linux-kernel - Re: [PATCH -mm 4/7] mm: make page colouring code generic

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Date:	Thu, 21 Jun 2012 09:24:19 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Borislav Petkov <bp@...64.org>
CC:	linux-mm@...ck.org, akpm@...ux-foundation.org, aarcange@...hat.com,
	peterz@...radead.org, minchan@...il.com, kosaki.motohiro@...il.com,
	andi@...stfloor.org, hannes@...xchg.org, mel@....ul.ie,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH -mm 4/7] mm: make page colouring code generic

On 06/21/2012 08:37 AM, Borislav Petkov wrote:

>> -unsigned long align_addr(unsigned long addr, struct file *filp,
>> -			 enum align_flags flags)
>> +unsigned long arch_align_addr(unsigned long addr, struct file *filp,
>> +			unsigned long pgoff, unsigned long flags,
>> +			enum mmap_allocation_direction direction)
>
> Arguments vertical alignment too, not only addr alignment :-)

Will do.

>>   {
>> -	unsigned long tmp_addr;
>> +	unsigned long tmp_addr = PAGE_ALIGN(addr);
>
> I'm guessing addr coming from arch_get_unmapped_area(_topdown) might not
> be page-aligned in all cases?

That is my guess, too :)

In some places arch_get_unmapped_area(_topdown) called
PAGE_ALIGN(addr), so we should make sure it is called.

It is probably masking bugs in some old old application,
and calling it here really should not hurt.

>> -	if (!(current->flags&  PF_RANDOMIZE))
>> -		return addr;
>> +	/* Always allow MAP_FIXED. Colouring is a performance thing only. */
>> +	if (flags&  MAP_FIXED)
>> +		return tmp_addr;
>
> Why here? Maybe we should push this MAP_FIXED check up in the
> arch_get_unmapped_area(_topdown) and not call arch_align_addr() for
> MAP_FIXED requests?
>
> Or do you want to save some code duplication?

The problem is that certain other architectures have
data cache alignment requirements, where mis-aligning
somebody's mmap of a file could result in actual data
corruption.

This means that, for those architectures, we have to
refuse non-colour-aligned MAP_FIXED mappings.

On x86 we can allow them, so we do. But that decision
needs to be taken in architecture specific code, not
in the shared arch_get_unmapped_area(_topdown) :)

>> +	/*
>> +	 * When aligning down, make sure we did not accidentally go up.
>> +	 * The caller will check for underflow.
>> +	 */
>
> Can we add this comment to the x86-64 version of arch_align_addr too pls?

Will do.

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