filemap: Drop the refcount while waiting for page lock

Commit bd8a1f3655 ("mm/filemap: support readpage splitting a page")
changed the read_iter path to drop the refcount while waiting for the
page lock.  However, it missed the same pattern in read_mapping_page()
and friends.  Use the same pattern in do_read_cache_folio() that is
used in filemap_update_page().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>

mm/filemap.c

@@ -3460,45 +3460,12 @@ filler:
 	if (folio_test_uptodate(folio))
 		goto out;
 
-	/*
-	 * Page is not up to date and may be locked due to one of the following
-	 * case a: Page is being filled and the page lock is held
-	 * case b: Read/write error clearing the page uptodate status
-	 * case c: Truncation in progress (page locked)
-	 * case d: Reclaim in progress
-	 *
-	 * Case a, the page will be up to date when the page is unlocked.
-	 *    There is no need to serialise on the page lock here as the page
-	 *    is pinned so the lock gives no additional protection. Even if the
-	 *    page is truncated, the data is still valid if PageUptodate as
-	 *    it's a race vs truncate race.
-	 * Case b, the page will not be up to date
-	 * Case c, the page may be truncated but in itself, the data may still
-	 *    be valid after IO completes as it's a read vs truncate race. The
-	 *    operation must restart if the page is not uptodate on unlock but
-	 *    otherwise serialising on page lock to stabilise the mapping gives
-	 *    no additional guarantees to the caller as the page lock is
-	 *    released before return.
-	 * Case d, similar to truncation. If reclaim holds the page lock, it
-	 *    will be a race with remove_mapping that determines if the mapping
-	 *    is valid on unlock but otherwise the data is valid and there is
-	 *    no need to serialise with page lock.
-	 *
-	 * As the page lock gives no additional guarantee, we optimistically
-	 * wait on the page to be unlocked and check if it's up to date and
-	 * use the page if it is. Otherwise, the page lock is required to
-	 * distinguish between the different cases. The motivation is that we
-	 * avoid spurious serialisations and wakeups when multiple processes
-	 * wait on the same page for IO to complete.
-	 */
-	folio_wait_locked(folio);
-	if (folio_test_uptodate(folio))
-		goto out;
+	if (!folio_trylock(folio)) {
+		folio_put_wait_locked(folio, TASK_UNINTERRUPTIBLE);
+		goto repeat;
+	}
 
-	/* Distinguish between all the cases under the safety of the lock */
-	folio_lock(folio);
-
-	/* Case c or d, restart the operation */
+	/* Folio was truncated from mapping */
 	if (!folio->mapping) {
 		folio_unlock(folio);
 		folio_put(folio);