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Message-ID: <20100720162517.GB1940@barrios-desktop>
Date: Wed, 21 Jul 2010 01:25:17 +0900
From: Minchan Kim <minchan.kim@...il.com>
To: Mel Gorman <mel@....ul.ie>
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 Tue, Jul 20, 2010 at 04:28:09PM +0100, Mel Gorman wrote:
> 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?
>
Good point.
If we can make sure pg->private is zero, we can fix it.
I will check it.
> >
> > #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 :)
Okay. If we call it in only free_memmap, we can change naming following as.
ex) mark_invalid_memmap(start, end);
Will change.
>
> > +#ifdef CONFIG_SPARSEMEM_HAS_HOLE
>
> Why not use CONFIG_ARCH_HAS_HOLES_MEMORYMODEL ?
As I mentioned my previous mail(reply of Hannes), if the problen can happen
in FLATMEM, CONFIG_ARCH_HAS_HOLES_MEMORYMODEL is right.
Please confirm this, Mel. :)
>
> > +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.
In fact, I tried it with SECTION_MAP_LAST_BIT as you suggested.
But stucked. That's because now we can use 2 bit of section_mem_map.
And we have used 2 bit all with different meaning.
I have a dumb question. Is there any case section have SECTION_MARKED_PRESENT
but don't have SECTION_HAS_MEM_MAP except section populated time?
I mean can't we remove SECTION_MARKED_PRESENT totally?
If it is, we can the 1 bit for marking hole section.
If it isn't, I think it seems we can use lower bit of pageblock_flags.
although it's not a good.
Thanks for careful review, Mel.
>
> >
> > 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
--
Kind regards,
Minchan Kim
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