Re: [PATCH] mm,vmscan: free pages if compaction_suitable tells us to

[Jaegeuk Hanse <jaegeuk.hanse@gmail.com>]

On 11/26/2012 06:44 AM, Johannes Weiner wrote:
> On Sun, Nov 25, 2012 at 01:29:50PM -0500, Rik van Riel wrote:
>> On Sun, 25 Nov 2012 17:57:28 +0100
>> Johannes Hirte <johannes.hirte@fem.tu-ilmenau.de> wrote:
>>
>>> With kernel 3.7-rc6 I've still problems with kswapd0 on my laptop
>>> And this is most of the time. I've only observed this behavior on the
>>> laptop. Other systems don't show this.
>> This suggests it may have something to do with small memory zones,
>> where we end up with the "funny" situation that the high watermark
>> (+ balance gap) for a particular zone is less than the low watermark
>> + 2<<order pages, which is the number of free pages required to keep
>> compaction_suitable happy.
>>
>> Could you try this patch?
> It's not quite enough because it's not reaching the conditions you
> changed, see analysis in https://lkml.org/lkml/2012/11/20/567
>
> But even fixing it up (by adding the compaction_suitable() test in
> this preliminary scan over the zones and setting end_zone accordingly)
> is not enough because no actual reclaim happens at priority 12 in a

The preliminary scan is in the highmem->dma direction, it will miss high 
zone which not meet compaction_suitable() test instead of lowest zone.

> small zone.  So the number of free pages is not actually changing and
> the compaction_suitable() checks keep the loop going.
>
> The problem is fairly easy to reproduce, by the way.  Just boot with
> mem=800M to have a relatively small lowmem reserve in the DMA zone.
> Fill it up with page cache, then allocate transparent huge pages.
>
> With your patch and my fix to the preliminary zone loop, there won't
> be any hung task warnings anymore because kswapd actually calls
> shrink_slab() and there is a rescheduling point in there, but it still
> loops forever.
>
> It also seems a bit aggressive to try to balance a small zone like DMA
> for a huge page when it's not a GFP_DMA allocation, but none of these
> checks actually take the classzone into account.  Do we have any
> agreement over what this whole thing is supposed to be doing?
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b99ecba..f7e54df 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2412,6 +2412,9 @@ static void age_active_anon(struct zone *zone, struct scan_control *sc)
>    *     would need to be at least 256M for it to be balance a whole node.
>    *     Similarly, on x86-64 the Normal zone would need to be at least 1G
>    *     to balance a node on its own. These seemed like reasonable ratios.
> + *
> + * The kswapd source code is brought to you by Advil(R).  "For today's
> + * tough pain, one might not be enough."
>    */
>   static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
>   						int classzone_idx)
>
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