Re: [PATCH 7/8] pipe: make account_pipe_buffers() return a value, and use it

On 08/19/2016 09:36 PM, Vegard Nossum wrote:

On 08/19/2016 07:25 AM, Michael Kerrisk (man-pages) wrote:

quoted

This is an optional patch, to provide a small performance improvement.
Alter account_pipe_buffers() so that it returns the new value in
user->pipe_bufs. This means that we can refactor too_many_pipe_buffers_soft()
and too_many_pipe_buffers_hard() to avoid the costs of repeated use of
atomic_long_read() to get the value user->pipe_bufs.

[...]

quoted

@@ -627,17 +625,18 @@ struct pipe_inode_info *alloc_pipe_info(void)
  	struct pipe_inode_info *pipe;
  	unsigned long pipe_bufs = PIPE_DEF_BUFFERS;
  	struct user_struct *user = get_current_user();
+	unsigned long num_bufs;

Maybe user_bufs would be more descriptive since num_bufs is a bit
ambiguous without the context.

Okay -- changed.

quoted

  	pipe = kzalloc(sizeof(struct pipe_inode_info), GFP_KERNEL_ACCOUNT);
  	if (pipe == NULL)
  		goto out_free_uid;

-	if (too_many_pipe_buffers_soft(user))
+	if (too_many_pipe_buffers_soft(atomic_long_read(&user->pipe_bufs)))
  		pipe_bufs = 1;

-	account_pipe_buffers(user, 0, pipe_bufs);
+	num_bufs = account_pipe_buffers(user, 0, pipe_bufs);

-	if (too_many_pipe_buffers_hard(user))
+	if (too_many_pipe_buffers_hard(num_bufs))
  		goto out_revert_acct;

  	pipe->bufs = kcalloc(pipe_bufs, sizeof(struct pipe_buffer),

Why not structure it like this?

num_bufs = account_pipe_buffers(user, 0, pipe_bufs);
if (too_many_pipe_buffers_soft(num_bufs)) {
	num_bufs = account_pipe_buffers(user, pipe_bufs, 1);
	pipe_bufs = 1;
}
if (too_many_pipe_buffers_hard(num_bufs))
	goto out_revert_acct;

Otherwise you still have the case that somebody makes it past
too_many_pipe_buffers_soft() before the accounting is done.

Ahh -- thanks! I knew there was a small glitch there, but decided to 
ignore it because didn't see the obvious solution. Fixed in 6/8 patch.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/