Johannes Weiner wrote: > > Hi, > > 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. > > > > 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. This patch is suggested by KAMEZAWA-san. > > I just fixed compile error and change some naming. > > I did not like it, because it messes up the whole concept of a > section. > > But most importantly, we already have a crutch for ARM in place, > namely memmap_valid_within(). Looking at Kukjin's bug report, > wouldn't it be enough to use that check in > setup_zone_migrate_reserve()? > > Your approach makes every pfn_valid() more expensive, although the > extensive checks are not not needed everywhere (check the comment > above memmap_valid_within): vm_normal_page() for example can probably > assume that a PTE won't point to a hole within the memory map. > > OTOH, if the ARM people do not care, we could probably go with your > approach, encode it all into pfn_valid(), and also get rid of > memmap_valid_within() completely. But I would prefer doing a bugfix > first and such a conceptual change in a different patch, would you > agree? > > Kukjin, does the appended patch also fix your problem? > Yes, did not happen problem with your patch. But already Minchan requested test on the board with same patch. And you can find it in following thread about that. http://lists.infradead.org/pipermail/linux-arm-kernel/2010-July/020199.html I'm not sure which approach is better to us right now. Hmm... Thanks. Best regards, Kgene. -- Kukjin Kim <kgene.kim@xxxxxxxxxxx>, Senior Engineer, SW Solution Development Team, Samsung Electronics Co., Ltd. > Hannes > > --- > From: Johannes Weiner <hannes@xxxxxxxxxxx> > Subject: mm: check mem_map backing in setup_zone_migrate_reserve > > Kukjin encountered kernel oopsen when changing > /proc/sys/vm/min_free_kbytes. The problem is that his sparse memory > layout on ARM is the following: > > section 0 section 1 section 2 > 0x20000000-0x25000000, 0x40000000-0x50000000, 0x50000000-0x58000000 > SECTION_SIZE_BITS 28(256M) > > where there is a memory hole at the end of section 0. > > Since section 0 has _some_ memory, pfn_valid() will return true for > all PFNs in this section. But ARM releases the mem_map pages of this > hole and pfn_valid() alone is not enough anymore to ensure there is a > valid page struct behind a PFN. > > We acknowledged that ARM does this already and have a function to > double-check for mem_map in cases where we do PFN range walks (as > opposed to coming from a page table entry, which should not point to a > memory hole in the first place e.g.). > > setup_zone_migrate_reserve() contains one such range walk which does > not have the extra check and was also the cause of the oopsen Kukjin > encountered. > > This patch adds the needed memmap_valid_within() check. > > Reported-by: Kukjin Kim <kgene.kim@xxxxxxxxxxx> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> > --- > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 0b0b629..cb6d6d3 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -3168,6 +3168,10 @@ static void setup_zone_migrate_reserve(struct zone > *zone) > continue; > page = pfn_to_page(pfn); > > + /* Watch out for holes in the memory map */ > + if (!memmap_valid_within(pfn, page, zone)) > + continue; > + > /* Watch out for overlapping nodes */ > if (page_to_nid(page) != zone_to_nid(zone)) > continue; -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>