From: Petr Tesarik <ptesarik@suse.cz>
Organization: SUSE LINUX, s.r.o.
To: wujianguo <wujianguo106@gmail.com>
Subject: Re: [PATCH]mm: fix-up zone present pages
Date: Tue, 21 Aug 2012 05:55:40 +0200
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Dne Po 20. srpna 2012 08:38:10 wujianguo napsal(a):
> From: Jianguo Wu <wujianguo@huawei.com>
> 
> Hi all,
> 	I think zone->present_pages indicates pages that buddy system can
> management, it should be:
> 	zone->present_pages = spanned pages - absent pages - bootmem pages,
> but now:
> 	zone->present_pages = spanned pages - absent pages - memmap pages.
> spanned pages：total size, including holes.
> absent pages: holes.
> bootmem pages: pages used in system boot, managed by bootmem allocator.
> memmap pages: pages used by page structs.

Absolutely. The memory allocated to page structs should be counted in.

> This may cause zone->present_pages less than it should be.
> For example, numa node 1 has ZONE_NORMAL and ZONE_MOVABLE,
> it's memmap and other bootmem will be allocated from ZONE_MOVABLE,
> so ZONE_NORMAL's present_pages should be spanned pages - absent pages,
> but now it also minus memmap pages(free_area_init_core), which are actually
> allocated from ZONE_MOVABLE. When offline all memory of a zone, This will
> cause zone->present_pages less than 0, because present_pages is unsigned
> long type, it is actually a very large integer, it indirectly caused
> zone->watermark[WMARK_MIN] become a large
> integer(setup_per_zone_wmarks()), than cause totalreserve_pages become a
> large integer(calculate_totalreserve_pages()), and finally cause memory
> allocating failure when fork process(__vm_enough_memory()).
> 
> [root@localhost ~]# dmesg
> -bash: fork: Cannot allocate memory
> 
> I think bug described in http://marc.info/?l=linux-mm&m=134502182714186&w=2
> is also caused by wrong zone present pages.

And yes, I can confirm that the bug I reported is caused by a too low number 
for the present pages counter. Your patch does fix the bug for me.

Thanks!
Petr Tesarik
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