Re: [Xen-devel] [PATCH 03 of 12] x86/mm: Add per-page locking for memory sharing, when audits are disabled

["Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>]

> At 21:56 -0500 on 15 Jan (1326664583), Andres Lagar-Cavilla wrote:
>> @@ -510,26 +684,63 @@ int mem_sharing_share_pages(struct domai
>>      smfn = get_gfn(sd, sgfn, &smfn_type);
>>      cmfn = get_gfn(cd, cgfn, &cmfn_type);
>>
>> -    ret = XEN_DOMCTL_MEM_SHARING_S_HANDLE_INVALID;
>> -    spage = mem_sharing_lookup(mfn_x(smfn));
>> -    if ( spage == NULL )
>> +    /* This tricky business is to avoid two callers deadlocking if
>> +     * grabbing pages in opposite client/source order */
>
> I think you need to delete the XXX comment just above. :)
>
Well, that XXX comment applies if get_gfn locks the p2m, and then what
we're grabbing in mem_sharing_lookup are lock per mfn, essentially. To
prevent deadlock, we will want to sort those p2m locks by ascending
domain, and then by ascending gfn within the same domain, if and when the
p2m lock is broken into fine-grained bits.

That is surely a battle for another day, I hope!
Andres
> Tim.
>
>



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