Re: [PATCH] mm/compaction : check the watermark when cc->order is -1

[Huang Shijie <b32955@freescale.com>]

> On Sat, Dec 31, 2011 at 02:18:43PM +0800, Huang Shijie wrote:
>> We get cc->order is -1 when user echos to /proc/sys/vm/compact_memory.
>> In this case, we should check that if we have enough pages for
>> the compaction in the zone.
>>
>> If we do not check this, in our MX6Q board(arm), i ever observed
>> COMPACT_CLUSTER_MAX pages were compaction failed in per migrate_pages().
>> That's mean we can not alloc any pages by the free scanner in the zone.
>>
>> This patch checks the watermark to avoid this problem.
>>
>> Signed-off-by: Huang Shijie<b32955@freescale.com>
>> ---
>>   mm/compaction.c |   14 ++++++++++++--
>>   1 files changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 899d956..0f12cc9 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -442,8 +442,13 @@ static int compact_finished(struct zone *zone,
>>   	 * order == -1 is expected when compacting via
>>   	 * /proc/sys/vm/compact_memory
>>   	 */
>> -	if (cc->order == -1)
>> +	if (cc->order == -1) {
>> +		/* Check if we have enough pages now. */
>> +		watermark = low_wmark_pages(zone) + COMPACT_CLUSTER_MAX * 2;
>> +		if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
>> +			return COMPACT_SKIPPED;
>>   		return COMPACT_CONTINUE;
>> +	}
>>
> We already do the watermark check in compact_finished. Would moving
> the cc->order == -1 check below it not be functionally equivalent? This


Now, I think these code for compact_finish() is not needed. These lines 
can be removed.

The watermark check in the compaction_suitable() is enough to avoid the 
problem.


Huang Shijie

> would be preferable to duplicating the code for the watermark check.
>
> Same for the second check in your patch.
>


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