mm/readahead: do not allow order-1 folio

The THP machinery does not support order-1 folios because it requires meta
data spanning the first 3 `struct page`s.  So order-2 is the smallest
large folio that we can safely create.

There was a theoretical bug whereby if ra->size was 2 or 3 pages (due to
the device-specific bdi->ra_pages being set that way), we could end up
with order = 1.  Fix this by unconditionally checking if the preferred
order is 1 and if so, set it to 0.  Previously this was done in a few
specific places, but with this refactoring it is done just once,
unconditionally, at the end of the calculation.

This is a theoretical bug found during review of the code; I have no
evidence to suggest this manifests in the real world (I expect all
device-specific ra_pages values are much bigger than 3).

Link: https://lkml.kernel.org/r/20231201161045.3962614-1-ryan.roberts@arm.comSigned-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/readahead.c

@@ -511,16 +511,14 @@ void page_cache_ra_order(struct readahead_control *ractl,
 		unsigned int order = new_order;
 
 		/* Align with smaller pages if needed */
-		if (index & ((1UL << order) - 1)) {
+		if (index & ((1UL << order) - 1))
 			order = __ffs(index);
-			if (order == 1)
-				order = 0;
-		}
 		/* Don't allocate pages past EOF */
-		while (index + (1UL << order) - 1 > limit) {
-			if (--order == 1)
-				order = 0;
-		}
+		while (index + (1UL << order) - 1 > limit)
+			order--;
+		/* THP machinery does not support order-1 */
+		if (order == 1)
+			order = 0;
 		err = ra_alloc_folio(ractl, index, mark, order, gfp);
 		if (err)
 			break;