2000-11-30 10:13:51

by uaca

[permalink] [raw]
Subject: innofensive BUG and fix: area->map's size is not calculated ok

In a 2.4 kernel, mm/page_alloc.c:free_area_init_core(), in the following

bitmap_size = size >> i;

...makes bitmap_size be the double of the needed size, this calculum
assumes that with a 512 byte map size the kernel is mapping 8*512= 4096 chunks
of memory, that is: one bit of the map is used for each chunk of memory,
and that's not true.

Really one bit of area->map is used to map two chunks of memory, so in the
example above an area->map of just 256 bytes is really needed, the other 256
bytes are _never accessed_.

So the righ thing would be to do is:

bitmap_size = size >> (i+1);

also I tested it and it works ok, so I believe I'm right...

Please CC: your replies to (I'm not subscribed to this list)

Ulisses Alonso Camar? <[email protected]>

PD: my nick on #kernelnewbies is despistao

Debian GNU/Linux: a dream come true
"Computers are useless. They can only give answers." Pablo Picasso

---> Visita http://www.valux.org/ para saber acerca de la <---
---> Asociaci?n Valenciana de Usuarios de Linux <---