grep: remove --mmap

mmap is a bad idea for sequentially accessed file because it will cause
a page fault for every read page.  Just consider it a failed experiment,
and ignore --mmap while accepting it for backwards compatibility.

* configure.ac (AC_FUNC_MMAP): Remove.
* doc/grep.texi (Other options): Say --mmap is ignored.
* src/grep.c (mmap_option): Remove.
(long_options): Do not reference it.
(bufmapped, initial_bufoffset): Remove.
(reset, fillbuf): Remove HAVE_MMAP code.
(grepfile): Remove bufmapped reference.
(usage): Say --mmap is ignored.

1 files changed, 4 insertions, 6 deletions

diff --git a/doc/grep.texi b/doc/grep.texi
index c2a494d1..87480de4 100644
--- a/doc/grep.texi
+++ b/doc/grep.texi
@@ -667,12 +667,10 @@ This can cause a performance penalty.
 @item --mmap
 @opindex --mmap
 @cindex memory mapped input
-If possible, use the @code{mmap} system call to read input,
-instead of the default @code{read} system call.
-In some situations, @samp{--mmap} yields better performance.
-However, @samp{--mmap} can cause undefined behavior (including core dumps)
-if an input file shrinks while @command{grep} is operating,
-or if an I/O error occurs.
+This option is ignored for backwards compatibility.  It used to read
+input with the @code{mmap} system call, instead of the default @code{read}
+system call.  On modern systems, @samp{--mmap} rarely if ever yields
+better performance.
 
 @item -U
 @itemx --binary