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

[Steven Price]

On 28/02/2019 19:00, Dave Hansen wrote:
> 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?

That is the only use I'm currently aware of.

>> 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.

If no-one has any objections to dropping the holes in the output, then I
can rebase on something like below and drop this 'depth' patch.

Steve

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