+ mm-readahead-do-not-allow-order-1-folio.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/readahead: do not allow order-1 folio
has been added to the -mm mm-unstable branch.  Its filename is
     mm-readahead-do-not-allow-order-1-folio.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-readahead-do-not-allow-order-1-folio.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Ryan Roberts <ryan.roberts@arm.com>
Subject: mm/readahead: do not allow order-1 folio
Date: Fri, 1 Dec 2023 16:10:45 +0000

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.com
Signed-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 |   14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

--- a/mm/readahead.c~mm-readahead-do-not-allow-order-1-folio
+++ a/mm/readahead.c
@@ -511,16 +511,14 @@ void page_cache_ra_order(struct readahea
 		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;
_

Patches currently in -mm which might be from ryan.roberts@arm.com are

mm-readahead-do-not-allow-order-1-folio.patch