linux-kernel - Re: Raw spinlocks and memory allocation

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Date:   Fri, 31 Jul 2020 13:38:34 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     paulmck@...nel.org
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, hannes@...xchg.org, willy@...radead.org,
        urezki@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Raw spinlocks and memory allocation

On Thu, 30 Jul 2020 16:12:05 -0700 "Paul E. McKenney" <paulmck@...nel.org> wrote:

> So, may we add a GFP_ flag that will cause kmalloc() and friends to return
> NULL when they would otherwise need to acquire their non-raw spinlock?
> This avoids adding any overhead to the slab-allocator fastpaths, but
> allows callback invocation to reduce cache misses without having to
> restructure some existing callers of call_rcu() and potential future
> callers of kfree_rcu().

We have eight free gfp_t bits so that isn't a problem.

Adding a test-n-branch to the kmalloc() fastpath may well be a concern.

Which of mm/sl?b.c are affected?

A doesnt-need-to-really-work protopatch would help us understand the
potential cost?