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Date: Mon, 30 May 2016 10:59:05 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: Thomas Gleixner <tglx@linutronix.de>, LKML <linux-kernel@vger.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Darren Hart <darren@dvhart.com>, Ingo Molnar <mingo@kernel.org>,
        Michael Kerrisk <mtk.manpages@googlemail.com>,
        Davidlohr Bueso <dave@stgolabs.net>, Chris Mason <clm@fb.com>,
        "Carlos O'Donell" <carlos@redhat.com>,
        Torvald Riegel <triegel@redhat.com>,
        Eric Dumazet <edumazet@google.com>
Subject: Re: [patch V2 3/7] futex: Add op for hash preallocation
Message-ID: <20160530085905.GO3192@twins.programming.kicks-ass.net>
References: <20160505204230.932454245@linutronix.de>
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 <20160519122558.GC3192@twins.programming.kicks-ass.net>
 <20160527172756.GD28561@breakpoint.cc>
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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.