Re: [PATCH 05/25] slab: make create_boot_cache() work with 32-bit sizes

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 06, 2018 at 12:34:05PM -0600, Christopher Lameter wrote:
> On Mon, 5 Mar 2018, Alexey Dobriyan wrote:
> 
> > struct kmem_cache::size has always been "int", all those
> > "size_t size" are fake.
> 
> They are useful since you typically pass sizeof( < whatever > ) as a
> parameter to kmem_cache_create(). Passing those values onto other
> functions internal to slab could use int.

Sure, but:

struct foo {
	int n;
	char *p;
};
int f(unsigned int x);

int g(void)
{
	return f(sizeof(struct foo));
}

gives:

   0:   bf 10 00 00 00          mov    $0x10,%edi
   5:   e9 00 00 00 00          jmpq   a <g+0xa>

Changing the prototype to "int f(unsigned long x)" produces _exactly the
same assembly_.  Why?  Because mov to %edi will zero out the upper 32-bits
of %rdi.  I consider it one of the flaws in the x86 instruction set that
mov %di doesn't zero out the upper 16 bits of %edi (and correspondingly
the upper 48 bits of %rdi), as it'd save an awful lot of bytes in the
instruction stream by replacing 32-bit constants with 16-bit constants.

There's just no difference between these two.  Unless you want to talk
about a structure exceeding 4GB in size, and then I'm afraid we have
bigger problems.

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