Re: [PATCH 1/2] <linux/hash.h>: Make hash_64(), hash_ptr() return 32 bits

From: Linus Torvalds
Date: Mon May 02 2016 - 17:41:25 EST

On Mon, May 2, 2016 at 2:19 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> The reason that "hash_len" is a single 64-bit field rather than two
> 32-bit fields, for example, is that that way it takes on _register_
> when we do the has lookup. Some of that code was tuned to inline - and
> _not_ inline in particular patterns.

Actually, I think the tuning for no stack frame etc was mostly for the
permission testing with the selinux AVC code.

The filename hashing does have some of it too - like making sure that
the call to ->d_hash() is in an unlikely path etc and doesn't pollute
the case that actually matters. But I don't think any of it is simple
enough to avoid a stack frame.

The hash_len thing did make the innermost hash lookup loop smaller,
which was noticeable at some point.

What I really wanted to do there was actually have a direct-mapped "L1
dentry hash cache", that didn't have a hash loop at all (it would fall
back to the slow case for that). I had a patch for that (which worked
*beautifully*, partly because it also moved the hot entries to that
hash cache and thus effectively moved the active entries to the head
of the queue), but I couldn't get the L1 cache update to be coherent
without locking, which killed the thing.

Anyway, I suspect that your mixing function changes should be fine.
That link_path_walk() is important, but a couple of shifts and xors
shouldn't kill it.