On Tue, 24 Jul 2007, Peter Zijlstra wrote:
> > There is another exception for __GFP_DMA.
>
> non of the zone specifiers are
__GFP_DMA is handled in a similar way to __GFP_ZERO though. Its explicitly
listed in BUG_ON() because it can be specified in the gfpflags to kmalloc
but also set by having created a slab with SLAB_DMA. It is also cleared
by the & GFP_LEVEL_MASK.
> > > Anybody else got a preference?
> >
> > > #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
> > >
> > > -/* if you forget to add the bitmask here kernel will crash, period */
> > > +/*
> > > + * If you forget to add the bitmask here kernel will crash, period!
> > > + *
> > > + * GFP_LEVEL_MASK is used to filter out the flags that are to be passed to the
> > > + * page allocator.
> > > + *
> >
> > GFP_LEVEL_MASK is also used in mm/vmalloc.c. We need a definition that
> > goes beyond slab allocators.
>
> Right, bugger.
Lets get rid of the cryptic sentence there and explain it in a better way.
GFP_LEVEL_MASK contains the flags that are passed to the page allocator
by derived allocators (such as slab allocators and vmalloc, maybe the
uncached allocator may use it in the future?).
__get_vm_area_node also relies on GFP_LEVEL_MASK to clear the __GFP_ZERO
flag. Otherwise the kmalloc_node there would needlessly return zeroed
memory (or have failed in the past).
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