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

["Vishal Moola (Oracle)" <vishal.moola@gmail.com>]

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.

I recall making sure we had a memcg_data member in ptdesc so that ptdesc
could directly reference it. Until we have a generic version, reverting
this patch should be the right way to go.

> This regression is visible in master as well as 6.18 stable, so CC
> stable too.
> 
> Fixes: f0c92726e89f ("ptdesc: remove references to folios from __pagetable_ctor() and pagetable_dtor()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>