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

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2025-02-04 at 14:21 +0100, Oleg Nesterov wrote:
> 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;

Oh _anonymous_ pipes. You can disregard my earlier questions:

Reviewed-by: Jeff Layton <jlayton@kernel.org>