Re: error at compaction (Re: mmotm 2010-04-15-14-42 uploaded

[Minchan Kim <minchan.kim@gmail.com>]

On Tue, Apr 20, 2010 at 4:39 AM, Mel Gorman <mel@csn.ul.ie> wrote:
> On Mon, Apr 19, 2010 at 07:14:42PM +0100, Mel Gorman wrote:
>> On Mon, Apr 19, 2010 at 07:01:33PM +0900, KAMEZAWA Hiroyuki wrote:
>> >
>> > mmotm 2010-04-15-14-42
>> >
>> > When I tried
>> >  # echo 0 > /proc/sys/vm/compaction
>> >
>> > I see following.
>> >
>> > My enviroment was
>> >   2.6.34-rc4-mm1+ (2010-04-15-14-42) (x86-64) CPUx8
>> >   allocating tons of hugepages and reduce free memory.
>> >
>> > What I did was:
>> >   # echo 0 > /proc/sys/vm/compact_memory
>> >
>> > Hmm, I see this kind of error at migation for the 1st time..
>> > my.config is attached. Hmm... ?
>> >
>> > (I'm sorry I'll be offline soon.)
>>
>> That's ok, thanks you for the report. I'm afraid I made little progress
>> as I spent most of the day on other bugs but I do have something for
>> you.
>>
>> First, I reproduced the problem using your .config. However, the problem does
>> not manifest with the .config I normally use which is derived from the distro
>> kernel configuration (Debian Lenny). So, there is something in your .config
>> that triggers the problem. I very strongly suspect this is an interaction
>> between migration, compaction and page allocation debug.
>
> I unexpecedly had the time to dig into this. Does the following patch fix
> your problem? It Worked For Me.

Nice catch during shot time. Below is comment.

>
> ==== CUT HERE ====
> mm,compaction: Map free pages in the address space after they get split for compaction
>
> split_free_page() is a helper function which takes a free page from the
> buddy lists and splits it into order-0 pages. It is used by memory
> compaction to build a list of destination pages. If
> CONFIG_DEBUG_PAGEALLOC is set, a kernel paging request bug is triggered
> because split_free_page() did not call the arch-allocation hooks or map
> the page into the kernel address space.
>
> This patch does not update split_free_page() as it is called with
> interrupts held. Instead it documents that callers of split_free_page()
> are responsible for calling the arch hooks and to map the page and fixes
> compaction.

Dumb question. Why can't we call arch_alloc_page and kernel_map_pages
as interrupt disabled? It's deadlock issue or latency issue?
I don't found any comment about it.
It should have added the comment around that functions. :)

And now compaction only uses split_free_page and it is exposed by mm.h.
I think it would be better to map pages inside split_free_page to
export others.(ie, making generic function).
If we can't do, how about making split_free_page static as static function?
And only uses it in compaction.

-- 
Kind regards,
Minchan Kim

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