Re: [PATCH 06/16] x86/mm/ldt: Reserve high address-space range for the LDT

[Waiman Long]

On 01/16/2018 12:31 PM, Peter Zijlstra wrote:
> On Tue, Jan 16, 2018 at 06:13:43PM +0100, Joerg Roedel wrote:
>> Hi Peter,
>>
>> On Tue, Jan 16, 2018 at 05:52:13PM +0100, Peter Zijlstra wrote:
>>> On Tue, Jan 16, 2018 at 05:36:49PM +0100, Joerg Roedel wrote:
>>>> From: Joerg Roedel <jroedel@xxxxxxx>
>>>>
>>>> Reserve 2MB/4MB of address space for mapping the LDT to
>>>> user-space.
>>> LDT is 64k, we need 2 per CPU, and NR_CPUS <= 64 on 32bit, that gives
>>> 64K*2*64=8M > 2M.
>> Thanks, I'll fix that in the next version.
> Just lower the max SMP setting until it fits or something. 32bit is too
> address space starved for lots of CPU in any case, 64 CPUs on 32bit is
> absolutely insane.

Maybe we can just scale the amount of reserved space according to the
current NR_CPUS setting. In this way, we won't waste more memory than is
necessary.