Rik van Riel wrote:
On Tue, 14 Jun 2005, Jim Cromie wrote:
IIUC, only fork sets up the new process with COW-able pages, which may
or may not be copied-on-write, dependent upon whether a write happens to
a given page.
There's an additional source for COW pages: MAP_PRIVATE mmap
of files, especially shared libraries. When an application
maps a shared library, this is done with a writable MAP_PRIVATE
mmap. This means that the application can write to the mmaped
region, but the writes will not be written back to the file.
Instead, the writes will go to a private copy of the page
that was dirtied. A COW page...
is this a common thing ?
or, why would shared-libs be written to, except for self-modifying code ?
linking of dynamically loaded libs ( ie loaded mid-run ) ?
heres the /proc/$$/maps taken from a bash shell.
[root@harpo 6761]# more maps
00a6e000-00a83000 r-xp 00000000 03:02 2125172 /lib/ld-2.3.5.so
00a84000-00a85000 r-xp 00015000 03:02 2125172 /lib/ld-2.3.5.so
00a85000-00a86000 rwxp 00016000 03:02 2125172 /lib/ld-2.3.5.so
00a88000-00bab000 r-xp 00000000 03:02 2125180 /lib/tls/libc-2.3.5.so
00bab000-00bac000 --xp 00123000 03:02 2125180 /lib/tls/libc-2.3.5.so
00bac000-00bae000 r-xp 00123000 03:02 2125180 /lib/tls/libc-2.3.5.so
00bae000-00bb0000 rwxp 00125000 03:02 2125180 /lib/tls/libc-2.3.5.so
00bb0000-00bb2000 rwxp 00bb0000 00:00 0
00bd9000-00bdb000 r-xp 00000000 03:02 2126327 /lib/libdl-2.3.5.so
00bdb000-00bdc000 r-xp 00001000 03:02 2126327 /lib/libdl-2.3.5.so
00bdc000-00bdd000 rwxp 00002000 03:02 2126327 /lib/libdl-2.3.5.so
00dbe000-00dc1000 r-xp 00000000 03:02 2126336 /lib/libtermcap.so.2.0.8
00dc1000-00dc2000 rwxp 00002000 03:02 2126336 /lib/libtermcap.so.2.0.8
08047000-080d8000 r-xp 00000000 03:02 659092 /bin/bash
080d8000-080de000 rwxp 00090000 03:02 659092 /bin/bash
080de000-080e3000 rwxp 080de000 00:00 0
09b20000-09b62000 rwxp 09b20000 00:00 0 [heap]
b7d80000-b7d89000 r-xp 00000000 03:02 2121646 /lib/libnss_files-2.3.5.so
b7d89000-b7d8a000 r-xp 00008000 03:02 2121646 /lib/libnss_files-2.3.5.so
b7d8a000-b7d8b000 rwxp 00009000 03:02 2121646 /lib/libnss_files-2.3.5.so
b7d9e000-b7da0000 rwxp b7d9e000 00:00 0
b7da0000-b7da6000 r-xs 00000000 03:02 39351
/usr/lib/gconv/gconv-modules.cache
b7da6000-b7fa6000 r-xp 00000000 03:02 2532494
/usr/lib/locale/locale-archive
b7fa6000-b7fa8000 rwxp b7fa6000 00:00 0
b7fc3000-b7fc4000 r-xp b7fc3000 00:00 0
bfdaf000-bfdc4000 rw-p bfdaf000 00:00 0 [stack]
I presume all the w pages are COW-able
to try to answer myself, I did this:
[root@harpo proc]# more 6761/maps |wc
26 151 1669
[root@harpo proc]# sort {6697,6761}/maps |wc
52 302 3338
[root@harpo proc]# sort -u {6697,6761}/maps |wc
36 208 2297
so 5 mappings in each process have apparently been altered,
unsprisingly including [heap] and [stack]
The rest of the COW-able maps havent been COW-ed yet.
09b20000-09b62000 rwxp 09b20000 00:00 0 [heap]
09b96000-09bd8000 rwxp 09b96000 00:00 0 [heap]
b7d2a000-b7d30000 r-xs 00000000 03:02 39351
/usr/lib/gconv/gconv-modules.cache
b7d30000-b7f30000 r-xp 00000000 03:02 2532494
/usr/lib/locale/locale-archive
b7da0000-b7da6000 r-xs 00000000 03:02 39351
/usr/lib/gconv/gconv-modules.cache
b7da6000-b7fa6000 r-xp 00000000 03:02 2532494
/usr/lib/locale/locale-archive
b7f30000-b7f32000 rwxp b7f30000 00:00 0
b7f4d000-b7f4e000 r-xp b7f4d000 00:00 0
b7fa6000-b7fa8000 rwxp b7fa6000 00:00 0
b7fc3000-b7fc4000 r-xp b7fc3000 00:00 0
bfd39000-bfd4e000 rw-p bfd39000 00:00 0 [stack]
bfdaf000-bfdc4000 rw-p bfdaf000 00:00 0 [stack]
I can see why a cache is COW-ed, but why a locale-archive ?
and what are those nameless maps ?
For everyone else reading along, the questions may be dumb, but the
answers are *authorititative*
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