linux-kernel - Re: [PATCH 2/5] lib: add bitmap_{from,to}

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Date:   Tue, 3 May 2022 12:56:13 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 2/5] lib: add bitmap_{from,to}_arr64

On Mon, May 02, 2022 at 01:06:56PM -0700, Yury Norov wrote:
> On Fri, Apr 29, 2022 at 08:45:35AM -0700, Yury Norov wrote:
> > On Fri, Apr 29, 2022 at 03:59:25PM +0300, Andy Shevchenko wrote:
> > > On Thu, Apr 28, 2022 at 01:51:13PM -0700, Yury Norov wrote:

...

> > > > +void bitmap_to_arr64(u64 *buf, const unsigned long *bitmap, unsigned int nbits)
> > > > +{
> > > > +	const unsigned long *end = bitmap + BITS_TO_LONGS(nbits);
> > > > +
> > > > +	while (bitmap < end) {
> > > > +		*buf = *bitmap++;
> > > > +		if (bitmap < end)
> > > > +			*buf |= (u64)(*bitmap++) << 32;
> > > > +		buf++;
> > > > +	}
> > > >  
> > > > +	/* Clear tail bits in last element of array beyond nbits. */

in the last

> > > > +	if (nbits % 64)
> > > > +		buf[-1] &= GENMASK_ULL(nbits, 0);
> > > 
> > > Hmm... if nbits is > 0 and < 64, wouldn't be this problematic, since
> > > end == bitmap? Or did I miss something?
> > 
> > BITS_TO_LONGS(0) == 0
> > BITS_TO_LONGS(1..32) == 1
> > BITS_TO_LONGS(33..64) == 2
> > 
> > The only potential problem with buf[-1] is nbits == 0, but fortunately
> > (0 % 64) == 0, and it doesn't happen.

I see, perhaps adding a small comment would be nice to have to explain that -1
index is safe.

> Are there any other concerns? If no, I'll fix formatting and append it to
> bitmap-for-next.

Nope.

-- 
With Best Regards,
Andy Shevchenko