Show real swap usage for each individual process

Update: the kernel patch is already in 2.6.34-rcN.

Currently, in top, the SWAP column only shows the difference between VIRT and RES:

    case P_SWP:
     MKCOL(scale_num(PAGES_TO_KB(p->size - p->resident), w, s));

So currently the value is of no use.

As a matter of fact, a recent enough kernel has provided the information which is needed to calculate the swap usage for each process. It is in /proc/$PID/smaps. But using smaps may affect top's performance because you have to calculate the sum yourself.

But with this patch https://patchwork.kernel.org/patch/56929/, the kernel will keep track of each process's swap usage for you. This patch (maybe not exactly in the same shape) is already in mmotm tree (http://userweb.kernel.org/~akpm/mmotm/) 2.6.34-rcN.

However, checking the /proc/$PID/status file directly may not be a thing which people would like to do. So I decided to make a patch for ps and top. Now it seems to be working.

This is the vanilla ps:
$ ps -o m_swap,comm -e | grep firefox
- firefox

This is modified ps:
$ ./ps/ps -o m_swap,comm -e | grep firefox
2984 firefox

This is a picture showing a modified top:


The patch can be found here:
http://dev.gentoo.org/~r0bertz/per-process-swap-usage.patch