Re: [Xen-devel] [PATCH v2 19/20] x86/mem_sharing: reset a fork

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.12.2019 01:15, Tamas K Lengyel wrote:
> On Wed, Dec 18, 2019 at 4:02 PM Julien Grall <julien@xxxxxxx> wrote:
>> On 18/12/2019 22:33, Tamas K Lengyel wrote:
>>> On Wed, Dec 18, 2019 at 3:00 PM Julien Grall <julien@xxxxxxx> wrote:
>>>> You also have multiple loop on the page_list in this function. Given the
>>>> number of page_list can be quite big, this is a call for hogging the
>>>> pCPU and an RCU lock on the domain vCPU running this call.
>>>
>>> There is just one loop over page_list itself, the second loop is on
>>> the internal list that is being built here which will be a subset. The
>>> list itself in fact should be small (in our tests usually <100).
>>
>> For a first, nothing in this function tells me that there will be only
>> 100 pages. But then, I don't think this is right to implement your
>> hypercall based only the  "normal" scenario. You should also think about
>> the "worst" case scenario.
>>
>> In this case the worst case scenario is have hundreds of page in page_list.
> 
> Well, this is only an experimental system that's completely disabled
> by default. Making the assumption that people who make use of it will
> know what they are doing I think is fair.

FWIW I'm with Julien here: The preferred course of action is to make
the operation safe against abuse. The minimum requirement is to
document obvious missing pieces for this to become supported code.

Jan

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