linux-kernel - Re: Raw spinlocks and memory allocation

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Message-ID: <20200731205933.GT23808@casper.infradead.org>
Date:   Fri, 31 Jul 2020 21:59:33 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        hannes@...xchg.org, urezki@...il.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: Raw spinlocks and memory allocation

On Fri, Jul 31, 2020 at 01:48:55PM -0700, Paul E. McKenney wrote:
> On Fri, Jul 31, 2020 at 01:38:34PM -0700, Andrew Morton wrote:
> > 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.
> 
> Whew!!!  ;-)
> 
> > Adding a test-n-branch to the kmalloc() fastpath may well be a concern.
> > 
> > Which of mm/sl?b.c are affected?
> 
> None of them, it turns out.  The initial patch will instead directly
> invoke __get_free_page().  So we could just leave sl?b.c alone.

Isn't that spelled GFP_NOWAIT?