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

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Date:	Fri, 01 Apr 2011 00:51:32 +0200
From:	"Michal Nazarewicz" <mina86@...a86.com>
To:	"Dave Hansen" <dave@...ux.vnet.ibm.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, 01 Apr 2011 00:26:51 +0200, Dave Hansen <dave@...ux.vnet.ibm.com>  
wrote:

> On Fri, 2011-04-01 at 00:18 +0200, Michal Nazarewicz wrote:
>> On Thu, 31 Mar 2011 23:14:38 +0200, Dave Hansen wrote:
>> > We BUG_ON() in bootmem.  Basically if we try to allocate an early-boot
>> > structure and fail, we're screwed.  We can't keep running without an
>> > inode hash, or a mem_map[].
>> >
>> > This looks like it's going to at least get partially used in drivers,  
>> at
>> > least from the examples.  Are these kinds of things that, if the  
>> driver
>> > fails to load, that the system is useless and hosed?  Or, is it
>> > something where we might limp along to figure out what went wrong  
>> before
>> > we reboot?
>>
>> 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.

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.

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michal "mina86" Nazarewicz    (o o)
ooo +-----<email/xmpp: mnazarewicz@...gle.com>-----ooO--(_)--Ooo--
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