GFP_KERNEL or SLAB_KERNEL?

[Posted February 18, 2003 by corbet]

The low-level kernel memory allocation functions take a set of flags describing how that allocation is to be performed. Among other things, these GFP_ ("get free page") flags control whether the allocation process can sleep and wait for memory, whether high memory can be used, and so on. See this article for the full set.

The kernel slab allocator is an additional layer built on top of the low-level code; it handles situations where numerous objects of the same size are frequently allocated and freed. The slab code, too, has a set of flags describing how memory allocation is to happen. They look suspiciously like the low-level flags, but they have different names; instead of GFP_KERNEL, for example, user of the slab code are expected to say SLAB_KERNEL.

Underneath it all, however, the two sets of flags are the same. As a result, many calls to the slab code just use the GFP_ flags, rather than the SLAB_ flags. William Lee Irwin decided it was time to fix that; he posted a patch converting several slab users over to the SLAB_ flags. It looked like a fairly standard, freeze-stage kernel cleanup.

The question came up, however: why bother? Not everybody, it seems, thinks that the separate SLAB_flags are worth the trouble. William responded with another patch which gets rid of the SLAB_flags altogether. So far, neither patch has been merged. But they do raise a worthwhile question: why do we need a separate set of flags if the callers have nothing different to say?