Re: how does kernel get the "current" task struct?

[Jamie Lokier]

Linus Torvalds wrote:
> > I'm lost.  I know you need aligned allocations.  I don't see why you
> > have to align a zone to get aligned allocations.
> 
> You don't have to. However, if you don't align the zone, you have to
> still offset a lot of the calculations by the "alignment factor". Which
> bacially means that you in practice _do_ end up aligning it after all,
> it's just that you're doing it at run-time instead of setup-time. No
> real difference, and doing it at setup is clearer I think.

Fair enough.  I see that for the bitmap index in particular.

I suppose a small amount of memory could be saved by noting that, as
some parts of a not-really-aligned zone are never allocated, those parts
of the bitmap don't have to exist.

Slightly different topic: I think I see a bug.  This code in
free_pages_ok:

     map_nr &= mask;
...
             buddy = list(map_nr ^ -mask);
...
             mask <<= 1;
...
             map_nr &= mask;

map_nr here is relative to the first page in mem_map, whichever zone is
used.  That means the first page in mem_map had better have as much
alignment as any zone, otherwise the masking operations on map_nr aren't
going to work nicely at all.  This only affects the folks who are asking
about systems with memory that doesn't start at address zero.

enjoy,
-- Jamie