fix PSS graph on recent kernels

[amurzeau]

It seems the smaps file layout in /proc changed in some kernel version. This commit fix it by:

• Using smaps_rollup instead of smaps to avoid having to loop over all entries (smaps_rollup being a summary of smaps)
• Searching "Pss:" instead of using hardcoded offsets (which will probably go wrong again in the future).

Fixes: #46

Note that the issue mentions the entropy graph too, but this one is working fine, only the PSS graph was corrupted (because of random value, some of them negative).

[bluca]

Is this change compatible with both old and new kernels?

[amurzeau]

The "Pss:" term exists since the PSS implementation (2.6.25, torvalds/linux@ec4dd3e), but the use of smaps_rollup requires a 4.14 kernel.
smaps_rollup was introduced here: torvalds/linux@493b0e9

To avoid the requirement of the 4.14 kernel, the PR needs to be changed to use smaps instead and compute the sum of Pss: for all occurrences.

[bluca]

Why not just check if the smaps_rollup file exists, and if so use the new implementation, and if not fallback to the previous one?

[amurzeau]

The previous way was assuming a fixed number of byte for each block in smaps.
There is no real reason except code it was easier to do like this, but this check can be implemented for sure.

[amurzeau]

I've introduced the check for the presence of smaps_rollup.
I've also replaced continue case of failure to open smaps with goto catch_rename to not skip remaining sampling operations.