David Hildenbrand wrote:
[..]
> > However, raises the question if fixing it here is sufficient for other
> > ZONE_DEVICE folio cases. I did not immediately find a place where other
> > ZONE_DEVICE users might be calling prep_compound_page() and leaving
> > stale tail page metadata lying around. Alistair?
>
> We only have to consider this when splitting folios (putting buddy
> freeing aside). clear_compound_head() is what to search for.
So I do not think there is a problem for the DEVICE_PRIVATE case since
that hits this comment in free_zone_device_folio()
/*
* Note: we don't expect anonymous compound pages yet. Once supported
* and we could PTE-map them similar to THP, we'd have to clear
* PG_anon_exclusive on all tail pages.
*/
The p2p-dma use case does not map into userspace, and the device-dax
case has static folio order for all potential folios. So I think this
fix is only needed for fsdax.
> We don't need it in mm/hugetlb.c because we'll only demote large folios
> to smaller-large folios and effectively reset the order/nr_pages for all
> involved folios.
I also now feel better about a local fs/dax.c fix because clearing
_nr_pages in free_zone_device_folio() would require static folio
metadata cases like device-dax to start re-inializing that field.
I.e. this seems to be the only ZONE_DEVICE case doing this demote to
order-0.
