Re: [PATCH v2 03/10] mm, page_alloc: split smallest stolen page in fallback

From: Mel Gorman
Date: Mon Feb 13 2017 - 05:51:25 EST

On Fri, Feb 10, 2017 at 06:23:36PM +0100, Vlastimil Babka wrote:
> The __rmqueue_fallback() function is called when there's no free page of
> requested migratetype, and we need to steal from a different one. There are
> various heuristics to make this event infrequent and reduce permanent
> fragmentation. The main one is to try stealing from a pageblock that has the
> most free pages, and possibly steal them all at once and convert the whole
> pageblock. Precise searching for such pageblock would be expensive, so instead
> the heuristics walks the free lists from MAX_ORDER down to requested order and
> assumes that the block with highest-order free page is likely to also have the
> most free pages in total.
> Chances are that together with the highest-order page, we steal also pages of
> lower orders from the same block. But then we still split the highest order
> page. This is wasteful and can contribute to fragmentation instead of avoiding
> it.

The original intent was that if an allocation request was stealing a
pageblock that taking the largest one would reduce the likelihood of a
steal in the near future by the same type.

> This patch thus changes __rmqueue_fallback() to just steal the page(s) and put
> them on the freelist of the requested migratetype, and only report whether it
> was successful. Then we pick (and eventually split) the smallest page with
> __rmqueue_smallest(). This all happens under zone lock, so nobody can steal it
> from us in the process. This should reduce fragmentation due to fallbacks. At
> worst we are only stealing a single highest-order page and waste some cycles by
> moving it between lists and then removing it, but fallback is not exactly hot
> path so that should not be a concern. As a side benefit the patch removes some
> duplicate code by reusing __rmqueue_smallest().
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

But conceptually this is better so

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Mel Gorman