Re: [PATCH v3 27/34] mm: pagewalk: Add 'depth' parameter to pte_hole

[Dave Hansen <dave.hansen@intel.com>]

On 2/28/19 3:28 AM, Steven Price wrote:
> static int get_level(unsigned long addr, unsigned long end)
> {
> 	/* Add 1 to account for ~0ULL */
> 	unsigned long size = (end - addr) + 1;
> 	if (size < PMD_SIZE)
> 		return 4;
> 	else if (size < PUD_SIZE)
> 		return 3;
> 	else if (size < P4D_SIZE)
> 		return 2;
> 	else if (size < PGD_SIZE)
> 		return 1;
> 	return 0;
> }
> 
> There are two immediate problems with that:
> 
>  * The "+1" to deal with ~0ULL is fragile
> 
>  * PGD_SIZE isn't what you might expect, it's not defined for most
> architectures and arm64/x86 use it as the size of the PGD table.
> Although that's easy enough to fix up.
> 
> Do you think a function like above would be preferable?

The question still stands of why we *need* the depth/level in the first
place.  As I said, we obviously need it for printing out the "name" of
the level.  Is that it?

> The other option would of course be to just drop the information from
> the debugfs file about at which level the holes are. But it can be
> useful information to see whether there are empty levels in the page
> table structure. Although this is an area where x86 and arm64 differ
> currently (x86 explicitly shows the gaps, arm64 doesn't), so if x86
> doesn't mind losing that functionality that would certainly simplify things!

I think I'd actually be OK with the holes just not showing up.  I
actually find it kinda hard to read sometimes with the holes in there.
I'd be curious what others think though.