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SubjectRe: Uninline kcalloc
On 2012-02-15 15:17 -0500, Nick Bowler wrote:
> On 2012-02-14 15:24 -0600, Christoph Lameter wrote:
> > On Tue, 14 Feb 2012, Nick Bowler wrote:
> >
> > > On 2012-02-14 13:37 -0600, Christoph Lameter wrote:
> > > > This patch still preserves kcalloc. But note that if kcalloc returns NULL
> > > > then multiple conditions may have caused it. One is that the array is
> > > > simply too large. The other may be that such an allocation is not possible
> > > > due to fragmentation.
> > > [...]
> > > > +static inline long calculate_array_size(size_t n, size_t size)
> > > > +{
> > > > + if (size != 0 && n > ULONG_MAX / size)
> > > > +
> > > > + return 0;
> > >
> > > This isn't right. The above tests whether or not the result of the
> > > multiplication will not be representable in an 'unsigned long'...
> >
> > Yes and so does the current kcalloc.
>
> Well, the current kcalloc doesn't assign the result to a signed long.
> However, it does assign the result to a size_t, which makes one wonder
> why it's not testing against SIZE_MAX.

Of course, right after I send this I realize that we do not appear to
define SIZE_MAX in the kernel. So s/SIZE_MAX/((size_t)-1)/g.

> If size_t has the same range as unsigned long on all architectures,
> then this confusion doesn't matter, but is that actually the case?
>
> > > > + return n * size;
> > >
> > > but then the result is assigned to a (signed) long, which may overflow
> > > if it's greater than LONG_MAX.
> >
> > That can happen?
>
> Yes, because LONG_MAX (the maximum value of your return type) is
> strictly less than ULONG_MAX (what you test against). It's not hard to
> pick input numbers that multiply to something between LONG_MAX and
> ULONG_MAX, which will cause your function to return a negative value
> (standard C leaves the result of such a conversion implementation-
> defined, but I'll assume for now that it works this way for everything
> that compiles Linux).
>
> Admittedly, your kcalloc change then assigns this negative value to a
> size_t, which will result in the correct positive value assuming
> SIZE_MAX == ULONG_MAX, but that's gratuitously roundabout.
>
> [...]
> > > [...]
> > > > void *kcalloc(size_t n, size_t size, gfp_t flags)
> > > > {
> > > > - if (size != 0 && n > ULONG_MAX / size)
> > > > - return NULL;
> > > > - return __kmalloc(n * size, flags | __GFP_ZERO);
> > > > + size_t s = calculate_array_size(n, size);
> > > > +
> > > > + if (s)
> > > > + return kzalloc(s, flags);
> > > > +
> > > > + return NULL;
> > > > }
> > >
> > > This hunk changes the behaviour of kcalloc if either of the two size parameters
> > > is 0.
> >
> > You want ZERO_PTR returns?
> >
> > NULL is one permissible return value of calloc() if size == 0. So we are
> > now deviating from user space conventions.
>
> Sort of. While standard C leaves it implementation-defined whether
> successful zero-sized allocations are possible, all sane implementations
> let them succeed. Hence, portable C apps need to handle 0 as a special
> case, because there are insane implementations out there. There's no
> reason for the kernel to be one of them.
>
> Regardless, this was still a (presumably unintentional) change from
> the previous behaviour.
>
> Cheers,
--
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)



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