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

[Rik van Riel]

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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