On 20.01.25 10:26, Vlastimil Babka wrote:
On 1/16/25 10:19, Fuad Tabba wrote:
Hi Ackerley,
On Thu, 16 Jan 2025 at 00:35, Ackerley Tng <ackerleytng@xxxxxxxxxx> wrote:
Registration of the folio_put() callback only happens if the VMM
actually tries to do vcpu_run(). For 4K folios I think this is okay
since the 4K folio can be freed via the transition state K -> state I,
but for hugetlb folios that have been split for sharing with userspace,
not getting a folio_put() callback means never putting the hugetlb folio
together. Hence, relying on vcpu_run() to add the folio_put() callback
leaves a way that hugetlb pages can be removed from the system.
I think we should try and find a path forward that works for both 4K and
hugetlb folios.
I agree, this could be an issue, but we could find other ways to
trigger the callback for huge folios. The important thing I was trying
to get to is how to have the callback and be able to register it.
IIUC page._mapcount and page.page_type works as a union because
page_type is only set for page types that are never mapped to userspace,
like PGTY_slab, PGTY_offline, etc.
In the last guest_memfd sync, David Hildenbrand mentioned that that
would be a temporary restriction since the two structures would
eventually be decoupled, work being done by Matthew Wilcox I believe.
Note the "temporary" might be few years still, it's a long-term project.
Right, nobody knows how long it will actually take. Willy thinks the
part that would be required here might be feasible in the nearer future:
"'d like to lay out some goals for the coming year. I think we can
accomplish a big goal this year" [1]
[1] https://lore.kernel.org/all/Z37pxbkHPbLYnDKn@xxxxxxxxxxxxxxxxxxxx/T/#u
--
Cheers,
David / dhildenb
