linux-kernel - Re: [PATCH next v4 1/5] minmax: Add umin(a, b) and umax(a, b)

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Message-ID: <af0bfe8a-8b29-43df-b385-a23f0da54a7b@moroto.mountain>
Date: Thu, 18 Jan 2024 13:30:52 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: David Laight <David.Laight@...lab.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	'Andy Shevchenko' <andriy.shevchenko@...ux.intel.com>,
	'Andrew Morton' <akpm@...ux-foundation.org>,
	"'Matthew Wilcox (Oracle)'" <willy@...radead.org>,
	'Christoph Hellwig' <hch@...radead.org>,
	"'Jason A. Donenfeld'" <Jason@...c4.com>
Subject: Re: [PATCH next v4 1/5] minmax: Add umin(a, b) and umax(a, b)

On Fri, Jan 12, 2024 at 02:26:33PM +0000, David Laight wrote:
> From: Dan Carpenter
> > Sent: 12 January 2024 14:03
> > 
> > On Fri, Jan 12, 2024 at 01:40:30PM +0000, David Laight wrote:
> > > From: Dan Carpenter
> > > > Sent: 12 January 2024 12:50
> > > >
> > > > On Mon, Sep 18, 2023 at 08:16:30AM +0000, David Laight wrote:
> > > > > +/**
> > > > > + * umin - return minimum of two non-negative values
> > > > > + *   Signed types are zero extended to match a larger unsigned type.
> > > > > + * @x: first value
> > > > > + * @y: second value
> > > > > + */
> > > > > +#define umin(x, y)	\
> > > > > +	__careful_cmp((x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull, <)
> > > >
> > > > Why do we match "a larger unsigned type" instead of ULL_MAX?  Presumably
> > > > it helps performance somehow...  I agree that it's probably fine but I
> > > > would be more comfortable if it skipped UINT_MAX and jumped directly to
> > > > ULONG_MAX.  These days 4 gigs is small potatoes.  The vmalloc() function
> > > > can allocate 4G so we've had integer overflow bugs with this before.
> > >
> > > The '+ 0ul*' carefully zero extend signed values without changing
> > > unsigned values.
> > > The compiler detects when it has zero-extended both sides and
> > > uses the smaller compare.
> > > In essence:
> > > 	x + 0u converts 'int' to 'unsigned int'.
> > > 		Avoids the sign extension adding 0ul on 64bit.
> > > 	x + 0ul converts a 'long' to 'unsigned long'.
> > > 		Avoids the sign extension adding 0ull on 32bit
> > > 	x + 0ull converts a 'long long' to 'unsigned long long'.
> > > You need all three to avoid sign extensions and get an unsigned
> > > compare.
> > 
> > So unsigned int compares are faster than unsigned long compares?
> > 
> > It's just sort of weird how it works.
> > 
> > 	min_t(unsigned long, -1, 10000000000)); => 10000000000
> > 	umin(umin(-1, 10000000000)); => UINT_MAX
> > 
> > UINT_MAX is just kind of a random value.  I would have prefered
> > ULONG_MAX, it's equally random but it's more safe because nothing can
> > allocate ULONG_MAX bytes.
> 
> umin() is only defined for non-negative values.

I'm so confused by this.  To me the big selling point of min_t() was
that it clamps things to between zero and the max.

> So that example is really outside the domain of the function.
> 
> Consider:
> 	int x = some_positive_value;
> 	unsigned long long y;
> then:
> 	min_t(unsigned long long, x, y);
> 	Does (unsigned long long)x which is (unsigned long long)(long long)x
> 	and requires that x be sign extended to 64bits.
> 	On 32bit that is quite horrid.

I wasn't saying jump straight to ull.  I was suggesting jump to ul then
ull, but skip uint.  On a 32bit system, you can't allocate ULONG_MAX
bytes, so it still ends up being quite a safe number.

regards,
dan carpenter