Re: [Xen-devel] "Deprecate" order>0 allocations? (was: [PATCH 1 of 2] Global virq for low memory situations)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.02.12 at 17:20, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Wednesday, February 29, 2012 1:54 AM
>> To: Dan Magenheimer
>> Cc: ian.campbell@xxxxxxxxxx; ian.jackson@xxxxxxxxxx; adin@xxxxxxxxxxxxxx; 
> andres@xxxxxxxxxxxxxx;
>> Andres Lagar-Cavilla; xen-devel; tim@xxxxxxx 
>> Subject: RE: [Xen-devel] [PATCH 1 of 2] Global virq for low memory situations
>> 
>> >>> On 29.02.12 at 00:50, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> wrote:
>> > Or maybe Jan's recent work has eliminated all but the corner
>> > cases of order>0 allocations?  (/me crosses fingers)
>> 
>> I'm unaware of remaining violators of this, but I'm also not constantly
>> monitoring.
>> 
>> Jan
> 
> As various "move memory back and forth between domains"
> solutions are becoming increasingly fashionable, use
> of order>0 allocations remains a ticking timebomb.
> 
> Does it make sense then now to somehow "deprecate" the
> code in the allocator that allows order>0 allocations?
> Not quite sure how, so just brainstorming.  Some ideas:
> 
> 1) At minimum, a very big comment warning about the
>    silent pitfalls due to fragmentation and a WARN_ON_ONCE
>    that occurs when order>1 and debug=1 (and for xen-unstable).
> 2) Add a new routine for order>0 allocations (with
>    a WARN_ON and a big comment) and BUG_ON order>0
>    allocations attempted via the old interface.

No, that doesn't sound feasible. When creating HVM guests, we
will want to be able to attempt large page allocations without
getting warned about, as the caller can handle the failure. That
might also be the case in other situations.

> IIRC, last I heard, some of the PCI-passthrough code
> still uses order>0 allocations, so this (and any
> other remaining culprits) might need to be grandfathered
> in somehow.

When was this "last I heard"? I thought I caught them all (largely
by storing per-interrupt information in a radix tree of per-interrupt
data structures rather than in nr_pirqs or nr_irqs sized arrays).

Jan


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