Re: [PATCH v6 02/30] mm: Add set/end/wait functions for PG_private_2

[Matthew Wilcox]

On Thu, Apr 08, 2021 at 03:04:20PM +0100, David Howells wrote:
> +static inline void set_page_private_2(struct page *page)
> +{
> +	get_page(page);
> +	SetPagePrivate2(page);

PAGEFLAG(OwnerPriv1, owner_priv_1, PF_ANY)

So we can set Private2 on any tail page ...

> +void end_page_private_2(struct page *page)
> +{
> +	page = compound_head(page);
> +	VM_BUG_ON_PAGE(!PagePrivate2(page), page);
> +	clear_bit_unlock(PG_private_2, &page->flags);
> +	wake_up_page_bit(page, PG_private_2);

... but when we try to end on a tail, we actually wake up the head ...

> +void wait_on_page_private_2(struct page *page)
> +{
> +	while (PagePrivate2(page))
> +		wait_on_page_bit(page, PG_private_2);

... although if we were waiting on a tail, the wake up won't find us ...

if only we had a way to ensure this kind of bug can't happen *cough,
lend your support to the page folio patches*.