Re: [PATCH] mm/migrate.c: Rework migration_entry_wait() to not take a pageref

[Matthew Wilcox]

On Thu, Nov 04, 2021 at 09:33:38PM +1100, Alistair Popple wrote:
> +++ b/mm/filemap.c
> @@ -1356,6 +1356,88 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
>  	return wait->flags & WQ_FLAG_WOKEN ? 0 : -EINTR;
>  }
>  
> +/**
> + * migration_entry_wait_on_locked - Wait for a migration entry to be removed
> + * @page: page referenced by the migration entry.
> + * @ptep: mapped pte pointer. This function will return with the ptep unmapped.
> + * @ptl: already locked ptl. This function will drop the lock.
> + *
> + * Wait for a migration entry referencing the given page to be removed. This is
> + * equivalent to put_and_wait_on_page_locked(page, TASK_UNINTERRUPTIBLE) except
> + * this can be called without taking a reference on the page. Instead this
> + * should be called while holding the ptl for the migration entry referencing
> + * the page.
> + *
> + * Returns after unmapping and unlocking the pte/ptl with pte_unmap_unlock().
> + *
> + * This follows the same logic as wait_on_page_bit_common() so see the comments
> + * there.
> + */
> +void migration_entry_wait_on_locked(struct page *page, pte_t *ptep,
> +				spinlock_t *ptl)
> +{
> +	struct wait_page_queue wait_page;
> +	wait_queue_entry_t *wait = &wait_page.wait;
> +	bool thrashing = false;
> +	bool delayacct = false;
> +	unsigned long pflags;
> +	wait_queue_head_t *q;
> +
> +	q = page_waitqueue(page);

You're going to need to update this patch to apply to Linus' current
tree; page_waitqueue() went away in favour of folio_waitqueue().

It seems like it would look simpler if this were a patch which modified
folio_wait_bit_common() instead of doing a manual inline of it into
this function.