On 07/31/2015 09:11 AM, Mel Gorman wrote:
On Fri, Jul 31, 2015 at 02:54:07PM +0900, Joonsoo Kim wrote:
Hello, Mel.
On Mon, Jul 20, 2015 at 09:00:18AM +0100, Mel Gorman wrote:
From: Mel Gorman <mgorman@xxxxxxx>
High-order watermark checking exists for two reasons -- kswapd high-order
awareness and protection for high-order atomic requests. Historically we
depended on MIGRATE_RESERVE to preserve min_free_kbytes as high-order free
pages for as long as possible. This patch introduces MIGRATE_HIGHATOMIC
that reserves pageblocks for high-order atomic allocations. This is expected
to be more reliable than MIGRATE_RESERVE was.
I have some concerns on this patch.
1) This patch breaks intention of __GFP_WAIT.
__GFP_WAIT is used when we want to succeed allocation even if we need
to do some reclaim/compaction work. That implies importance of
allocation success. But, reserved pageblock for MIGRATE_HIGHATOMIC makes
atomic allocation (~__GFP_WAIT) more successful than allocation with
__GFP_WAIT in many situation. It breaks basic assumption of gfp flags
and doesn't make any sense.
Currently allocation requests that do not specify __GFP_WAIT get the
ALLOC_HARDER flag which allows them to dip further into watermark reserves.
It already is the case that there are corner cases where a high atomic
allocation can succeed when a non-atomic allocation would reclaim.
I think (and said so before elsewhere) is that the problem is that we
don't currently distinguish allocations that can't wait (=are really
atomic and have no order-0 fallback) and allocations that just don't
want to wait (=they have fallbacks). The second ones should obviously
not access the current ALLOC_HARDER watermark-based reserves nor the
proposed highatomic reserves.
Well we do look at __GFP_NO_KSWAPD flag to treat allocation as
non-atomic, so that covers THP allocations and two drivers. But the
recent networking commit fb05e7a89f50 didn't add the flag and nor does
Joonsoo's slub patch use it. Either we should rename the flag and employ
it where appropriate, or agree that access to reserves is orthogonal
concern to waking up kswapd, and distinguish non-atomic non-__GFP_WAIT
allocations differently.
A MIGRATE_HIGHORDER pageblock is created when an allocation request steals
a pageblock but limits the total number to 10% of the zone.
When steals happens, pageblock already can be fragmented and we can't
fully utilize this pageblock without allowing order-0 allocation. This
is very waste.
If the pageblock was stolen, it implies there was at least 1 usable page
of the correct order. As the pageblock is then reserved, any pages that
free in that block stay free for use by high-order atomic allocations.
Else, the number of pageblocks will increase again until the 10% limit
is hit.
It's however true that many of the "any pages free in that block" may be
order-0, so they both won't be useful to high-order atomic allocations,
and won't be available to other allocations, so they might remain unused.
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