Re: [patch 2/2] mm: handle unaligned zones

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andy Whitcroft wrote:

> Ok.  I agree that that unaligned zones should be opt-in, it always was

Yes.

> However, this patch here seems redundant.  The requirement from the
> buddy allocator has been an aligned node_mem_map out to MAX_ORDER either
> side of the zones in that node.  With the recent patch from Bob Picco it
> is now allocated that way always.  So we will always have a page* from
> either the adjoining zone or from the node_mem_map padding to examine
> when we are looking for a buddy to coelesce with.  It should always be
> safe to examine that page*'s flags to see if its free to coelesce.  For
> pages outside any zone PG_buddy will never be true, for those in another
> zone the page_zone_idx() check is sufficient.

That's true - does this cover all architectures that do not define
CONFIG_HOLES_IN_ZONE ?

> With the page_zone_idx check enabled and the node_mem_map aligned, I
> cannot see why we would also need to check the zone pfn numbers too?  If
> we did need to check them, then there would be no benefit in checking
> the page_zone_idx as that check would always succeed.

Yes. BTW. are the struct pages outside the nodes going to be correctly
aligned? Either way, I think we should also check that everything has
been set up in the way we expect at meminit time (see my debug function).

> 
> I think the smallest, lightest weight set of changes for this problem is
> the node_mem_map alignement patch from Bob Picco, plus the changes to
> add just the page_zone_idx checks to the allocator.  If the stack that

Yes, that sounds fine.

> makes this an opt-out option is too large, a two liner to check just
> page_zone_idx always would be a good option for stable.

I think it is more a question of time for all arch maintainers to verify
rather than size.

If you just mean: you want to negate the meaning of the CONFIG_ option,
and go through and define it in all architectures, I'd be fine with that
too (by opt-in I just mean the check should be turned on until proven
otherwise)

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