Re: [RFC PATCH 10/34] x86: mm: carve out the generic compile-time folded pgtable case in effective_prot()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/13/26 22:27, Dave Hansen wrote:
> On 7/13/26 12:49, David Hildenbrand (Arm) wrote:
>>> I really do think this series is changing things, not keeping them the same.
>> Looking at it from that angle, I tend do agree: it further moves into the
>> direction that actually the p4d_present() carries value, and the previous
>> pgd_present() is just a dummy.
>>
>> I can see that some more work is required to untangle the mess we already have.
>>
>> I don't know why pgd_present() was made a dummy function and not p4d_present(),
>> but I'm sure there is a good reason to it. Changing that now ... omg.
> 
> Yeah, the p4d addition added even more wrinkles because it was the first
> time we tried runtime folding in addition to the historical compile-time
> folding.
> 
> There were certainly some ugly corners of it that we never found a nice
> way to fix. I know Kiryl tried pretty hard.
> 
> But, zooming back out... I don't think this diffstat:
> 
>  31 files changed, 329 insertions(+), 206 deletions(-)
> 
> is nearly justified to eliminate one virtually guaranteed cached memory
> read instruction. "Unnecessary", sure. But, worth the diffstat? I'm not
> quite there.

Just to emphasize, this prevents common code from being converted to pgdp_get()
and friends, as it would make folded page tables more expensive in other page
table walkers, and other arch maintainers rightfully complained about that.

I don't want to be stuck with this inconsistent mess of some common code page
walkers using pgp_get(), while others are stopped from being converted due to
inefficiency concerns.

*that* makes our common MM code worse, and it grinds my gears.

And I really care more about common MM code being clean and consistent than some
rusty old 32bit code being slightly worse.

As discussed, we can keep the set_pgd() monstrosity from working in all places
that don't use something like set_pgd(pgp_get()), bust as I raised off-list,
setting something that is unconditionally pgd_present() -- a dummy -- is just
extremely ugly. But ugly seems to be the theme in folded page table land.

That would certainly be less churn, while leaving this questionable code in
place, I'm fine with that as long as we don't add silent bugs when some arch
start doing e.g., set_pgd(pgp_get()).

> 
> Also, I think a series that adds code like this:
> 
>> -	if (level > 0) {
>> +	/* Ignore folded levels ... */
>> +	if (((level == 0) && mm_p4d_folded(st->mm)) ||
>> +	    ((level == 1) && mm_pud_folded(st->mm)) ||
>> +	    ((level == 2) && mm_pmd_folded(st->mm)))
>> +		return;
>> +
>> +	/* ... and make the actual first level remember the protection. */
>> +	if (((level == 0)) ||
>> +	    ((level == 1) && mm_p4d_folded(st->mm)) ||
>> +	    ((level == 2) && mm_pud_folded(st->mm)) ||
>> +	    ((level == 3) && mm_pmd_folded(st->mm)))
>> +		first_level = true;
> 
> fails the sniff test of being an effective refactor. It unquestionably
> makes _that_ code worse, not better.

As I said, we can improve that part. But we'll have to stop faking that each
level exist, when it's really folded.

That's all this code does, and the first part of this logic could be had in
common code easily (just don't call on folded page tables).

> 
> If the series was:
> 
> 	Subject: Refactor page table folding in all architectures
> 
> and we had a discussion about what the best overall thing to do for all
> architectures is, I think we might have a better path forward. It would
> also be a smaller series.

It took me too long to understand why a "nop4d.h" file defines PGD functions to
be dummies and assumes an arch to implement p4d functions.

So yes, in a perfect world we'd rewrite folded page table handling entirely.

BUT

Let's focus on the real problem:

Common code cannot consistently use pgdp_get() etc. as it harms configs with
folded page tables.

And that problem shouldn't require refactoring/rewriting all of page table
folding at this point.

> 
> To me, the double READ_ONCE() is a non-issue.

To other arch maintainers, it is!

> Having a nice, coherent
> page table API in the kernel is much more important to talk about.

While I agree, we are talking about avoiding degrading folded page table configs
as we make common code more consistent.

I hate our folded page table code when looking it from an arch perspective
(common code walking just works), and I agree that it should be improved. But I
disagree that it should be blocking of core-mm using page table getters
consistently.

-- 
Cheers,

David