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? 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>