Function to do end-writeback in bulk

[David Howells <dhowells@redhat.com>]

Hi Willy,

Can you take a look at the function below?  This is what I've come up with for
the moment for a basic bulk end-writeback (and I'll need something similar for
bulk unlock).  At the moment, it returns true/false depending on whether it
made progress, but I'm thinking that it might be better if it returns the
remaining size of any folio that partially overlaps the end position and so
didn't get unlocked.  This would could allow me to set a minimum amount to
accrue before I call it again.  Whether progress was made can also be
determined by comparing @from before and after.

Thanks,
David
---
/**
 * folio_end_writeback_range - End writeback for the folios within the range
 * @mapping: The pagecache to modify
 * @from: Pointer to the starting position (updated)
 * @to: The end position (exclusive)
 * @cleaner_func: Function to clean up the folios in the range
 * @cleaner_priv: Private data for the cleaner func
 *
 * Unlock folios that are entirely within in the given range, where @from is
 * included in the range, but @to is excluded from the range.
 *
 * Return: True if at least one folio got cleaned, false otherwise.  @from will
 * be updated to point past the last folio cleaned.
 */
static inline
bool folio_end_writeback_range(struct address_space *mapping,
			       uoff_t *from, uoff_t to,
			       void (*cleaner_func)(struct folio *folio,
						    void *cleaner_priv),
			       void *cleaner_priv)
{
	struct folio *folio;
	XA_STATE(xas, &mapping->i_pages, *from / PAGE_SIZE);
	bool cleaned = false;

	rcu_read_lock();
	xas_for_each(&xas, folio, (to - 1) / PAGE_SIZE) {
		uoff_t fend;

		if (xas_retry(&xas, folio))
			continue;

		fend = folio_next_pos(folio);
		if (fend > to)
			break;

		cleaner_func(folio, cleaner_priv);
		folio_end_writeback(folio);
		*from = fend;
		cleaned = true;
	}
	rcu_read_unlock();
	return cleaned;
}