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:
>>>
>>>   
>>>       
>>>> 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.
>   
>> 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...