linux-kernel - Re: [PATCH 04/12] mm: alloc_contig_freed

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Message-ID: <1301666596.30870.176.camel@nimitz>
Date:	Fri, 01 Apr 2011 07:03:16 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Michal Nazarewicz <mina86@...a86.com>
Cc:	Marek Szyprowski <m.szyprowski@...sung.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-media@...r.kernel.org,
	linux-mm@...ck.org, Kyungmin Park <kyungmin.park@...sung.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ankita Garg <ankita@...ibm.com>,
	Daniel Walker <dwalker@...eaurora.org>,
	Johan MOSSBERG <johan.xx.mossberg@...ricsson.com>,
	Mel Gorman <mel@....ul.ie>, Pawel Osciak <pawel@...iak.com>
Subject: Re: [PATCH 04/12] mm: alloc_contig_freed_pages() added

On Fri, 2011-04-01 at 00:51 +0200, Michal Nazarewicz wrote:
> On Fri, 01 Apr 2011 00:26:51 +0200, Dave Hansen <dave@...ux.vnet.ibm.com>  
> wrote:
> >> Bug in the above place does not mean that we could not allocate  
> >> memory.  It means caller is broken.
> >
> > Could you explain that a bit?
> >
> > Is this a case where a device is mapped to a very *specific* range of
> > physical memory and no where else?  What are the reasons for not marking
> > it off limits at boot?  I also saw some bits of isolation and migration
> > in those patches.  Can't the migration fail?
> 
> The function is called from alloc_contig_range() (see patch 05/12) which
> makes sure that the PFN is valid.  Situation where there is not enough
> space is caught earlier in alloc_contig_range().
> 
> alloc_contig_freed_pages() must be given a valid PFN range such that all
> the pages in that range are free (as in are within the region tracked by
> page allocator) and of MIGRATETYPE_ISOLATE so that page allocator won't
> touch them.

OK, so it really is a low-level function only.  How about a comment that
explicitly says this?  "Only called from $FOO with the area already
isolated."  It probably also deserves an __ prefix.

> That's why invalid PFN is a bug in the caller and not an exception that
> has to be handled.
> 
> Also, the function is not called during boot time.  It is called while
> system is already running.

What kind of success have you had running this in practice?  I'd be
worried that some silly task or a sticky dentry would end up in the
range that you want to allocate in.  


-- Dave

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