limits / PIPE_BUF?

[Vadim Lobanov <vlobanov@speakeasy.net>]

Hi,

I was reading through the include/linux/limits.h file in order to
generate a cleanup patch for it -- a large number of #defines within
that file are no longer being used, as I surmise that they are simply
remnants of earlier implementations.

A snippet from include/linux/limits.h:
#define PIPE_BUF        4096    /* # bytes in atomic write to a pipe */

PIPE_BUF is a bit of an oddity. It is defined there, then redefined in
the arm header files, even though those header files are never included
anywhere. Also, PIPE_BUF is never referenced by name in any of the Linux
code. And yet, it is still being mentioned in some Big And Scary
Warnings (tm): fs/autofs4/waitq.c or include/linux/pipe_fs_i.h, for
example.

What's the deal with PIPE_BUF? Is its value used in the code indirectly,
so that more comments would help make this fact obvious? Or is it now
deprecated, and therefore a viable target for include/linux/limits.h
cleanups?

- Vadim Lobanov