On 07/05/2016 11:01 PM, David Rientjes wrote:

On Thu, 30 Jun 2016, Vlastimil Babka wrote:

quoted

quoted

 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?

No.

Acked-by: Vlastimil Babka <redacted>

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

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