Re: [RFC Patch 2/2] kexec: allow to shrink reserved memory

[Amerigo Wang <amwang@redhat.com>]

Eric W. Biederman wrote:
> Amerigo Wang <amwang@redhat.com> writes:
>
>   
>> Eric W. Biederman wrote:
>>     
>>> Amerigo Wang <amwang@redhat.com> writes:
>>>
>>>   
>>>       
>>>> Eric W. Biederman wrote:
>>>>     
>>>>         
>>>>> Amerigo Wang <amwang@redhat.com> writes:
>>>>>
>>>>>         
>>>>>           
>>>>>> This patch implements shrinking the reserved memory for crash kernel,
>>>>>> if it is more than enough.
>>>>>>
>>>>>> For example, if you have already reserved 128M, now you just want 100M,
>>>>>> you can do:
>>>>>>
>>>>>> # echo $((100*1024*1024)) > /sys/kernel/kexec_crash_size
>>>>>>             
>>>>>>             
>>>>> This patch looks like a reasonable start.
>>>>>
>>>>> However once a crash kernel image is loaded we have already told that
>>>>> image about the memory that is available and what you are doing here
>>>>> will go and stop on the memory that is reserved but not yet used,
>>>>> totally breaking the DMA protections.  AKA we know the memory is safe
>>>>> from ongoing DMAs because it has lain fallow since boot up.
>>>>>
>>>>> The only safe thing to do is to reduce the memory size before (possibly
>>>>> just before) we load the crash kernel.  Which means we should only
>>>>> be allowed to shrink the size when nothing is loaded, exactly the
>>>>> opposite of what you have implemented.
>>>>>
>>>>>         
>>>>>           
>>>> Confused, why just loading the crash kernel makes it unsafe?
>>>> DMA should be avoided when reserving that memory during boot, shouldn't it?
>>>>     
>>>>         
>>> Yes.  But you are removing the reservation and starting DMA on memory
>>> we have told the crash kernel it can use.
>>>   
>>>       
>> We can modify the info given to the crash kernel.
>>     
>
> Only by unloading and reloading.
>
>   
>>>> I know I missed the part that freeing memory before loading, but if it is safe
>>>> before loading, how can it be unsafe after that?
>>>>     
>>>>         
>>> We tell the crash kernel when loading it, it can use all of the reserved memory.
>>>   
>>>       
>> Yeah, but we should reload the kernel after shrinking the memory, it is not
>> surprised that doing this is necessary...
>>     
>
> So unload the crash kernel first.  If you don't you open a race where
> many of the guarantees we make for the crash kernel about the state of
> the memory it might be using are not true.
>
> In general I expect we will be able to do this all before we load the
> crash kernel the first time.  But at least we should not need to reboot
> things if there is a problem.
>   

Ok, thanks for your explanation.

I will drop patch 1/2, since we will move it to kexec.
I will put patch 2/2 into my "crashkernel=auto" patch set, and resend 
them all.

Thanks.