Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and fix blkcg percpu alloc deadlock

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 27 Dec 2011 13:44:21 -0800
Tejun Heo <tj@kernel.org> wrote:

> I'm not convinced trying to put this into GFP_KERNEL context would
> work.  Short of that, the next best thing would be making percpu
> allocator useable from memory reclaim path, right?

Well..  All allocations which are weaker than GFP_KERNEL are to be
discouraged.  That being said...

>  But that would
> involved a lot more churn and complexity without much added benefit,
> given that this type of use cases aren't expected to be common - and
> I'm fairly sure it isn't given track record of past few years.

I don't think it would be too hard to add an alloc_percpu_gfp().  Add
the gfp_t to a small number of functions (two or three?) then change
pcpu_mem_zalloc() to always use kzalloc() if (flags & GFP_KERNEL !=
GFP_KERNEL).  And that's it?

But the question is: is this a *good* thing to do?  It would be nice if
kernel developers understood that GFP_KERNEL is strongly preferred and
that they should put in effort to use it.  But there's a strong
tendency for people to get themselves into a sticky corner then take
the easy way out, resulting in less robust code.  Maybe calling the
function alloc_percpu_i_really_suck() would convey the hint.