On Sun, 2009-02-15 at 13:55 -0800, Andrew Morton wrote: > On Sun, 15 Feb 2009 15:43:14 -0600 Matt Mackall <mpm@xxxxxxxxxxx> wrote: > > > On Sun, 2009-02-15 at 13:36 -0800, Andrew Morton wrote: > > > On Thu, 12 Feb 2009 17:55:04 +0200 Pekka Enberg <penberg@xxxxxxxxxxxxxx> wrote: > > > > > > > On Thu, Feb 12, 2009 at 12:45:21PM +0200, Pekka Enberg wrote: > > > > > > > > > > > > Because the API was being widely abused in the nommu code, for example. > > > > > > I'd rather not add it back for this special case which can be handled > > > > > > otherwise. > > > > > > > > On Thu, 2009-02-12 at 18:50 +0800, Herbert Xu wrote: > > > > > I'm sorry but that's like banning the use of heaters just because > > > > > they can abused and cause fires. > > > > > > > > > > I think I've said this to you before but in networking we very much > > > > > want to use ksize because the standard case of a 1500-byte packet > > > > > has loads of extra room given by kmalloc which all goes to waste > > > > > right now. > > > > > > > > > > If we could use ksize then we can stuff loads of metadata in that > > > > > space. > > > > > > > > OK, fair enough, I applied Kirill's patch. Thanks. > > > > > > > > > > Could we please have more details regarding this: > > > > > > > The ksize() function is not exported to modules because it has non-standard > > > > behavour across different slab allocators. > > > > > > How does the behaviour differ? It this documented? Can we fix it? > > > > SLAB and SLUB support calling ksize() on objects returned by > > kmem_cache_alloc. > > > > SLOB only supports it on objects from kmalloc. This is because it does > > not store any size or type information in kmem_cache_alloc'ed objects. > > Instead, it infers them from the cache argument. > > OK. This is really bad, isn't it? No. There are very few ksize callers and very few of those are making this particular category error. And it -is- a category error. The fact that kmalloc is implemented on top of kmem_cache_alloc is an implementation detail that callers should not assume. They shouldn't call kfree() on kmem_cache_alloc objects (even though it might just happen to work), nor should they call ksize(). > > Ideally SLAB and SLUB would complain about using ksize inappropriately > > when debugging was enabled. > > > > OK, thanks. > > Ideally we would support ksize() for both kmalloc() and > kmem_cache_alloc() memory across all implementations. There's never a good reason to call ksize on a kmem_cache_alloced object. You -must- statically know what type of object you have already to be able to free it later with kmem_cache_free, ergo, you can statically know how big it is too. Another alternative to the above is to throw sparse at it, and have it track what allocators a pointer might have come through. But as far as I'm aware, there's only been one actual bug in this area: nommu was calling ksize on pointers of all kinds, including stuff allocated at compile time. > Gee this sucks. Biggest mistake I ever made. Are we working hard > enough to remove some of these sl?b implementations? Would it help if > I randomly deleted a couple? Again, I think there's a strong argument for having two. We can't reasonably expect one allocator to work well on supercomputers and phones. One will likely value performance significantly higher than memory usage and vice-versa. I think most of the pain here is actually peripheral. SLUB in particular has churned a lot of interfaces. But we would have had that had we instead decided to throw a lot of effort into making SLAB better. -- http://selenic.com : development and support for Mercurial and Linux -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
