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