linux-kernel - Re: [PATCH] mm/page_isolation: let caller take the zone lock for test_pages

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Date:	Thu, 17 Mar 2016 18:33:40 +0100
From:	Lucas Stach <l.stach@...gutronix.de>
To:	Joonsoo Kim <js1304@...il.com>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Rientjes <rientjes@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, kernel@...gutronix.de,
	patchwork-lst@...gutronix.de
Subject: Re: [PATCH] mm/page_isolation: let caller take the zone lock for
 test_pages_isolated

Am Freitag, den 18.03.2016, 02:18 +0900 schrieb Joonsoo Kim:
> 2016-03-17 1:49 GMT+09:00 Lucas Stach <l.stach@...gutronix.de>:
> > This fixes an annoying race in the CMA code leading to lots of "PFNs busy"
> > messages when CMA is used concurrently. This is harmless normally as CMA
> > will just retry the allocation at a different place, but it might lead to
> > increased fragmentation of the CMA area as well as failing allocations
> > when CMA is under memory pressure.
> >
> > The issue is that test_pages_isolated checks if the range is free by
> > checking that all pages in the range are buddy pages. For this to work
> > the start pfn needs to be aligned to the higher order buddy page
> > including the start pfn if there is any.
> >
> > This is not a problem for the memory hotplug code, as it always offlines
> > whole pageblocks, but CMA may want to isolate a smaller range. So for
> > the check to work correctly it down-aligns the start pfn to the higher
> > order buddy page. As the zone is not yet locked at that point a
> > concurrent page free might coalesce the pages to be checked into an
> > even bigger buddy page, causing the check to fail, while all pages are
> > in fact buddy pages.
> >
> > By moving the zone locking to the caller of the test function, it's
> > possible to do it before CMA tries to find the proper start page and stop
> > any concurrent page coalescing to happen until the check is finished.
> 
> I think that this patch cannot prevent the same race on
> isolate_freepages_range(). If buddy merging happens after we
> passed test_pages_isolated(), isolate_freepages_range() cannot see
> buddy page and will fail.
> 
Your analysis seems correct. I'll fix this patch to hold the zone lock
across isolate_freepages_range.

Thanks,
Lucas