linux-kernel - [PATCH] mm: shmem: fix minor off-by-one in shrinkable calculation

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Message-ID: <d8e75079-af2d-8519-56df-6be1dccc247a@google.com>
Date: Sun, 25 Aug 2024 15:42:45 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
    Baolin Wang <baolin.wang@...ux.alibaba.com>, 
    Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org
Subject: [PATCH] mm: shmem: fix minor off-by-one in shrinkable calculation

There has been a long-standing and very minor off-by-one, where
shmem_get_folio_gfp() decides if a large folio extends beyond i_size
far enough to leave a page or more for freeing later under pressure.

This is not something needed for stable: but it will be proportionately
more significant as support for smaller large folios is added, and is
best fixed before duplicating the check in other places.

Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure")
Signed-off-by: Hugh Dickins <hughd@...gle.com>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 85e3bd3e709e..37c300f69baf 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2326,7 +2326,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	alloced = true;
 	if (folio_test_large(folio) &&
 	    DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE) <
-					folio_next_index(folio) - 1) {
+					folio_next_index(folio)) {
 		struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 		struct shmem_inode_info *info = SHMEM_I(inode);
 		/*
-- 
2.35.3