Splitting pinned folios

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 12, 2024 at 06:23:43PM -0700, Jane Chu wrote:
> I noticed this recently

OK, this is entirely different, so I'm going to start a new thread ;-)

>  * GUP pin and PG_locked transferred to @page. Rest subpages can be freed if
>  * they are not mapped.
>  *
>  * Returns 0 if the hugepage is split successfully.
>  * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under
>  * us.
>  */
> int split_huge_page_to_list(struct page *page, struct list_head *list)
> {
> 
> I have a test case with poisoned shmem THP page that was mlocked and
> 
> GUP pinned (FOLL_LONGTERM|FOLL_WRITE), but the split succeeded.

I'm going to blame John for this!  There's no reference to pincount
anywhere in huge_memory.c, so I have no clue how this comment is even
close to true, nor do I understand how it could be done, since we don't
know which pages in a folio are pinned.

I think we have to prohibit splits of folios that are GUP pinned.