmalloc_section_memmap(unsigned long pnum, int nid,
> --
> 1.7.0.5
>
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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