[PATCH] pipe: don't update {a,c,m}time for anonymous pipes

[Oleg Nesterov <oleg@redhat.com>]

These numbers are visible in fstat() but hopefully nobody uses this
information and file_accessed/file_update_time are not that cheap.
Stupid test-case:

	#include <stdio.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <assert.h>
	#include <sys/ioctl.h>
	#include <sys/time.h>

	static char buf[17 * 4096];
	static struct timeval TW, TR;

	int wr(int fd, int size)
	{
		int c, r;
		struct timeval t0, t1;

		gettimeofday(&t0, NULL);
		for (c = 0; (r = write(fd, buf, size)) > 0; c += r);
		gettimeofday(&t1, NULL);
		timeradd(&TW, &t1, &TW);
		timersub(&TW, &t0, &TW);

		return c;
	}

	int rd(int fd, int size)
	{
		int c, r;
		struct timeval t0, t1;

		gettimeofday(&t0, NULL);
		for (c = 0; (r = read(fd, buf, size)) > 0; c += r);
		gettimeofday(&t1, NULL);
		timeradd(&TR, &t1, &TR);
		timersub(&TR, &t0, &TR);

		return c;
	}

	int main(int argc, const char *argv[])
	{
		int fd[2], nb = 1, loop, size;

		assert(argc == 3);
		loop = atoi(argv[1]);
		size = atoi(argv[2]);

		assert(pipe(fd) == 0);
		assert(ioctl(fd[0], FIONBIO, &nb) == 0);
		assert(ioctl(fd[1], FIONBIO, &nb) == 0);

		assert(size <= sizeof(buf));
		while (loop--)
			assert(wr(fd[1], size) == rd(fd[0], size));

		struct timeval tt;
		timeradd(&TW, &TR, &tt);
		printf("TW = %lu.%03lu TR = %lu.%03lu TT = %lu.%03lu\n",
			TW.tv_sec, TW.tv_usec/1000,
			TR.tv_sec, TR.tv_usec/1000,
			tt.tv_sec, tt.tv_usec/1000);

		return 0;
	}

Before:
	# for i in 1 2 3; do /host/tmp/test 10000 100; done
	TW = 8.047 TR = 5.845 TT = 13.893
	TW = 8.091 TR = 5.872 TT = 13.963
	TW = 8.083 TR = 5.885 TT = 13.969
After:
	# for i in 1 2 3; do /host/tmp/test 10000 100; done
	TW = 4.752 TR = 4.664 TT = 9.416
	TW = 4.684 TR = 4.608 TT = 9.293
	TW = 4.736 TR = 4.652 TT = 9.388

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/pipe.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 94b59045ab44..baaa8c0817f1 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -247,6 +247,11 @@ static inline unsigned int pipe_update_tail(struct pipe_inode_info *pipe,
 	return tail;
 }
 
+static inline bool is_pipe_inode(struct inode *inode)
+{
+	return inode->i_sb->s_magic == PIPEFS_MAGIC;
+}
+
 static ssize_t
 pipe_read(struct kiocb *iocb, struct iov_iter *to)
 {
@@ -404,7 +409,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 	if (wake_next_reader)
 		wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
 	kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
-	if (ret > 0)
+	if (ret > 0 && !is_pipe_inode(file_inode(filp)))
 		file_accessed(filp);
 	return ret;
 }
@@ -604,11 +609,13 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 	kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 	if (wake_next_writer)
 		wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
-	if (ret > 0 && sb_start_write_trylock(file_inode(filp)->i_sb)) {
-		int err = file_update_time(filp);
-		if (err)
-			ret = err;
-		sb_end_write(file_inode(filp)->i_sb);
+	if (ret > 0 && !is_pipe_inode(file_inode(filp))) {
+		if (sb_start_write_trylock(file_inode(filp)->i_sb)) {
+			int err = file_update_time(filp);
+			if (err)
+				ret = err;
+			sb_end_write(file_inode(filp)->i_sb);
+		}
 	}
 	return ret;
 }
@@ -1108,7 +1115,7 @@ static void wake_up_partner(struct pipe_inode_info *pipe)
 static int fifo_open(struct inode *inode, struct file *filp)
 {
 	struct pipe_inode_info *pipe;
-	bool is_pipe = inode->i_sb->s_magic == PIPEFS_MAGIC;
+	bool is_pipe = is_pipe_inode(inode);
 	int ret;
 
 	filp->f_pipe = 0;
-- 
2.25.1.362.g51ebf55