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

[Amerigo Wang]

Eric W. Biederman wrote:
> Amerigo Wang <amwang@xxxxxxxxxx> 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?

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?

> You patch also plays with global kexec variables outside of the mutex
> before calling into shrink_crash_memory.   If my memory serves just
> doing mutx_lock(&kexec_mutex) on this code path should be fine.  The
> mutex_trylock on the other code paths is about having non-blocking
> behavior that you don't need here.
>   

Hmm, yes, I will fix it... Thanks!

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