On Tue, 4 Jul 2023, Alexander Gordeev wrote:

On Sat, Jul 01, 2023 at 09:32:38PM -0700, Hugh Dickins wrote:

quoted

On Thu, 29 Jun 2023, Hugh Dickins wrote:

Hi Hugh,

...

quoted

No, not quite the same rules as before: I came to realize that using
list_add_tail() for the HH pages would be liable to put a page on the
list which forever blocked reuse of PP list_add_tail() pages after it
(could be solved by a list_move() somewhere, but we have agreed to
prefer simplicity).

Just to make things more clear for me: do I understand correctly that this
was an attempt to add HH fragments to pgtable_list from pte_free_defer()?

Yes, from page_table_free() called from pte_free_defer(): I had claimed
they could be put on the list (or not) without needing to consider their
HH-ness, apart from wanting to list_add_tail() rather than list_add() them.

But then realized that this category of list_add_tail() pages would block
access to the others.

But I think I was mistaken then to say "could be solved by a list_move()
somewhere"; because "somewhere" would have had to be __tlb_remove_table()
when it removes PP-bits, which would bring us back to the issues of
getting a spinlock from an mm which might already be freed.

Hugh