[Devel] Re: [PATCH 2/4] Add a __GFP_SLABMEMCG flag

[James Bottomley]

On Fri, 2012-06-08 at 14:31 -0500, Christoph Lameter wrote:
> On Fri, 8 Jun 2012, Glauber Costa wrote:
> 
> >   */
> >  #define __GFP_NOTRACK_FALSE_POSITIVE (__GFP_NOTRACK)
> >
> > -#define __GFP_BITS_SHIFT 25	/* Room for N __GFP_FOO bits */
> > +#define __GFP_BITS_SHIFT 26	/* Room for N __GFP_FOO bits */
> >  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
> 
> Please make this conditional on CONFIG_MEMCG or so. The bit can be useful
> in particular on 32 bit architectures.

I really don't think that's at all a good idea.  It's asking for trouble
when we don't spot we have a flag overlap.  It also means that we're
trusting the reuser to know that their use case can never clash with
CONFIG_MEMGC and I can't think of any configuration where this is
possible currently.

I think making the flag define of __GFP_SLABMEMCG conditional might be a
reasonable idea so we get a compile failure if anyone tries to use it
when !CONFIG_MEMCG.

James