linux-kernel - [PATCH] vfs: replace current_kernel

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Message-Id: <20180620150138.49380-1-arnd@arndb.de>
Date:   Wed, 20 Jun 2018 17:01:24 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     y2038@...ts.linaro.org, Andi Kleen <andi.kleen@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jeff Layton <jlayton@...hat.com>, Jan Kara <jack@...e.cz>,
        Brian Foster <bfoster@...hat.com>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] vfs: replace current_kernel_time64 with ktime equivalent

current_time is one of the few callers of current_kernel_time64(), which
is a wrapper around ktime_get_coarse_real_ts64(). This calls the latter
directly for consistency with the rest of the kernel that is moving to
the ktime_get_ family of time accessors.

An open questions is whether we may want to actually call the more
accurate ktime_get_real_ts64() for file systems that save high-resolution
timestamps in their on-disk format. This would add a small but measurable
overhead to each update of the inode stamps but lead to inode timestamps
to actually have a usable resolution better than one jiffy (1 to 10
milliseconds normally).

I traced the original addition of the current_kernel_time() call to set
the nanosecond fields back to linux-2.5.48, where Andi Kleen added a
patch with subject "nanosecond stat timefields". This adds the original
call to current_kernel_time and the truncation to the resolution of the
file system, but makes no mention of the intended accuracy.  At the time,
we had a do_gettimeofday() interface that on some architectures could
return a microsecond-resolution timestamp, but there was no interface
for getting an accurate timestamp in nanosecond resolution, neither inside
the kernel nor from user space. This makes me suspect that the use of
coarse timestamps was never really a conscious decision but instead
a result of whatever API was available 16 years ago.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 fs/inode.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 2c300e981796..e27bd9334939 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2133,7 +2133,9 @@ EXPORT_SYMBOL(timespec64_trunc);
  */
 struct timespec64 current_time(struct inode *inode)
 {
-	struct timespec64 now = current_kernel_time64();
+	struct timespec64 now;
+
+	ktime_get_coarse_real_ts64(&now);

 	if (unlikely(!inode->i_sb)) {
 		WARN(1, "current_time() called with uninitialized super_block in the inode");
-- 
2.9.0