linux-kernel - Re: [powerpc] Fix bogus BUG_ON() in in hugetlb_get_unmapped

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Message-Id: <EE6A12AA-3BE6-43DE-AEB2-7477CDAD21DC@kernel.crashing.org>
Date:	Fri, 22 Dec 2006 19:51:09 +0100
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	David Gibson <david@...son.dropbear.id.au>
Cc:	Andrew Morton <akpm@...l.org>,
	libhugetlbfs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	William Lee Irwin <wli@...omorphy.com>,
	linuxppc-dev@...abs.org, Paul Mackerras <paulus@...ba.org>
Subject: Re: [powerpc] Fix bogus BUG_ON() in in hugetlb_get_unmapped_area()

>>> +	if (len > TASK_SIZE)
>>> +		return -ENOMEM;
>>
>> Shouldn't that be addr+len instead?  The check looks incomplete
>> otherwise.  And you meant ">=" I guess?
>
> No.  Have a look at the other hugetlb_get_unmapped_area()
> implementations.  Because this is in the get_unmapped_area() path,
> 'addr' is just a hint,

Ah I missed this vital piece of information, thanks for the
explanation.  Care putting in a code comment pointing this out?

> so checking addr+len would give bogus
> failures.  This test is, I believe, essentially an optimization - if
> it fails, we're never going to find a suitable addr, so we might as
> well give up now.

Yes, it all makes sense now.

>>> -		/* Paranoia, caller should have dealt with this */
>>> -		BUG_ON((addr + len) > 0x100000000UL);
>>> -
>>
>> Any real reason to remove the paranoia check?  If it's trivially
>> always satisfied, the compiler will get rid of it for you :-)
>
> Yes - this is the very bug on which was causing crashes - the "caller
> should have dealt with this" comment is wrong.  The test has been
> moved into htlb_check_hinted_area() and now simply fails (and so falls
> back to searching for a suitable address), rather than BUG()ing.

Yep.

Cheers,


Segher

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