linux-kernel - [PATCH] shmem: use monotonic time for i

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Date:   Wed, 20 Jun 2018 10:25:39 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>
Cc:     y2038@...ts.linaro.org, Arnd Bergmann <arnd@...db.de>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] shmem: use monotonic time for i_generation

get_seconds() is deprecated because it will lead to a 32-bit overflow
in 2038 or 2106. We don't need the i_generation to be strictly
monotonic anyway, and other file systems like ext4 and xfs just use
prandom_u32(), so let's use the same one here.

If this is considered too slow, we could also use ktime_get_seconds()
or ktime_get_real_seconds() to keep the previous behavior.
Both of these return a time64_t and are not deprecated, but only
return a unique value once per second, and are predictable.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 mm/shmem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 2cab84403055..387ae5323f56 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -29,6 +29,7 @@
 #include <linux/pagemap.h>
 #include <linux/file.h>
 #include <linux/mm.h>
+#include <linux/random.h>
 #include <linux/sched/signal.h>
 #include <linux/export.h>
 #include <linux/swap.h>
@@ -2187,7 +2188,7 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
 		inode_init_owner(inode, dir, mode);
 		inode->i_blocks = 0;
 		inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
-		inode->i_generation = get_seconds();
+		inode->i_generation = prandom_u32();
 		info = SHMEM_I(inode);
 		memset(info, 0, (char *)inode - (char *)info);
 		spin_lock_init(&info->lock);
-- 
2.9.0