linux-kernel - Re: [patch V2 3/7] futex: Add op for hash preallocation

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Message-ID: <20160530085905.GO3192@twins.programming.kicks-ass.net>
Date:	Mon, 30 May 2016 10:59:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch V2 3/7] futex: Add op for hash preallocation

On Fri, May 27, 2016 at 07:27:57PM +0200, Sebastian Andrzej Siewior wrote:
> On 2016-05-19 14:25:58 [+0200], Peter Zijlstra wrote:
> > On Thu, May 05, 2016 at 08:44:05PM -0000, Thomas Gleixner wrote:
> > > +static int futex_preallocate_hash(unsigned int slots)
> > > +{
> > > +#ifdef CONFIG_FUTEX_PRIVATE_HASH
> > > +	struct mm_struct *mm = current->mm;
> > > +	struct futex_hash_bucket *hb;
> > > +	unsigned int bits;
> > > +
> > > +	/* Try to allocate the requested nr of slots */
> > > +	bits = order_base_2(slots);
> > > +
> > > +	if (bits < FUTEX_MIN_HASH_BITS)
> > > +		bits = FUTEX_MIN_HASH_BITS;
> > > +
> > > +	if (bits > futex_max_hash_bits)
> > > +		bits = futex_max_hash_bits;
> > > +
> > > +	futex_populate_hash(bits);
> > 
> > Should we not simply fail if the provided number of slots is not a power
> > of 2 ?
> 
> We could if it is worth doing so. The procfs interface which limits the
> upper / lower limit is bits based. This is slot based which then gets
> converted to the number if bits.
> If we align this interface with proc's limits then we would expect the
> number of bits instead slots - now check for power of two anymore.
> Anyone?

I'm all for consistent and strict when it comes to things like this.