Re: Number of arguments in vmalloc.c

[Matthew Wilcox <willy@infradead.org>]

On Mon, Dec 03, 2018 at 02:04:41PM -0800, Nadav Amit wrote:
> On Dec 3, 2018, at 8:13 AM, Matthew Wilcox <willy@infradead.org> wrote:
> > On Mon, Dec 03, 2018 at 02:59:36PM +0100, Vlastimil Babka wrote:
> >> On 11/28/18 3:01 PM, Matthew Wilcox wrote:
> >>> Some of the functions in vmalloc.c have as many as nine arguments.
> >>> So I thought I'd have a quick go at bundling the ones that make sense
> >>> into a struct and pass around a pointer to that struct.  Well, it made
> >>> the generated code worse,
> >> 
> >> Worse in which metric?
> > 
> > More instructions to accomplish the same thing.
> > 
> >>> so I thought I'd share my attempt so nobody
> >>> else bothers (or soebody points out that I did something stupid).
> >> 
> >> I guess in some of the functions the args parameter could be const?
> >> Might make some difference.
> >> 
> >> Anyway this shouldn't be a fast path, so even if the generated code is
> >> e.g. somewhat larger, then it still might make sense to reduce the
> >> insane parameter lists.
> > 
> > It might ... I'm not sure it's even easier to program than the original
> > though.
> 
> My intuition is that if all the fields of vm_args were initialized together
> (in the same function), and a 'const struct vm_args *' was provided as
> an argument to other functions, code would be better (at least better than
> what you got right now).
> 
> I’m not saying it is easily applicable in this use-case (since I didn’t
> check).

Your intuition is wrong ...

   text	   data	    bss	    dec	    hex	filename
   9466	     81	     32	   9579	   256b	before.o
   9546	     81	     32	   9659	   25bb	.build-tiny/mm/vmalloc.o
   9546	     81	     32	   9659	   25bb	const.o

indeed, there's no difference between with or without the const, according
to 'cmp'.

Now, only alloc_vmap_area() gets to take a const argument.
__get_vm_area_node() intentionally modifies the arguments.  But feel
free to play around with this; you might be able to make it do something
worthwhile.