On Mon, 2012-06-11 at 09:24 -0500, Christoph Lameter wrote: > On Sat, 9 Jun 2012, James Bottomley wrote: > > > 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. > > Flag overlap can be avoided using the same method as we have done with the > page flags (which uses an enum). There are other uses of N bits after > GFP_BITS_SHIFT. On first look this looks like its 4 right now so we cannot > go above 28 on 32 bit platforms. It would also be useful to have that > limit in there somehow so that someone modifying the GFP_BITS sees the > danger. But if there's no possible configuration that can use a flag and depends on !CONFIG_MEMGC then why bother? The main problem is that unless you get two configurations which exactly cancel each other and require a GFP flag, you end up eventually with unbuildable configurations that need >32 flags. > > 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. > > Ok that is another reason to do so. A reason to make it conditional, not a reason to go to the trouble of making the flags reusable. James -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>