Let's assume a single subpage of a large folio is no longer mapped.
Then, we'd have:
nr_pages == folio_nr_pages(folio) - 1.
You could simply map+reuse most of the folio without COWing.
yes. This is good but the pte which is no longer mapped could be
anyone within the nr_pages PTEs. so it could be quite tricky for
set_ptes.
The swap batching logic should take care of that, otherwise it would be
buggy.
Once we support COW reuse of PTE-mapped THP we'd do the same. Here, it's
just easy to detect that the folio is exclusive (folio_ref_count(folio)
== 1 before mapping anything).
If you really want to mimic what do_wp_page() currently does, you should
have:
exclusive || (folio_ref_count(folio) == 1 && !folio_test_large(folio))
I actually dislike the part that do_wp_page() handles the reuse of a large
folio which is entirely mapped. For example, A forks B, B exit, we write
A's large folio, we get nr_pages CoW of small folios. Ideally, we can
reuse the whole folios for writing.
Yes, see the link I shared to what I am working on. There isn't really a
question if what we do right now needs to be improved and all these
scenarios are pretty obvious clear.
Personally, I think we should keep it simple here and use:
exclusive || folio_ref_count(folio) == 1
I feel this is still better than
exclusive || (folio_ref_count(folio) == 1 && !folio_test_large(folio))
as we reuse the whole large folio. the do_wp_page() behaviour
doesn't have this.
Yes, but there is the comment "Same logic as in do_wp_page();". We
already ran into issues having different COW reuse logic all over the
place. For this case here, I don't care if we leave it as
"exclusive || folio_ref_count(folio) == 1"
But let's not try inventing new stuff here.
--
Cheers,
David / dhildenb
