On 2/3/26 1:31 AM, Thomas Hellström wrote:
...
> So this is what I had in mind:
>
> I think certainly this would work regardless of whether pincount is
> implemented by means of refcount with a bias or not, and AFAICT it's
> also consistent with
>
> https://docs.kernel.org/core-api/pin_user_pages.html
>
> But it would not work if some part of core mm grabs a page refcount and
> *expects* that to pin a page in the sense that it should not be
> migrated. But you're suggesting that's actually the case?
Yes. The migration code itself, in fact, uses folio_get() with the
expectation that this blocks migration:
migrate_vma_collect_pmd():
/*
* By getting a reference on the folio we pin it and that blocks
* any kind of migration. Side effect is that it "freezes" the
* pte.
*
* We drop this reference after isolating the folio from the lru
* for non device folio (device folio are not on the lru and thus
* can't be dropped from it).
*/
folio = page_folio(page);
folio_get(folio);
I'm experiencing a bit of local-mind-boggle at decoupling refcount
from pincount, but part of that is probably just bias towards "but
it's always been that way!". haha :)
thanks,
--
John Hubbard
