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SubjectRe: [patch for-4.7] mm, compaction: prevent VM_BUG_ON when terminating freeing scanner
On 07/05/2016 11:01 PM, David Rientjes wrote:
> On Thu, 30 Jun 2016, Vlastimil Babka wrote:
>>> Note: I really dislike the low watermark check in split_free_page() and
>>> consider it poor software engineering. The function should split a free
>>> page, nothing more. Terminating memory compaction because of a low
>>> watermark check when we're simply trying to migrate memory seems like an
>>> arbitrary heuristic. There was an objection to removing it in the first
>>> proposed patch, but I think we should really consider removing that
>>> check so this is simpler.
>> There's a patch changing it to min watermark (you were CC'd on the series). We
>> could argue whether it belongs to split_free_page() or some wrapper of it, but
>> I don't think removing it completely should be done. If zone is struggling
>> with order-0 pages, a functionality for making higher-order pages shouldn't
>> make it even worse. It's also not that arbitrary, even if we succeeded the
>> migration and created a high-order page, the higher-order allocation would
>> still fail due to watermark checks. Worse, __compact_finished() would keep
>> telling the compaction to continue, creating an even longer lag, which is also
>> against your recent patches.
> I'm suggesting we shouldn't check any zone watermark in split_free_page():
> that function should just split the free page.
> I don't find our current watermark checks to determine if compaction is
> worthwhile to be invalid, but I do think that we should avoid checking or
> acting on any watermark in isolate_freepages() itself. We could do more
> effective checking in __compact_finished() to determine if we should
> terminate compaction, but the freeing scanner feels like the wrong place
> to do it -- it's also expensive to check while gathering free pages for
> memory that we have already successfully isolated as part of the
> iteration.
> Do you have any objection to this fix for 4.7?


Acked-by: Vlastimil Babka <>

> Joonson and/or Minchan, does this address the issue that you reported?

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