linux-kernel - Re: [PATCH 09/10] mm, page_alloc: Reserve pageblocks for high-order atomic allocations on demand

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Date:	Fri, 31 Jul 2015 17:30:31 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Mel Gorman <mgorman@...hsingularity.net>,
	Mel Gorman <mgorman@...e.com>, Linux-MM <linux-mm@...ck.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Pintu Kumar <pintu.k@...sung.com>,
	Xishi Qiu <qiuxishi@...wei.com>, Gioh Kim <gioh.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/10] mm, page_alloc: Reserve pageblocks for high-order
 atomic allocations on demand

On Fri, Jul 31, 2015 at 09:25:13AM +0200, Vlastimil Babka wrote:
> 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@...e.de>
> >>>
> >>>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.

Yes, I agree. If we distinguish such cases, I'm not sure that this
kinds of reservation is needed. It is better that atomic high-order
allocation in IRQ context should prepare their own fallback such as
reserving memory or low-order allocation.

> 
> 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.

Agreed.

Thanks.
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