Re: [Q] mm/memblock.c: cast truncates bits from RED_INACTIVE

[Pekka Enberg]

On 6/21/11 3:31 AM, H Hartley Sweeten wrote:
> On Monday, June 20, 2011 5:03 PM, Andrew Morton wrote:
> > On Tue, 14 Jun 2011 19:47:19 -0500 H Hartley Sweeten wrote:
> >
> > > Hello all,
> > >
> > > Sparse is reporting a couple warnings in mm/memblock.c:
> > >
> > > 	warning: cast truncates bits from constant value (9f911029d74e35b becomes 9d74e35b)
> > >
> > > The warnings are due to the cast of RED_INACTIVE in memblock_analyze():
> > >
> > > 	/* Check marker in the unused last array entry */
> > > 	WARN_ON(memblock_memory_init_regions[INIT_MEMBLOCK_REGIONS].base
> > > 		!= (phys_addr_t)RED_INACTIVE);
> > > 	WARN_ON(memblock_reserved_init_regions[INIT_MEMBLOCK_REGIONS].base
> > > 		!= (phys_addr_t)RED_INACTIVE);
> > >
> > > And in memblock_init():
> > >
> > > 	/* Write a marker in the unused last array entry */
> > > 	memblock.memory.regions[INIT_MEMBLOCK_REGIONS].base = (phys_addr_t)RED_INACTIVE;
> > > 	memblock.reserved.regions[INIT_MEMBLOCK_REGIONS].base = (phys_addr_t)RED_INACTIVE;
> > >
> > > Could this cause any problems?  If not, is there anyway to quiet the sparse noise?
> >
> > It's all just a debugging check and that check will continue to work OK
> > despite this bug.
> >
> > But yes, it's ugly and should be fixed.
> >
> > I don't think that mm/memblock.c should have reused RED_INACTIVE.
> > That's a slab thing and wedging it into a phys_addr_t was
> > inappropriate.
> >
> > In fact I don't think RED_INACTIVE should exist.  It's just inviting
> > other subsystems to (ab)use it.  It should be replaced by a
> > slab-specific SLAB_RED_INACTIVE, as slub did with SLUB_RED_INACTIVE.
> >
> >
> > I'd suggest something like the below, which I didn't test.  Feel free to
> > send it back at me, or ignore it ;)
> >
> >
> > diff -puN include/linux/poison.h~a include/linux/poison.h
> > --- a/include/linux/poison.h~a
> > +++ a/include/linux/poison.h
> > @@ -40,6 +40,12 @@
> >   #define	RED_INACTIVE	0x09F911029D74E35BULL	/* when obj is inactive */
> >   #define	RED_ACTIVE	0xD84156C5635688C0ULL	/* when obj is active */
> >
> > +#ifdef CONFIG_PHYS_ADDR_T_64BIT
> > +#define MEMBLOCK_INACTIVE	0x3a84fb0144c9e71bULL
> > +#else
> > +#define MEMBLOCK_INACTIVE	0x44c9e71bUL
> > +#endif
> > +
> >   #define SLUB_RED_INACTIVE	0xbb
> >   #define SLUB_RED_ACTIVE		0xcc
> >
> > diff -puN mm/memblock.c~a mm/memblock.c
> > --- a/mm/memblock.c~a
> > +++ a/mm/memblock.c
> > @@ -758,9 +758,9 @@ void __init memblock_analyze(void)
> >
> >   	/* Check marker in the unused last array entry */
> >   	WARN_ON(memblock_memory_init_regions[INIT_MEMBLOCK_REGIONS].base
> > -		!= (phys_addr_t)RED_INACTIVE);
> > +		!= MEMBLOCK_INACTIVE);
> >   	WARN_ON(memblock_reserved_init_regions[INIT_MEMBLOCK_REGIONS].base
> > -		!= (phys_addr_t)RED_INACTIVE);
> > +		!= MEMBLOCK_INACTIVE);
> >
> >   	memblock.memory_size = 0;
> >
> > @@ -786,8 +786,8 @@ void __init memblock_init(void)
> >   	memblock.reserved.max	= INIT_MEMBLOCK_REGIONS;
> >
> >   	/* Write a marker in the unused last array entry */
> > -	memblock.memory.regions[INIT_MEMBLOCK_REGIONS].base = (phys_addr_t)RED_INACTIVE;
> > -	memblock.reserved.regions[INIT_MEMBLOCK_REGIONS].base = (phys_addr_t)RED_INACTIVE;
> > +	memblock.memory.regions[INIT_MEMBLOCK_REGIONS].base = MEMBLOCK_INACTIVE;
> > +	memblock.reserved.regions[INIT_MEMBLOCK_REGIONS].base = MEMBLOCK_INACTIVE;
> >
> >   	/* Create a dummy zero size MEMBLOCK which will get coalesced away later.
> >   	 * This simplifies the memblock_add() code below...
>
> FWIW, your patch above quiet's the sparse warnings on my system (arm ep93xx) and
> the system boots and runs fine.
>
> If you want it..
>
> Tested-by: H Hartley Sweeten<hsweeten@xxxxxxxxxxxxxxxxxxx>

Acked-by: Pekka Enberg <penberg@xxxxxxxxxx>
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