linux-kernel - [PATCH] shm: Fix unlikely() test of info->seals to test only for WRITE and GROW

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Message-ID: <20170203105656.7aec6237@gandalf.local.home>
Date:   Fri, 3 Feb 2017 10:56:56 -0500
From:   "Steven Rostedt (VMware)" <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Herrmann <dh.herrmann@...il.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: [PATCH] shm: Fix unlikely() test of info->seals to test only for
 WRITE and GROW

From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>

Running my likely/unlikely profiler, I discovered that the test in
shmem_write_begin() that tests for info->seals as unlikely, is
always incorrect. This is because shmem_get_inode() sets info->seals to
have F_SEAL_SEAL set by default, and it is unlikely to be cleared when
shmem_write_begin() is called. Thus, the if statement is very likely.

But as the if statement block only cares about F_SEAL_WRITE and
F_SEAL_GROW, change the test to only test those two bits.

Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index bb53285..ef4cdbb 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2194,7 +2194,7 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
 	pgoff_t index = pos >> PAGE_SHIFT;
 
 	/* i_mutex is held by caller */
-	if (unlikely(info->seals)) {
+	if (unlikely(info->seals & (F_SEAL_WRITE | F_SEAL_GROW))) {
 		if (info->seals & F_SEAL_WRITE)
 			return -EPERM;
 		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
-- 
2.9.3