Re: [PATCH] Revert "ptdesc: remove references to folios from __pagetable_ctor() and pagetable_dtor()"

[David Hildenbrand (Arm)]

On 2/25/26 17:03, Matthew Wilcox wrote:
> On Tue, Feb 24, 2026 at 04:24:34PM -0800, Axel Rasmussen wrote:
>> This change swapped out mod_node_page_state for lruvec_stat_add_folio.
>> But, these two APIs are not interchangeable: the lruvec version also
>> increments memcg stats, in addition to "global" pgdat stats.
>>
>> So after this change, the "pagetables" memcg stat in memory.stat always
>> yields "0", which is a userspace visible regression.
>>
>> I tried to look for a refactor where we add a variant of
>> lruvec_stat_mod_folio which takes a pgdat and a memcg instead of a
>> folio, to try to adhere to the spirit of the original patch. But at the
>> end of the day this just means we have to call
>> folio_memcg(ptdesc_folio(ptdesc)) anyway, which doesn't really
>> accomplish much.
> 
> Thank you!  I hadn't been able to get a straight answer on this before.
> 
> You're right that there's no good function to call, but that just means
> we need to make one.  The principle here is that (eventually) different
> memdescs don't need to know about each other.  Obviously we're not there
> yet, but we can start disentangling them by not casting ptdescs back to
> folios (even though they're created that way).
> 
> Here's three patches smooshed together; I have them separately and I'll
> post them soon.

Should we just apply + backport the revert for now and re-do it based on
the revert?

-- 
Cheers,

David