Re: bugs in page colouring code

[Rik van Riel]

On 06/14/2012 09:20 AM, Russell King - ARM Linux wrote:
> On Wed, Jun 13, 2012 at 03:29:36PM -0400, Rik van Riel wrote:
> > COLOUR_ALIGN_DOWN can use the pgoff % shm_align_mask either positively
> >     or negatively, depending on the address initially found by
> >     get_unmapped_area
> >
> > static inline unsigned long COLOUR_ALIGN_DOWN(unsigned long addr,
> >                                                unsigned long pgoff)
> > {
> >          unsigned long base = addr&  ~shm_align_mask;
> >          unsigned long off = (pgoff<<  PAGE_SHIFT)&  shm_align_mask;
> >
> >          if (base + off<= addr)
> >                  return base + off;
> >
> >          return base - off;
> > }
>
> Yes, that is bollocks code, introduced by this commit:
>
> commit 7dbaa466780a754154531b44c2086f6618cee3a8
> Author: Rob Herring<rob.herring@xxxxxxxxxxx>
> Date:   Tue Nov 22 04:01:07 2011 +0100

It's not just ARM that has this bug. It appears to
be cut'n'pasted from other architectures (MIPS? SPARC?).

> > The fix would be to return an address that is a whole shm_align_mask
> > lower: (((base - shm_align_mask)&  ~shm_align_mask) + off
>
> Yes, agreed.

I will make sure the arch-independent colouring
code does that.

> This brings up the question: should a MAP_PRIVATE mapping see updates
> to the backing file made via a shared mapping and/or writing the file
> directly?  After all, a r/w MAP_PRIVATE mapping which has been CoW'd
> won't see the updates.
>
> So I'd argue that a file mapped MAP_SHARED must be mapped according to
> the colour rules, but a MAP_PRIVATE is free not to be so.

OK, fair enough.

> > Secondly, MAP_FIXED never checks for page colouring alignment.
> > I assume the cache aliasing on AMD Bulldozer is merely a performance
> > issue, and we can simply ignore page colouring for MAP_FIXED?
> > That will be easy to get right in an architecture-independent
> > implementation.
>
> There's a whole bunch of issues with MAP_FIXED, specifically address
> space overflow has been discussed previously, and resulted in this patch:
>
> [PATCH 0/6] get rid of extra check for TASK_SIZE in get_unmapped_area

Turns out, get_unmapped_area_prot (the function
that calls arch_get_unmapped_area) checks for
these overflows, so we should be fine.
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