[PATCH v1] mm/readahead: Do not allow order-1 folio

[PATCH v1] mm/readahead: Do not allow order-1 folio

[Ryan Roberts]

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).

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 mm/readahead.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 6925e6959fd3..23620c57c122 100644
--- a/mm/readahead.c
+++ b/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;
--
2.25.1

Re: [PATCH v1] mm/readahead: Do not allow order-1 folio

[Matthew Wilcox]

On Fri, Dec 01, 2023 at 04:10:45PM +0000, Ryan Roberts wrote:
> 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).
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>

It's better code anyway!

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>