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.

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 <redacted>

It took me a second to understand what you're doing here, but this is
clever. Finding a suitable fallback still goes by biggest block to
make future stealing less probable, but when we do steal the entire
block and move_freepages_block() has migrated all the free chunks of
that block over to the new migratetype list, we might as well then try
to allocate from the smallest chunk available in the stolen block.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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