Re: [RFC] 2.3.39 zone balancing

[kanoj@google.engr.sgi.com (Kanoj Sarcar)]

> 
> On Wed, 12 Jan 2000, Kanoj Sarcar wrote:
> 
> >+There are two reasons to be requesting non __GFP_WAIT allocations:
> >+the caller can not sleep (typically intr context), or does not want
> >+to incur cost overheads of page stealing and possible swap io.
> 
> You may be in a place where you can sleep but you can't do I/O to avoid
> deadlocking and so you shouldn't use __GFP_IO and nothing more (it has
> nothing to do with __GFP_WAIT).

You are right: the documentation should read:

+Memory balancing is _only_ needed for non __GFP_WAIT and non __GFP_IO allocations.
+
+There are two reasons to be requesting non __GFP_WAIT allocations:
+the caller can not sleep (typically intr context), or does not want
+to incur cost overheads of page stealing and possible swap io.
+
+Non __GFP_IO allocations are requested to prevent filesystem deadlocks.

But I would not say __GFP_WAIT and __GFP_IO have no relationship. __GFP_IO
does not make sense if __GFP_WAIT is not set. 

> 
> But if it can sleep and there aren't deadlock conditons going on and it
> doesn't use __GFP_WAIT, it means it's buggy and has to be fixed.
> 

Well, I thought about that while coding the patch: you can not try to 
outsmart the programmer who writes that code. For example, I was 
looking at replace_with_highmem() which makes __GFP_HIGHMEM|__GFP_HIGH
requests, although I _think_ it can do __GFP_WAIT|__GFP_IO without
any problems. I just assumed that whoever coded it (you/Mingo?) had
some logic, like not wanting to waste time scanning for stealable pages
or incur disk swap to implement this performance optimization (that
would defeat the optimization).

Kanoj

> I have not read the rest and the patch yet (I'll continue ASAP).
> 
> Andrea
> 
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