Re: [PATCH] mm: add the block to the tail of the list in expand()

[Dave Hansen <dave.hansen@intel.com>]

On 07/31/2015 02:30 AM, Xishi Qiu wrote:
> __free_one_page() will judge whether the the next-highest order is free,
> then add the block to the tail or not. So when we split large order block, 
> add the small block to the tail, it will reduce fragment.

It's an interesting idea, but what does it do in practice?  Can you
measure a decrease in fragmentation?

Further, the comment above the function says:
 * The order of subdivision here is critical for the IO subsystem.
 * Please do not alter this order without good reasons and regression
 * testing.

Has there been regression testing?

Also, this might not do very much good in practice.  If you are
splitting a high-order page, you are doing the split because the
lower-order lists are empty.  So won't that list_add() be to an empty
list most of the time?  Or does the __rmqueue_fallback()
largest->smallest logic dominate?

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