Re: [Xen-devel] [PATCH] x86/NUMA: make init_node_heap() respect Xen heap limit

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.09.15 at 10:27, <ian.campbell@xxxxxxxxxx> wrote:
> On Fri, 2015-09-04 at 01:37 -0600, Jan Beulich wrote:
>> > > > On 03.09.15 at 22:58, <julien.grall@xxxxxxxxxx> wrote:
>> > And found why! The last xenheap_bits changed from 39 to 38.
>> > 
>> > On x-gene the last max mfn used for the xenheap is 0x4400000, which the
>> > new computation, it will give 38 bits which doesn't cover the entire
>> > xenheap range.
>> > 
>> > I have wrote a patch to fix the issue, but I'm not sure that it's
>> > the right things to do (see below).
>> 
>> No, this is wrong: xenheap_bits isn't meant to cover all RAM, it is
>> meant to indicate how much (as an exact power of 2) of RAM is
>> always accessible. I'm surprised anyway that ARM64 uses
>> xenheap_max_mfn() (and even unconditionally); I thought all RAM
>> is always accessible there. The invocation is off by one now in any
>> case, but rather than correcting it that way the proper fix likely
>> will involve more than just this simple an adjustment, as it looks
>> like its use was wrong from the beginning (commit 5263507b1b).
> 
> What is the correct thing which arm64 should be doing, given that today all
> RAM is indeed always mapped? Not call xenheap_max_mfn at all, therefore
> leaving xenheap_bits == 0?

Yes, if all memory is always accessible, then no limit should be
enforced at all, i.e. the call be dropped.

Just for the record - even with the call in place it escapes me why
this causes any problem: All it tells the allocator is to not hand out
pages above a certain limit when asked for Xen heap pages. Sadly
I wasn't able to interpret the dumped information (after the crash)
in a way telling me what actually went wrong. IOW - I'm afraid
there's a second problem here that's going to be hidden again
when the call gets removed.

Jan


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