Re: [RFC v2 0/3] iommu/intel: Free empty page tables on unmaps

[Pasha Tatashin]

On Fri, Apr 26, 2024 at 2:42 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 26.04.24 05:43, Pasha Tatashin wrote:
> > Changelog
> > ================================================================
> > v2: Use mapcount instead of refcount
> >      Synchronized with IOMMU Observability changes.
> > ================================================================
> >
> > This series frees empty page tables on unmaps. It intends to be a
> > low overhead feature.
> >
> > The read-writer lock is used to synchronize page table, but most of
> > time the lock is held is reader. It is held as a writer for short
> > period of time when unmapping a page that is bigger than the current
> > iova request. For all other cases this lock is read-only.
> >
> > page->mapcount is used in order to track number of entries at each page
> > table.
>
> I'm wondering if this will conflict with page_type at some point? We're
> already converting other page table users to ptdesc. CCing Willy.

Hi David,

This contradicts with the following comment in mm_types.h:
 * If your page will not be mapped to userspace, you can also use the four
 * bytes in the mapcount union, but you must call
page_mapcount_reset()
 * before freeing it.

Thank you,
Pasha